Adaptation of Unicellular Algae to Irradiance: an Analysis of Strategies by K

Home , Plant physiology, Pyrocystis fusiformis

New PhytoL (19S3) 93, 157-191 157

ADAPTATION OF UNICELLULAR ALGAE TO IRRADIANCE: AN ANALYSIS OF STRATEGIES BY K. RICHARDSON, J. BEARDALL* AND J. A. RAVEN Department of Biological Sciences, University of Dundee, Dundee DDI 4HN, Scotland, U.K.

(Accepted 1 October 1982)

CONTENTS

SUMMARY 157 INTRODUCTION 158 AN.^LYTICAL METHODS 159 LIGHT HARVESTING BY MICROALGAE 160 RANGE OF PHOTON FLUX DENSITIES ALLOWING GROWTH AND PHOTOSYNTHESIS IN PHOTOTHOPHIC MICROALGAE (GENOTYPIC ADAPTATION) 163 Growth 163 Photosynthesis 165 Photoinhibition 165 PHENOTYPIC ADAPTATION 168 Changes in amounts of pigments 168 Interpretation of the effects of pigment changes: models 169 Observed changes in P vs I curves 170 ENERGETIC CONSIDERATIONS 174 General 174 Reduction of capital costs 175 Reduction of maintenance costs 175 Energetic costs of changing the photosyothetic apparatus 177 S2-S3 decay 177 Proton leakage due to passive uniport 178 PHYLOGENETIC ASPECTS OF DIFFERENCES IN LIGHT RESPONSES OF MICROALGAL PHOTOSYNTHESIS AND PHOTOLITHOTROPHIC GROWTH 178 Phylogenetic diflerences in photosynthetic structures 178 Comparison of tbe photosynthetic characteristics of green algae and other pbototrophs 180 ECOLOGICAL CONSIDERATIONS 182 ACKNOWLEDGEMENTS 185 REFERENCES 185

SUMMARY Analysis of data in the literature relating to micrcalgal adaptations to different photon flux densities indicates that different algal classes have significantly different ligbt requirennents for growth and photosynthesis. Although there is some variability within each class, dinoflagellates and blue-green algae generally photosynthesize and grow best at low photon flux densities. Diatoms also tend to be able to grow at very low photon flux densities (growth for some species has been reported at less than 1 fi.E m"' s~'). Comparison of the photon flux densities at which photoinhibition occurs in dinoflagellates and diatoms suggests that the former often experience photoinhibition at comparatively low irradiances. In contrast, diatoms often can tolerate relatively high light environments. This tolerance of a large absolute range of photon flux densities may, in part, explain why diatoms are often associated with spring blooms. Green algae

* New address; Department of Botany, La Trobe University, Bundoora, Victoria, 3083 Australia.

tend to exhibit higher light compensation points than the other common microalgal classes and can tolerate very high light environments. The photosynthesis vs irradiance curves of high and low light adapted microalgae are discussed in relation to the theoretical models that other workers have put forward to describe phytoplankton photoadaptation as a function of either an increased number or an increase in the size of photosynthetic units in a cell. We conclude that while the models form a useful starting point for the study of photoadaptation, they cannot explain all of the strategies observed. Microalgal photoadaptation is discussed in terms of the possible mechanisms by which a cell could reduce its capital and maintenance costs under conditions of limiting energy supply. Phylogenetic aspects of the differences in light responses of tbe various microalgal classes and the ecological implications of different photoadaptive strategies are also considered.

INTRODUCTION By comparison with most vascular plants, unicellular microalgae occupy habitats characterized by very low photon flux densities. Most microalgae live in aquatic environments where light is attenuated exponentially with depth according to the Lambert-Beer Law: where d is depth, !„ is the incident radiation upon the surface of the vifater, I^ is the photon flux density at depth, d, and k is an extinction coefficient. Typically, clear temperate coastal waters exhibit a k for photosynthetically active radiation (PAR), 400 to 700 nm of about 0-15 while in the clearest oceanic waters k may be in the region of 0"04. In addition to attenuating light, water selectively filters the light passing through it and although the quality of light penetrating to depth varies in different water types, in all types red light (> 600 nm) is essentially undetectable at depths of 10 m or more. (For detailed discussions of light and the underwater environment, see Jerlov, 1968, 1976; Wheeler and Neushal, 1981). The ability of microalgae to survive and grow in habitats where they nnay experience exposure to very low photon flux densities must result from the interaction of structural, behavioural, physiological and biochemical factors. Unlike macroscopic algae and vascular plants, unicellular phytoplankton do not have large amounts of non-photosynthetic tissue to support. This structural difference between unicellular algae and higher plants tnust certainly make the former more suited to life at low photon fiux densities. Furthermore, although most macroscopic algae and vascular plants occupy a fixed topographic position and in order to survive must be able to tolerate all environmental extremes that that position experiences, some phytoplankton are able, at least to some extent, to regulate their position in the water column. This tneans that some species may be able to exert some control over their light environment as well. Microalgae are better suited for the study of photoadaptive strategies than macrophytes because the latter can adapt morphologically to changes in the light environment and, therefore, may be less reliant on cell-level changes than unicells. Also, terrestial macrophytes have more extreme thermo/water regulation problems in 'sun' than in 'shade' environments. Thus, photoadaptive strategies may be difficult to differentiate from temperature or dessication responses. To date, a large number of physiological and biochemical investigations have been carried out on the problem of the relationship between irradiance and algal growth and photosynthesis. Adaptation of unicellular algae 159 Most such studies of phytoplankton photoadaptation have examined responses of an individual species confronted with a change in its light environment. By analogy with vascular plants, investigators have attempted to differentiate between ' sun' and ' shade' species. Although a number of different adaptive strategies have been described, no clear overall picture of phytoplankton adaptations to changes in photon flux density has, as yet, emerged. The problems associated with analysis of light adaptation in microalgae are considerable. In the field, natural water movements make description of a phytoplankter's light environment and light history very difficult. In the laboratory, description of the light environment is possible. However, results of investigations may be affected by changes in algal physiological state precipitated by the culturing conditions employed (see Griffiths, 1973; Morris and Glover, 1974; Beardall and Morris, 1976). Furthermore, laboratory cultures are normally maintained at greater cell densities than would normally be encountered in nature. Failure to consider the extent to which algal suspensions absorb incident light complicates the analysis of photosynthesis vs photon flux density (P vs I) and grovi^th rate vs photon flux density (p, vs I) curves. Finally, problems surrounding the measurement of light have complicated the algal physiologist's understanding of light adaptation by phytoplankton. In recent years, the convention of describing the light environment in units of illumination (lux, footcandles) has gradually been replaced by expressing light either in units of energy (joules, ergs, gram calories) or in quanta (Einsteins). As photosynthesis at its most basic level is controlled by the absorption of photons, the expression of light in quanta (einsteins) would seem particularly suited to investigations into phytoplankton photoadaptation. Knowledge of the spectral distribution of the quanta received then allows calculation of the energy input. Conversion between units can be complicated and owing to differences in the nature of the measurements is, at best, imprecise when converting between units of illumination and energy or quanta. Analysis of the available literature reveals that many investigators (and reviewers) remain confused about the units in use and what is actually being measured. (For discussion, see Incoll, Long and Ashmore, 1981.) Many appear to lack a feeling for 'reasonable' values and some quite unrealistic light measurement values have found their way into print. We believe that further understanding of light/shade adaptation by phytoplankton cannot be achieved until a synthesis of the available data pertaining to microalgal light adaptations has been made. Therefore,, the present work is an attempt to collate and analyse the literature relating to phytoplankton light adaptation. By doing so we hope to clarify the present understanding of photoadaptation by phytoplankton and to identify the questions which should be asked in further research.

ANALYTICAL METHODS Data pertaining to algal growth and photosynthesis at differing photon flux densities were compiled from the published literature and from personal com- munications. All values for light measurement were converted to /lE m~^ s''- according to the conversions shown below.* In these units, 'full sunlight' has a photon flux density of about 1700 fcE m~^ s'^ (400' to 700 nm). When cultures were • 2Wm-= = 10/(Em-'s-'; 1 klx = 16-5/tE m"'s"'; 1 ly min"'= 4-184 J cm^'min"i; 1 ly min-'= 697 W m-"; ] ly min"' = 3485 /iB m"' s"'; 1 erg cm-'' s^' = 5-03 x 10-= /lE m-'^ s-'. 6-2 i6o K. RICHARDSON et al. grown on light/dark (L/D) cycles, values for the incident light have been corrected to the equivalent continuous illumination by tnultiplying by the fraction of a day that the algae were illuminated (i.e. for a 15/9 L/D cycle, the incident photon flux density would be multiplied by 15/24 or 0-625). This treatment assumes that it is the total amount of light received during the day which is important for phytoplankton survival. Some evidence to justify this assumption comes from the works of White (1974), Verity (1982b), Nelson, D'Elia and Guillard, (1979) and Heath (pers. comm.). However, there are clearly also species that respond quite differently to continuous illumination than to a L/D cycle (Foy, Gibson and Smith, 1976; Brand and Guillard, 1981). Particular attention was paid to finding values for the minimum incident photon flux density at which each species was able to (i) grow (as defined by measurable cell division) and (ii) undergo net photosynthesis. The photon flux density at which each species experienced a minimum growth rate (I,)—/tmin) proved particularly difficult to obtain as many workers fail to define this point accurately. When enough data points were available for a plot oi /i vs I, we extrapolated back to zero growth rate to find Io—/

LIGHT HARVESTING BY MICROALGAE The nature of the light harvesting machinery and the processes involved in the harvesting of light energy by phytoplankton have been dealt with in at least two recent reviews (Jeffrey, 1981; Prezelin, 1981) to which the reader is referred. However, it is relevant to this discussion to point out the basic differences between iight harvesting pigments found in phytoplankton and in higher plants. (See also section oo Phylogenetic Aspects of Differences in Light Responses of Microalgal Photosynthesis and Photolithotrophic Growth) Adaptation of unicellular algae

•OH + -I- +

ll st 2 S S.2- • s-=. £ •5 5 O U i62 K. RICHARDSON et al. Table 1 shows the distribution of the major photosynthetic pigments in various algal taxa and in higher ('green') plants. Notice that the most abundant marine microalgae, members of the Chrysophyta and Pyrrhophyta, all contain chlorophylls a and c. Carotenoids constitute a major proportion of the pigment composition in these algae. In contrast, green algae and higher plants contain chlorophylls a and b while carotenoids play a more minor role in light harvesting pigment composition. Red algae (Rhodophyceae) and blue-green algae (Cyanophyceae or Cyanobacteria) contain chlorophyll a and biliproteins. The presence of light harvesting pigments in aquatic algae that differ from those in higher plants has long been explained as an evolutionary adaptation which allows different algal classes to exploit light of varying quality. However, this hypothesis has never been proven under fleld conditions. Indeed, Yentsch (1974) points out that the theory seldom fits the facts of algal distributions and Dring (1981), in a very thorough study, concluded that it was adaptation to low irradiance rather than to light quality that allowed the survival of benthic algae at depth. As most phytoplankton species have pigment systems similar to those of benthic marine algae, this must give cause to question whether chromatic adaptation is important in the distribution and the survival of microalgae in aquatic environments. This is a problem that deserves more attention than it has received. Extracted chlorophyll pigments absorb both in the blue and the red region of the spectrum. Prezelin (1981) has pointed out that for chlorophyll a, the size of the absorption peak in the blue is approximately the same size as the peak in the red region. For chlorophyll 6, the ratio of peak height in the blue to peak height in the red is around 2-85 and for chlorophyll c, the ratio of absorbance in the blue to absorbance in the red is approximately 10. Thus, it would seem that chlorophyll c is the most blue-light adapted chlorophyll pigment. As blue light penetrates most readily into marine waters, we might predict that chlorophyll a,c/carotenoid containing microalgae would be better adapted to marine life than species containing chlorophylls a and b. The most common marine phytoplankton species do, in fact, contain chlorophylls a and c. However, it should be emphasized that in spite of their lack of chlorophyll c, the action spectra for a number of green algae indicate that photosynthetic performance is high in the blue region of the spectrum. Thus, it seems that members of most (if not all) algal classes can use the limited spectrum of light available in marine waters to meet their absolute energy requirements. Therefore, we believe that photon flux density plays a more important role than light quality in the control of microalgal distrihutions within the water column. Some experimental evidence suggests that this is so in nature (Wall and Briand, 1979). However, there are indications that light quality may be important in the regulation of certain cellular processes. Work with green plants has shown that light quality may affect the control of photosynthetic electron transport and enzyme activity (Voskresenskaya and Polyakov, 1975; Voskresenskaya, Drozdova and Krendeleva, 1977). Numerous pigment studies have revealed that members of the Cyanophyceae and Rhodo- phyceae alter intracellular ratios of phycobiliprotein to chlorophyll a in direct response to the spectrum of the light available (see Bogorad, 1975). Light quality related changes in pigment composition for the most abundant phytoplankton classes are less dramatic but a number of workers have suggested that they occur. Jeffrey and Vesk (1977) and Vesk and Jeffrey (1977) noted pigment increases in cells grown in blue-green light over those grown in white light in 14 Adaptation of unicellular algae 163

of 17 marine phytoplankton species studied. In six species,ultrastructural changes in the form of an increased number of thylakoids per chloroplast in blue-green light grown cells were reported. Wallen and Geen (1971b) suggested that blue-green light stimulates increases in chlorophylls a and c in the marine diatom Cyclotella nana. In both laboratory and field studies, these authors found that blue and green light bias the photosynthetic fixation of CO2 towards protein synthesis while white light favours carbohydrate production (1971a,b,c). Other workers (Shimura and Ichimura, 1973) have examined the photosynthetic characteristics of natural phytoplankton populations in relation to the spectral changes of light with depth but were unable to confirm a change in either pigment content or photosynthetic performance resulting from the light quality changes. We believe that light quality may be important in controlling certain metabolic processes in phytoplankton and its effects deserve further investigation. However in the context of this study, we have chosen to deal primarily with adaptations to changes in photon flux density.

RANGE OF PHOTON FLUX DENSITIES ALLOWING GROWTH AND PHOTOSYNTHESIS IN PHOTOTROPHIC MICROALGAE (GENOTYPIC ADAPTATION) Growth Every phototroph has a distinct range of photon flux densities over which growth and photosynthesis can occur. This range is determined by the photosynthetic and metabolic characteristics of each species. Therefore, this range must be genetically controlled and we can say that a given species is ' genotypically' adapted to a particular range of photon flux densities.* Within each algal class, species may exhibit large differences in the ranges of photon flux densities that they will tolerate. For example, in the Dinophyceae, Amphidinium carterae has been found by several workers to be genotypically adapted to low photon flux densities. Chan (1978) found cell division rates in this species to be 80 % of maximum at a continuous illumination of only 8 /(E m"^ s-*^ (21 °C). Richardson and Fogg (1982) report growth for this species at 2 /tE m^^' s~^ (18 °C). Hersey and Swift (1976) report cell division to occur at less than 1 /fE m-^ s-> (12:12; L/D and 20 to 22 °C). Fmally, Samuelsson and Richardson (1982) have shown photoinhibition to occur in A. carterae at a continuous photon flux density of only 80/iE m'^ s^^ (15 °C). In contrast, the dinoflagellate Gonyaulaxpolyedra has been reported to be unable to grow below about 30/(Em^^s-^ In the same study, growth rates were not saturated for this organism until about 130/iE m^^ sî (12:12; L/D; 23 °C) (Rivkin, Voytex and Seliger, 1982). Prezelin and Sweeney (197.8) report 'photo- stress ' and a decrease in all photosynthetically active pigments for this dinoflagellate below about ilSO fiSN cxrT'^ (ca. 63/tE m"^ s~'). These same workers report a lowering of Pâx fo^" Gonyaulax polyedra when cultured at 4000 /iW cm-^ (ca. 200 fiE m~^ s"') but they suggest that this is not a result of photoinhihition. In the study by Rivkin et al., photon flux densities for growth experiments exceeded 200 Ê m~^ s"' and no photoinhibition was noted. More examples of genotypic diflêrences in light requirements by algae within the * Recent studies (Brand, 1982; Gallagher, 1982) indicates that within each species, genetically distinct clones can be identified. These clones may vary with respect to light responses and requirements. However, for the purposes of this study, intraspecific differences in light requirements are considered to be small relative to interspecific differences and the latter only arc considered. 164 K. RICHARDSON et al. sanne class can be taken from Perry, Larson and Alberte, (1981). That study examines the photosynthetic responses of eight diatom species adapted to 4 and to 300 /tE m~* s~^ The extremes were Ditylum brightwellii which performed markedly better, both per cell and per unit chlorophyll a, when adapted to 4/tE m~^ s^^ than to 300, and Thalassiosira fiuviatilis which was virtually unable to photosynthesize following exposure to 4 fiE m^* s^^. In the latter species, there is also evidence of the destruction of photosynthetic pigments during the low light treatment. In spite of the differences in light tolerance that may be expressed within a single algal class, it is possible to nriake some generalizations about the light preferences of the various algal classes. In Table 2, we have assembled the data relating to the photon flux densities where minimum and maximum growth rates (Ij —/tmin ^nd IQ—/iâx) ^re attained for representatives of each algal class.

Table 2. Growth characteristics of microalgae

lo-/'niin (Sts.e.) Ia-/im^ (x±s.e,) Class (jitE m-2 s^') n QiE m"' s"") n

Cyanophyceae 5 1 38-8 + 6-2 14 Rhodophyceae — 0 78-7 + 19-7 3 Dinophyceae 6-6 ±0-09 6-6 ±0-09 16 46-6 ±6-6 17 Bacillariophyceae 6-4+ 0-9 17 84-0 + 8-1 22 Cryptophyceae 1-0 1 — 0 Chlorophyta 20-6 ±8-1 5 211±58 9

Corrections have been made for L/D cycles as described previously. References used in compiling tahle: Nordli (1957), Castenholz (1964), Jitts et al. (1964), McAllister, Shah and Strickland (1964), Thomas (1966). Brown and Richardson (1968), Smayda (1969), Myers (1970), Durbin (1974). Fairchild and Sheridan (1974), White (1974). Owens and Seliger (1975), Shear and Walsby (1975), Beardall (1976), Foy, Gibson and Smith (1976), Prezelin (1976), Swift and Meunier (1976), Aiba and Ogawa (1977), Kruger and Eloff (1977). Jur, Gons and van Liere (1977), Chan (1978), Fleischacker and Senger (1978), Prizelin and Sweeney (1978). Morgan and Kalff (1979), Nelson, D'Elia and Guillard (1979), van Liere and Mur (1979), Falkowski and Owens (1980), Brand and Guillard (1981), Morris and Glover (1981), Rhee and Gotham (1981), Schlesinger, Molot and Shuter (1981), Rivkin et al. (1982), Samuelsson and Richardson (1982), Verity (1982b) and Van Liere and Walsby (1982).

There is considerable variation about the mean values of Io—/temperatures may also contribute to the variability observed. Nevertheless, it is possible to make several points concerning the light tolerances of the various algal classes. Most obvious is the fact that all classes except those in the Chlorophyta are able to grow in very low light environments. As noted earlier, precise values for I(i—/*niij, are difficult to obtain for algae which grow at very low photon flux densities. Therefore, the means for Ij-/tmin recorded here for diatoms and dinoflagellates are undoubtedly higher than the actual. Even so, the mean value for IQ—/tjnin for the Chlorophyta is significantly different from those found for the dinoflagellates and the diatoms (P < 0-001 by ANOVA). The Io-/greens and diatoms to favour higher light regimes than blue-greens, dinofiagellates and other flagellates.

Photosynthesis The photosynthetic characteristics of the various microalgal classes reflect the same pattern as that derived from the growth data. Light compensation points for photosynthesis by chlorophyll a,c and carotenoid containing algae tend to be low. For Skeletonema costatum, a compensation point of around 0-2 fiE m"^ s"-' has been reported when the organism was grown over a range of photon flux densities (Falkowski and Owens, 1980). For other algae, changes have been recorded in the light compensation point depending upon the photon Rux density at which the algae were grown {Glenodinium sp. - Prezelin, 1976; Dunaliella tertiolecta- Falkowski and Owens, 1980; A. carterae - Samuelsson and Richardson, 1982). For the Chlorophyta, recorded light compensation points tend to be noticeably higher. For D. tertiolecta, the light compensation point for photosynthesis has been reported to vary between 4 and 20 fiE mr^ s^^ depending upon the photon fluence rate used for culturing the organism (Falkowski and Owens, 1980). Members of the chlorophyll a, c, carotenoid containing algae comprise the majority of the phytoplankton in temperate seas. In a recent study of the Bedford Basin, Nova Scotia (Lehman, 1981) dinoflagellates were found to constitute 68% of the total cell volume measured over an annual cycle; diatoms comprised 14% and flagellates 17%. From the photosynthetic and growth data available, it appears that many algae in these classes photosynthesize optimally at photon flux densities considerably below what might be considered 'full sunlight' (1500 to 2000/iE m-^ s'^; 400 to 700 nm) and that the photosynthetic apparatus of these species is not able to work effectively under supra-optimal conditions.

Photoinhibition A number of reports of photoinhibition of natural phytoplankton populations in surface waters exist (Ryther and Menzel, 1959; Kalff and Welch, 1974; Stadlemann, Moore and Picket, 1974; Ganf, 1975; Jewson, 1976; Belay, 1981). Harris (1978), in a review of data on natural populations, suggests that the onset of photoinhibition occurs at about 200 /lE rrr^ s"'. However, physiological data discussed in this work indicate that some species may become photoinhibited at even lower photon flux densities. Photoinhibition is known to be dependent upon light quality as well as intensity. Both u.v. and visible light have been identified as potentially damaging to i66 K. RICHARDSON et al.

Table 3. I,,—/tmax-"Io~/*min (^"^ If, for onset of photoinhibition for Bacillariophyceae and Dinophyceae*

If^ photoinhibition ) («E m-= s->) Reference

Bacillariophy ceae Phaeodactylum tricornutum -65 250(G)t Beardall and Morris (1976), Nelson etai. (1979) Skeletonetna costatum >100 Falkowski and Owens (1980) Cylindricotkec a -75 Chan (1978) Thalassiosira floridana -75 Chan(1978) Thalassiosira eccentrica -30 150(G) Chan (1978) Coscinodiscus sp. >100 White (1974) Cyclotella cryptica >100 White (1974) A.chna:nthes exigua -40 150(G) Fairchild and Sheridan (1974) Fragillaria sp. -50 Rhee and Gotham (1981) Thalassiosira notdenskioldii -75 Durhin (1974) Detonula conforvacea -95 Holl and Smayda(1974) Asterionella gladalis >100 Brand and Guillard (1981) Hemiautus hauckii 75{G) Brand and Guillard (1981) Corethron criophilum >33O(G) Brand and Guillard (1981) Ditylum brightwelU >33O(G) Brand and Guillard (1981) Thalassiosira sp. > 33O(G) Brand and Guillard (1981) Leptocylindricus danicus -75 Verity (1982h) Fragillaria striata -75 Castenholz (1964) Biddulphia striata -60 Castenholz (1964) Synedra tabulata -45 Castenholz (1964) Melosira monaformis -50 Castenholz (1964) Thalassiosira pseudonana >100 250(G) Nelson ef a/. (1979), Brand and Guillard (1981) X = 72-7, n =18 i = 233, K = 8 Dinophyceae Dissodinium lunula -30 Swift and Meunier (1976), Brand and Guillard (1981) Pyrocystis noctiluca -20 Swift and Meunier (1976) Pyrocystis fusiformis -10 50(G) Swift and Meunier (1976) Gonyaulax polyedra -50 Rivkin et al. (1982) A.mphidinium carterae -15 80(P) Samuelsson and Richardson (1981) Gymnodinium simplex -75 Chan (1978) Scrippsiella sweeneyae -30 80(G) Chan (1978) Ceratium Uneatum 150(G) Chan (1978) Prorocentrum micans -30 80(G) Brand and Guillard (1981), Chan (1978) Gymnodiniutn splendens -25 Owens and Seliger (1975) Peridinium cinctum -20 Prezelin and Sweeney (1979) Ceratium tripos -70 150(G) Nordli (1957) Ceratium fusus -35 83(G) Nordli (1957) Ceratium furca -35 83(G) Nordli (1957) Ceratium platicorne -70 >75-300(G) Brand and Guillard (1981) Ceratium ranipes >75-3OO(G) Brand and Guillard (1981) Ceratium candelabrum 19(G) Brand and Guillard (1981) Gymnodinium sp. (581) -80 Thomas (1966) X = 39-7, n = 15 X = 86, n = 9

* Corrected for L/D cycle. t (G) denotes photoinhibition of growth; (P) of photosynthesis.

photosynthetic systems (Ilmavirta and Hakala, 1972; Jitts, Morel and Saijo, 1976; Smith et al., 1980). The detrimental effects of u.v. irradiation of photosynthesis have been known for some time (Halldal, 1964, 1967; Halldal and Taube, 1972). The mechanisms of photoinhibition induced by visible light were first discussed Adaptation of unicellular algae 167 by Jones and Kok (1966a,b) in their studies of photoinhibition in spinach chloroplasts. They suggested two sites for photoinhibition caused by visible light, one located in PS II and the other in PS I. Studies with higher plants have suggested that exposure to excess photosynthetic excitation energy in the visible region of the spectrum may cause daniage at or near the PS II reaction centre (Fork, Oquist and Powles, 1981; Powles, Osmond and Thorne,, 1979). As a result, early indications of photoinhibition include the inhibition of PS II electron transport and the loss of variable fluorescenceemissio n from PS II (Powles et at, 1979; Powles and Critchley, 1980). In the laboratory data compiled here, reports of photoinhibition of growth or photosynthesis occur more often for dinoflagellates than for diatoms and in general, the onset of photoinhibition is at lower photon flux densities for dinoflagellates than for diatoms (see Table 3). Data relating to photosynthesis are more limited for the other algal classes. However, there are a number of reports of photoinhibition in the Cyanophyceae while in the Chlorophyta, photoinhibition seems to be rare at least until very high photon flux densities are attained. In the dinoflagellate, A. carterae, photoinhibition has been identified both by impairment of photosynthetic characteristics and by the loss of variable fluorescence associated with PS II (Samuelsson and Richardson, 1982). This same species has been shown to concentrate at approximately 30 fiE m~^ s"'' when exposed to a photon flux density gradient from 0 to 120 /iE m~^ s~' (Richardson et al., submitted for publication). The natural migrations of a number of dinoflagellates off the Baja California coast have been studied by Blasco (1978). She found that at noon, Gonyaulax polyedra maintained maximum numbers at the surface while the other species represented, Ceratium furca, Gymnodinium sp. C. dens, Gonyaulax digitals and Prorocentrum micans, all concentrated lower in the water column. During limno- iogical studies, Harris, Heany and Tailing (1979) have reported ' surface avoidance' by the dinoflagellate Ceratium hirundiella. Other workers have also noted that some dinoflagellate species avoid the surface waters during daylight hours (Hasle, 1950; Dodge and Hart-Jones, 1977). MuUer-Haeckel (1981) has suggested that the disappearance of a winter bloom of the dinoflagellate, Gonyaulax catenata coincides with the local ice break and the consequent increase in the ambient light energy within the water column. Finally, dinoflagellates have frequently been im;plicated in the formation of subsurface chlorophyll peaks (e.g. HoUigan and Harbour, 1976; Tyler and Seliger, 1978). There does seem to be the physiological evidence in the literature to suggest that the low photon flux densities associated with these regions of subsurface chlorophyll peaks may represent the preferred environment for photosynthesis by these organisms. Often, in those nnicroalgae that experience photoinhibition, the onset of this inhibition occurs at a photon flux density that appears to be only slightly higher than that at which growth or photosynthesis becomes saturated (see Table 3). At first, the small range of photon flux densities over which these algae are able to photosynthesize optimally may seem a disadvantage in that it should severely limit the habitats in which they can survive. However, since light is attenuated exponentially with depth, changes in the ambient light environment at low photon flux densities (i.e. at depth) will occur relatively slowly. Therefore, in a stable water column, there may be a considerable region in which the ambient light would support phytoplankton that photosynthesize optimally only over a limited absolute range. i68 K. RICHARDSON et al.

PHENOTYPIC ADAPTATION The phenotypic adaptation of microalgae to different photon flux densities has heen the suhject of a numher of investigations originating from the initial work hy Steemann-Nielsen and his colleagues (Steemann-Nielsen and Hansen, 1959; Steennann-Nielsenand Jergensen, 1962; 1968; Jorgensen, 1964,1966,1969,1970). These early reports suggested two different strategies of light adaptation in phytoplankton. These were termed the ' Cyclotella' type and the ' Chlorella' type. In the former, the adaptation response took the form of an increase in the rate of the enzymatic 'dark' reactions. In the Chlorella type, adaptation was supposed to take the form of an increase in pigment levels. The validity of this early interpretation of light adaptation by microalgae has been brought into question by the observation that similar results could be obtained merely by comparing photosynthetic characteristics of high and low light grown cells at different times during hatch growth (Beardall and Morris, 1976). With hindsight, we can also see that the models for phytoplankton photadaptation presented hy Steemann-Nielsen and co-workers greatly oversimplify the process of light adaptation by microalgae. Nevertheless, these workers were the first to acknowledge and to study in any detail the fact that there may be different strategies of phenotypic light adaptation io phytoplankton and their works represent an important historical point in the study of phytoplankton physiology. For this paper we have attempted, in so far as is possible, to take culturing conditions into account when collecting data for the analyses. When available, we have used data from experiments in which the algae have been cultured using continuous culturing techniques in order to avoid the problem of artifacts originating from culturing methods interfering with the interpretation of adaptive responses. Falkowski (1980) has recently reviewed much of the available literature relating to phenotypic light adaptation by phytoplankters. Therefore, tio attempt is made to do so here. Instead, we have tried to fit the available data to current hypotheses relating to light adaptation in the hope of identifying the strengths and weaknesses of popular theses.

Changes in amounts of pigments One general response of microalgae to reduced photon flux densities is an increase in cellular pigment content and/or a change in pigment composition. The degree to which different species respond may vary. For example, the chlorophyll a + c content of the marine diatom Skeletonema costatum changes only by a factor of 1-8 between 130 and 20 fiE m~^ s~^ while the chlorophyll a + i> content of the chlorophyte, Dunaliella tertiolecta, changes some fivefold between 30 and 600 fiE m~^ s~^ (Falkowski and Owens, 1980). Such pigment changes may be even more nnarked than in Dunaliella. For example. Ley and Mauzerall (1982) have shown the chlorophyll content of Chlorella vulgaris to vary from 1 -6 mol chl (a + &)/10" cells at 1258 fiE m'^ s'^ to 15 mol chl (a + 6)/10« cells at 0-75 ^E m^ s"-". Note, however, that the latter photon fiux density appears rather low to sustain steady state growth of a green alga (as discussed in the previous section). It has been suggested that the magnitude of this change in chlorophyll content per cell in response to changes in the light environment may be greater for green algae than for diatoms or dinofiagellates (Falkowski, 1980). In addition to quantitative changes in pigment content, the qualitative com- Adaptation of unicellular algae 16g position of pigments may change with the photon fiux density used for growth illumination. In the ahove example (taken from Falkovcski and Owens 1980), the chlorophyll a :b ratio in Dunaliella changed from 5-6 at the higher photon flux density to only 2-3 at Io--/tmin- That of Chlorella vulgaris changed from 6-7 to 2-8 upon transfer from high to lovy photon Oux density. The chlorophyll a :c ratio of Skeletonema as well as other diatoms and dinoflagellates shows a similar response upon changes in the light environment (Falkowski, 1980; Falkowski and Owens, 1980). In the dinoflagellate, Glenodinium sp., it has been shown that the peridinin: chlorophyll a ratio increases as well as the chlorophyll a:c ratio at low photon flux densities (Prezelin, 1976).

Interpretation of the effects of pigment changes : models The significance of decreased chlorophyll a :b ratios in shade adapted algae has heen discussed by Falkowski (1980) and is interpreted as signifying increases in light harvesting chlorophyll proteins (Kawamura, Mimuro and Fujita, 1979). Such qualitative and quantitative changes in pigment composition in response to changes in the light environment have led a numher of workers to interpret phytoplankton photoadaptation in terms of the photosynthetic unit*. Falkowski and Owens (1980), Prezelin and Sweeney (1979), Perry et at., 1981 and Ley and Mauzerall (1982) have all suggested that phytoplankton respond to decreased photon flux densities hy increasing either the size or the number of photosynthetic units within a cell. Prezelin and Sweeney (1979) have suggested that changing either the size or the number of PSUs will produce characteristic P vs I curves. A similar proposal has been made for macroalgae (Ramus, 1981). In 1981, Prezelin published modified versions of these P vs I curves and included a further predictive model to depict changes in P vs I curves for algae experiencing changes in photosynthetic enzymic reaction rates during exposure to different light environments. These and the earlier models are illustrated in Figure 1. Notice that there is ambiguity concerning the slopes of the P vs I curves in the predictive models. In the earlier versions, increases in the size of the PSU are depicted as leading to decreased light utilization efficiency on a per chlorophyll basis for low light adapted cells; in contrast in Prezelin's (1981) version of the model, the slopes for both high and low light adapted cells are depicted as being identical. Dr Prezelin (pers. comm.) has indicated that this discrepancy results from an error in the presentation of the figure published in 1981. To date, this point has not been clarified in print. Therefore, she has asked us to emphasize that the 1979 version of the P vs I curves for algae that change the size of their photosynthetic units in response to changes in the light environment are still considered to be those which are predicted by the model. For algae in which the number of PSUs changes in response to changes in irradiance, it is the slope (alpha) of the P vs I curves of photosynthesis on a per cell or biomass basis that is in question. In the earlier model, this slope is depicted to be identical for low and high light adapted cells. However, in the 1981 model, Prezelin shows the low light adapted cells as having the greater light utilization

* The term photosynthetic unit (PSU) is generally taken to refer to the functional photosynthetic structure conniprised of the reaction centers of PS I and PS II, and associated light harvesting chlorophylls (see Prezelin, 1981 for nnore detailed description). This definition assumes a 1:1 relationship hetween reaction centres I and II. However recent work has suggested this, tnay not be so for all algal classes (Kawamura et al., 1979; Mclis and Brown, 1980). K. RICHARDSON et al.

E H H+L .9 L o n o X) x: o o / L / / (o r CL =: / 1 o- A o / Q. Irradiance Increasing size of PSUs Increosing no, of PSUs

H L ^ ;»— H + L L H ,-- o (b) '/ GL P/cel l P/ch I e / / Irradiance Increosing size of PSUs Increasing no. of PSUs

H H _L o L a. / P/ce i Irradiance Altered phatosynthetic enzyme reac \ ons Fig. 1. Predictive depicting theoretical responses in the P m I curves of algae expressing different strategies of photoadaptation. H, high light adapted cells; L, low light adapted cells, (a) Prezelin and Sweeney (1979); Ramus (1981). (h) Prezelin (1981). efficiency. This point is not explicitly dealt with in her discussion but, in this case, the change between the figures published in 1979 and 1981 was intentional (Prezelin, pers. comm.); her argument being that even if the PSUs are light limited, increasing the total number of them per cell should increase the photosynthetic output of the cell proportionally. The discrepancy concerning the slope of the P Dj I curve in this latter case is especially important because the earlier models imply that low light' adapted' cells are no better able to harvest and utilize low light than their high light adapted counterparts.

Observed changes in P vs I curves Perhaps the best way to clarify the responses of alpha to changes in irradiance for the different strategies of photoadaptation is to examine actual data relating to the P CS I curves of different algal species. From our analysis of the relevant literature, we believe that there are at least five different types of P w I response. These types are differentiated on the basis of differences in the shape of the P vs I curve dependent upon whether P is expressed on a per cell or a per chlorophyll basis. A number of factors complicate compiling published data for such an analysis. These include the facts that (i) photosynthesis has been determined by different methods in different studies and, as a result, respiration will affect results to a different degree; (ii) there may be changes in cell size associated with adaptation to changed photon flux density and (iii) most workers are measuring incident rather than absorbed light. Adaptation of unicellular algae 171 When more controlled experiments have been conducted in an efTort to elucidate these different photoadaptive strategies, it may develop that the differences between some of the strategies naay not be as marked as they appear at this time. However, for the moment, we are left with the phenotypic responses to changes in photon flux density illustrated in Figure 2. These are discussed individually in the footnotes below. *t

C S

Irrodionce

Fig. 2, Observed responses in P vs I in various microalgae during photoadaptation. The curves for high light adapted algae are designated ' 1' and those for the lowest light adapted are labelled ' 3 ', Sources for the curves are identified in the text.

* Note that because of genotypic differences in the range of photon flux densities that individual species will tolerate, the algae exhibiting these phenotypic responses to changes in the light environment do not necessariiy do so over the same range of photon flux densities. Therefore, these strategies are discussed in relative terms, If actual values were to be considered, then ' high ' light for one organism might well be ' low' for another. f When photosynthesis has been determined by '*C incorporation, P ifs I curves are normally plotted through zero. When photosynthetic rates are determined by net oxygen exchange, respiration rates can be determined and P w I curves will intersect the ac-axis at a point significantly different from the origin (j.e. the light compensation point). In most cases, however, respiration rates relative to Pmax sn"^ s^i^ll (<10%), For the construction of Figure 2, responses A to D, oxygen determined P vs I curves have been corrected for respiration and brought through the origin. This correction did not alter the shape of the curves and allowed direct comparison of P vs 1 curves determined by both oxygen exchange and *^C incorporation. In response E, respiration rates for high light adapted algae have been reported to be three times the rate of Pmax' Therefore, correction for respiration would significantly alter the shape of the F vs I curves and has not been attempted. K. RICHARDSON et al. Response A. The marine diatom Phaeodactylum tricornutum has been reported to exhibit the Pvsl characteristics described for this strategy (Beardall and Morris, 1976). In addition, Leletkin, Zvalinsky and Titlyanov (1980) have reported zooxanthellae from the reef coral Podllopora verucosa to respond in the same way to changes in the light environment. Another coral sytnbiont (from Pavona praetorta) has been shown to behave according to this strategy at least on a photosynthesis/chlorophyll basis (Wethey and Porter, 1976). In this type of response, Pmax> °" ^ P^r '^^^ basis, remains the same when algae are adapted to high or intermediate photon flux densities but decreases when the algae are adapted to the very lowest photon flux densities. P^ax expressed on a chlorophyll basis increases with increasing irradiance. Alpha increases at decreased photon flux densities both per cell and per unit chlorophyll. The behaviour of alpha has been suggested to result from changes in the activity of the accessory pigments (e.g. fucoxanthin) (Beardall, 1976).

Response B. This type of response is the same, in effect, as that predicted by Prezelin (1981) for algae that change the size of their photosynthetic units in response to changes in photon flux density. The only exception is that at very low photon fiux densities, P^^^ per cell may be reduced relative to that for cells adapted to higher light environments. On a chlorophyll basis, no change in quantum efficiency is noted between algae adapted to different photon flux densities. Organisms exhibiting this response include Dunaliella tertiolecta (Beardall, 1976; Falkowski and Owens, 1980), Chaeotocerosgracilis (Verry et al.., 1981), Glenodinium sp. (Prezelin, 1976), Gonyaulax polyedra (Prezelin and Sweeney, 1978) and Scenedesmus obliquus (Fleischacker and Senger, 1978). Unlike Prezelin (1981), we have chosen not to include Skeletonema costatum as an organism that exhibits this photoadaptation strategy. The reasons are discussed under type D.

Response C. This phenotypic response is similar to Prezelin's (1981) model for algae increasing their number of PSUs in response to changes in the light environment. However, there is a marked difference in the slope of P »j I on a per chlorophyll as well as on a per cell basis. Pj^ax/chl a may be similar for low and intermediate light adapted algae but is markedly reduced for cells grown at high photon flux densities. We include A. carterae (Samuelsson and Richardson, 1982), Peridinium cinctum (Prezelin and Sweeney, 1979) and Ditylum brightwellii (Perry et al., 1981) as exhibiting this strategy. Samuelsson and Richardson (1982) have suggested that the impaired photosynthetic performance of A. carterae on both a per cell and per unit chlorophyll basis at the higher photon flux densities is a result of photoinhibitory damage at or near the reaction centre of PS II. Thus, algae exhibiting the response depicted in C may not be 'adapting' to low photon flux densities but rather they may be stressed by the higher light environments.

Response D. It is within this response type that we include the diatom Skeletonema costatum. The results of Falkowski and Owens (1980) clearly indicate a reduction in the slope ofPvs I (incident upon the cells) expressed on a chlorophyll basis for algae at low photon flux densities. Thus, this organism behaves, at least on a chlorophyll basis, similarly to the earlier models for algae that alter the size of their PSUs in response to changes in the light environment (Prezelin and Sweeney, 1979; Ramus, 1981). Pmax P^r cell increases with increasing photon flux density in this strategy but Adaptation of unicellular algae 173 alpha per cell does not appear to change for algae adapted to the different light environnaents. Data relating to changes in cell size of Skeletonema costatum at various photon flux densities are available from Falkowski and Owens (1980). When the decrease in cell size is taken into account, alpha on a per biomass basis increases for the algae grown at low photon flux densities. This is the only indication of actual adaptation to low light for this organism. Chaetoceros danicus (Perry et al. 1981) and, at least on a chlorophyll basis, Nannochloris atomus (Yentsch and Lee, 1966) have also been implicated as behaving according to this strategy. However, it is not known whether these species behave similarly to Skeletonema in changing cell size at low photon flux densities.

Response E. This type of response appears to be unique to a symbiont of the coral, Stylophora (Falkowski and Dubinsky, 1981). On a per cell basis, P^iax ^^^d alpha are greatest for low light adapted algae. However, on a chlorophyll basis, high light adapted algae exhibit an equal or slightly greater Pn,j,x than low light adapted. Alpha on a chlorophyll basis is greatest for low light adapted algae. The light compensation point for high light adapted algae is reported to he about 300 iiE m"^ s-i. It should be noted that the different phenotypic responses to changes in photon flux density described here do not conveniently align themselves to taxonomic groupings. Numerically, diatoms are the best studied class and they exhibit four of the flve responses illustrated. From comparison of the models describing theoretical changes in P its I curves at different photon flux densities and actual ¥ vs 1 curves for various species, it becomes apparent that the responses of microalgae to changes in the light environment are extremely complex. While it has served as a useful starting point to think of light adaptation processes in terms of changes in PSU size or number, such models clearly oversin:iplify microalgal photoadaptation. This is probably due to the fact that ' changes' in the size of the photosynthetic unit could include any combination of the following: (1) Changes in the ratio of light harvesting pigments i.e. changes in the capacity to intercept light; (2) changes in electron transport capacity and (3) changes in photosynthetic enzyn:ie activity. In addition, changes in cellular and physiological characteristics unrelated to the photosynthetic unit itself (such as cell volume, cellular carbon content, and respiration rate) may contribute to the overall strategy of photoadaptation of a particular organism. Prezelin (1981) considered several of these factors individually in her theoretical models of changes in P c:t I at varying photon flux densities. However, in order to understand the actual V vsl curves exhibited by microalgae it is now important to try and elucidate which factors are controlling the responses observed in each of the different strategies and how they interact. One important aspect of phenotypic adaptation to changes in photon flux density that we have not considered is the time scale over which these responses occur. This is clearly a vital consideration when estimating the production potential of phytoplankton in a mixed water column. Although it is a problem that has received too little attention, those reports that do exist suggest that adaptations to changes in photon fiux density may occur rapidly. Prezelin and Matlick (1980) found increases in the peridinin-chlorophyll-protein conaplex light harvesting component of the photosynthetic unit in the dinofiagellate, Genodinium sp. within 174 K. RICHARDSON et al. 3 h of transfer from high to low light. Rivkin et al. (1982) record a rate constant of 0-33 per day for the increase in chlorophyll a per cell upon the transfer of Pyrocystis noctiluca from high to low photon flux density. Other workers have also indicated rapid responses to changes in photon flux density by phytoplankters (Marra, 1978; Vincent, 1979, 1980).

ENERGETIC CONSIDERATIONS General This section deals with adaptation to irradiance by unicellular algae in the context of cell energetics. Our discussion is based on a number of concepts of general applicability in cell energetics. One such concept is the distinction between running costs and capital costs. Running costs relate to the energy requirements for the operation of particular pathways, i.e. the ATP and reductant input per unit net product. Biochemical and biophysical similarities between organisms are such that decreases in running costs do not seem to be an important aspect of adaptation to a restricted energy supply. Thus, a more efficient net conversion of COj, HjO, NO^", and SO^^^ into protein is unlikely to characterize a low light adapted unicell. Capital costs, by contrast, relate to the energy costs associated with the synthesis of catalytic and structural cell material. An organism adapted to a restricted energy supply has, teleologically, a smaller expectation of requirements for catalytic capacity on a per cell basis, and accordingly, may have a lower protein content per cell than an organism adapted to a higher energy supply. Thus, although the cost to a low-light adapted cell of making unit protein is not lower than for a high-light adapted cell, a reduction in the quantity of protein per cell would reduce the energy needed for cell doubling (cf. Table 2 of Raven, 1982). The discussion of running costs and capital costs implicitly relates to growth in that net uptake of nutrients and net synthesis of polymers is considered. These energy costs may accordingly be classified as growth costs. In addition to these energy costs, there are maintenance costs, related to the survival of the organism in the face of protein and pigment breakdown and the leakage of accumulated solutes. While these maintenance costs are not directly proportional to growth rate, they are unlikely to be invariate between unicellular algal genotypes or to be independent of growth conditions for a given genotype. Other things being equal, a higher protein content per cell implies a higher maintenance requirement related to protein degradation and resynthesis so that a low-protein cell could economize on maintenance as well as capital costs. Alternatively, or additionally, cells adapted to low energy environments could have lower rates of protein breakdown and resynthesis than those in high energy environments. Thus, a low energy adapted cell might have lower maintenance costs. Further, 'slippage' reactions (Raven and Beardall, 1982) might be less significant in low-light adapted cells; this is particularly important for efficient energy conversion at low photon flux densities when the dissipation of intermediates of oxygen evolution and of ATP synthesis might be quantitatively significant losses in the context of the total energy budget. Finally, photoinhibition may involve higher maintenance costs in ternas of repair of damaged parts of the photosynthetic apparatus. The discussion in this section explores some of these possibilities in more detail. Adaptation of unicellular algae 175

Reduction of capital costs The reduction of capital costs during exposure to low photon flux densities is an obvious, mechanism by which phytoplankton may enhance the chances of survival under such conditions. One strategy for reducing capital costs is to reduce the level of the photosynthetic enzymes maintained. Beardall and Morris (1976) found reduced levels of RuBP carhoxylase activity in Phaeodactylum tricornutum grown at low photon flux densities. Rivkin et al. (1982) found that Pmax ^^d RuBP carboxylase activity hoth decreased by a factor of 4 in low light grown cells of the oceanic dinoiiagellate, Pyrocystis noctiluca. A similar response has also been noted in a number of higher plants (Bjorkman, 1981) and to a somewhat lesser degree in Scenedesmus (Senger and Fleischacker, 1978). Teleologically, the strategy makes sense in that an organism unlikely to encounter photosynthetically saturating photon flux densities is not faced with the metabolic expenditure of maintaining the biochemical means of photosynthesizing at the maximum rate. It is unclear whether this approach of lowering photosynthetic enzyme activity at low photon flux densities is ubiquitous within the microalgae. Nor is there any indication of the time required for enzyme activity to be restored upon return to optimal light conditions. The growth rates of a number of dinoflagellate species upon transfer from low to high photon flux densities have been examined (Richardson, 1980; Richardson and Fogg, 1982; Rivkin et al., 1982; Richardson et al., submitted for publication). Several species have been observed to resume maximal growth rates immediately or very soon after transfer from a period of low to high light. While this does not necessarily indicate that these organisms retain the ability to photosynthesize maximally while at low photon flux densities, it does suggest that they maintain biochemical machinery in excess of their immediate demands at low light and/or that the recovery time of this biochemiical machinery upon return to high light is fast. Some species were noted (Richardson and Fogg, 1982) to exhibit a considerable lag period prior to resuming maximal growth rates upon transfer from low to high light. However, there were indications that ' low light' for these species was, in fact, below their light compensation points. Thus, the failure of these species to resume maximal growth rates as quickly as others may be a manifestation of a genotypic preference for high photon flux densities rather than an indication that the biochemical machinery of these species responds differently to a reduction in light.

Reduction of maintenance costs The rate of turnover of cellular components such as photosynthetic pigments and protein is another area that should be investigated in the consideration of the energy budgets of phytoplankton exposed to varying photon flux densities. There in evidence (Grumbach, Lichtenthaler and Erisman, 1978; Riper, Owens and Falkowski, 1979) that microalgae exhibit very fast rates of chlorophyll turnover (on the order of hours rather than days) when compared to higher plants (Sironval 1963; Lichtenthaler and Grumbach, 1975). However, there is, as yet, no indication of rates of chlorophyll turnover in varying photon flux densities. Turnover rates in low-light exposed phytoplankton would be interesting to study in the context of determining minimum cellular maintenance costs. Turnover rates at very high photon fluxdensitie s are intriguing in the sense that cells experiencing photodamage from receiving excess photosynthetic excitation energy may have additional energy expenses for the ' repair' of dam^aged photosystems. 176 K. RICHARDSON et al. There must be a number of mechanisms (including the reduction of photosynthetic enzymes and possible decreases in the rates of turnover of certain labile cellular components) that an organism can invoke to decrease capital and/or maintenance costs at low photon flux densities for there is evidence that this is a common trait in phytoplankters from several classes. In Table 4 we have presented values for carbon specific respiration rates (respiration/cell carbon) for four different species at Io —^min ^'^d IQ—/*max- The sources for the data used in the calculations are indicated in the Table legend. Notice that all of the species exhibit lower specific respiration rates at low photon flux densities than at high.

Table 4. Carbon specific rates of four phytoplankton species

Specific respiration rate

Dunaliella tertiolecta 0-081 0-027 Gonyaulax polyedra 0-073 0046 Skeletonema costatum 0-044 0-004 Leptocylindricus danicus 0-017 0-003

* Derived from extrapolation of R/C vs ft to /i = Q. Valuesfor Duftaliella and Skeletonema are cSLlculaXed from data presented by Falkowski and Owens (1980); for Leptacylindricus from Verity (1981a,b, 1982a,b) and for Gonyaulax from Prezelin and Sweeney (1978) and Rivkin et al. (1982).

It is also striking that the two organisms identified as being genotypically adapted to high light {Dunaliella, Io-/tmin=2O /iE.m-^s~^ L/D = 14:10 Falkowski and Owens, 1980) and Gonyaulax polyedra (Io~/*min ~ 30 /lE m^* sT^ L/D = 12:12; Rivkin, et al., 1982) exhibit considerably higher respiration rates overall than Skeletonema (Ij,—/t^in = 0-3 ^E m~* s^"^ L/D = 14:10; Falkowski and Owens, 1980) and Leptocylindricus danicus (Ij —^^j^ = 2-3/iE m"^ s~^; Verity, 1982b). Of these four algae, the two 'high' light organisms have flagella while the 'low' light ones do not. Falkowski and Owens (1978) have suggested that high R/C ratios may be typical of motile organisms; the implication being that flagella operation may present an organism with high energy demands. However, Raven and Beardall (1981) have analysed the energetic costs of flagella operation for the dinoflagellate Gonyaulax polyedra and concluded that they represent a minor proportion of the organisms's maintenance budget. Raven (1982) has considered the energetic cost of contractile vacuole operation for freshwater flagellates and concluded that while it is greater than flagellar operation costs, it still represents a minor proportion of the total energy expenditure by the cell. We were unable to find the necessary data to calculate the carbon specific respiration rate for a low light tolerant dinoflagellate. However, taking the data from Prezelin (1976) for Glenodinium sp. and assuming that this organism (i) has the same percentage carbon (on a volume basis) as Gonyaulax and (ii) that the carbon content/cell varies in the same proportion as it does for Gonyaulax polyedra at different photon fiux densities, we calculate a specific respiration rate of 0-002 for this dinoflageliate at If,—ftmin- This compares well with the values calculated for the two shade tolerant diatonas despite the fact that Glenodinium has flagella and the diatoms do not. Adaptation of unicellular algae 177 A number of workers have recorded Pmax/I^ values during studies of microalgal photoadaptation. In the simplest analysis (since the compensation depth of the water column is equal to the depth where gross photosynthesis = respiration) a low P^g^jj/R ratio would appear to be to an organism's disadvantage for survival in the water column. Dinoflagellates, in general, exhibit low ratios for Pmax/^ ^^'^ yet, the analysis of genotypic light preferences indicated tht dinoflagellates as a class prefer low photon flux densities. This suggests that carbon specific respiration rates may serve as a better tool for predicting the relative light tolerances of different species than a P/R ratio. While considering cellular maintenance costs, it is relevant to refer briefly to the eiffect of temperature on low-light adaptation by phytoplankton. A detailed review of tbe literature pertaining to temperature effects on microalgal respiration rates has been made by Harris (1978). He reports a Qj^ for respiration of about 2 to 2-5, but at subsaturating photon flux densities, photosynthesis is less markedly affected. Therefore, one would expect the lower limit for the light at which growth and photosynthesis can occur to decrease with decreasing temperature. This temperature effect on respiration may be relevant to the occurrence of autotrophic microalgae under ice.

Energetic costs of changing the photosynthetic apparatus The successful low ligbt phytoplankter has, then, to keep cellular energy costs as small as possible during exposure to low photon flux densities. In addition, this organism can adapt by increasing its light harvesting potential through alteration of the photosynthetic apparatus itself. As discussed in the last section, this alteration may take on two general forms: either an increase in the number of photosynthetic units per cell or an increase in tbe size of existing ones. It is possible to consider theoretically the implications of each of these strategies to cellular energetics. Increasing the number of PSUs within a cell should increase the possibility of slippage processes within that cell. The term ' slippage' implies ' a less than theoretical stoichiometry of some energy transduction process' (Raven and Beardall, 1982). It has been suggested (Raven and Beardall, 1982) that the most important slippage reactions when considering phytoplankton adaptation to low photon flux densities are: (i) the decay of the S2-S3 intermediates in the oxygen evolution pathway of photosynthesis (Delrieu, 1981; Sinclair and Cousineau, 1982) and (ii) the functioning of primary active proton porters as passive uniporters (i.e. leakage of protons leading to breakdown of the electrochemical gradient across the thylakoid membrane). In the absence of relevant microalgal data, it is possible to consider the theoretical loss in quantum yield of photosynthesis resulting from these two slippage processes using data for Phaseolus vulgaris (Raven, 1981). S2-S3 decay The & for S2-S3 decay is approx. 3 s (Radmer and Kok, 1977). This means that the quantum yield of gross photosynthesis is reduced to 50 % of the maximum value when each PS II reaction centre receives a quantum every 3 s. There are 2-4 |ttmo] chlorophyll {a + b) m~^ of thylakoid membrane in Phaseolus vulgaris. With 500 chlorophyll (a + 6)/PS II reaction centre, there are 4-8 nmol PS II reaction centres ni~*. Taking a specific absorption coefficient for chlorophyll of 10*m^ mol~^ m~' and assuming that half of the light absorbed goes to PS II, then at a photon flux density of 0-5 /(E m"^ s"\ the PS II reaction centre would receive 9 quanta 3 s"-'. This would correspond to a drop in efficiency of about 4%. K. RICHARDSON et al.

Proton leakage due to passive uniport {through CF^—CFi ATP synthetase) Assuming the thylakoid proton motive force is 170 mV and the proton conductance is 4 mS m~^, then the minimum H'*^ leak across the thylakoid membrane is 16 nmol m~^ s~'. Again, takinga value of 2-4/tmol chl{a + b) m~^ and a specific absorption coefficient for chlorophyll of 10'' m^ mol"-" m""- and assuming H^/quantum = 1 (H+/electron = 2, electron/quantum = 0-5) and a thickness for the thylakoid membrane of 10~" m, then the thylakoid membrane will absorb 0-0568 of the incident light. At a photon fiux density of 0-5 fiE m~^ s^\ the H+ flux is 0-0568 X 0-5 = 28-4 nmol m"^ s~^ With a minimum H+ leak of 16 nmol m~^ s^\ this corresponds to a drop in quantum yield of 44%. In Figure 3, we have continued this theoretical analysis of the percentage drop in quantum yield at various photon fiux densities owing to the two slippage processes. Notice that of the two processes, passive H^ leakage is predicted to affect the quantum yield most dramatically. Because of the reduced chance of slippage reactions occurring, it would seem that increasing the ratio of light harvesting pigments to energy transducing catalysts (i.e. increasing the size of the photosynthetic unit) would be a more appropriate response to low light exposure than increasing the number of PSUs especially as such a strategy may also serve to reduce the capital costs of the production of PSUs at low photon flux densities. Indeed, shade tolerant phototrophs do, in general, seem to have a higher ratio of energy transmitters (light harvesting pigments) to transducers (reaction centres) than high light requiring organisms (Raven and Beardall, 1982). However, there must come a point at which increasing the size of the light harvesting component of the PSU is no longer cost eflFective owing to a loss in efficiency of energy transfer within a large light harvesting apparatus. It has been postulated (Raven and Beardall, 1982) that this efficiency of energy transfer may be a controlling factor in determining the upper limit for the size of the photosynthetic unit.

PHYLOGENETIC ASPECTS OF DIEFERENCESIN LIGHT RESPONSES OF MicROALGAi. PHOTOSYNTHESIS AND PHOTOLITHOTKOPHIC GROWTH Phylogenetic differences in photosynthetic structures A major conclusion from our earlier discussion is that there are significant differences in the potential for photolithotropic growth at low photon flux densities among the microalgae. The organisms with phycobilins alone (Cyanobacteria, Rhodophyta) or chlorophyll(s) c (Bacillariophyceae, Dinophyceae) in their light harvesting pigment—protein complexes can have lower light requirements for growth than do the Chlorophyta which have chlorophyll b in their light harvesting pigment-protein conaplexes. These chemical differences are correlated with differences in the structure of, and kind of association between, the thylakoids (Whatley and Whatley, 1981). Cyanobacteria and Rhodophyta have their phycobilins in phycobilisomes, i.e. peripheral pigment-protein complexes on the N (Mitchell, 1979) side of the thylakoid membrane. Perhaps because of the presence of phycobilisomes, the thylakoids of these organisms occur singly. The chlorophyll c containing algae have their thylakoids in pairs in the case of the Cryptophyceae, which are also distinguished by the occurrence of phycobilins on the P side {Mitchell, 1979) of the thylakoid membrane. The remaining chlorophyll c containing algae, including Adaptation of unicellular algae 179

100

E 80 3

Q. 2 40

£ 20

0-2 0-3 0-4 0-5 0-6 0-7 0-8 Photon flux density (^mot m"^ s'' Fig. 3. Theoretical loss in quantum efficiency owing to ' slippage' processes; Passive proton leakage ( ) and decay of S2-S3 oxygen evolution intermediates (—).

tbe Bacillariopbyceae, Dinophyceae and Prymnesiopbyceae, have tbylakoids associated in stacks of three, although tbe association is less close than in the cbloropbyll b containing organisnns. The light harvesting pigment-protein complexes of these ' chromopbyte' algae are of varying bydrophobicity witb (presum- ably) a range of membrane locations between essentially peripheral and fully integral (Anderson, Barrett and Thorne, 1982). In tbe green plants in the strict sense, i.e. those containing chlorophyll b, the thylakoids are always stacked. The prokaryote, ProcMoron (Prochlorophyta or Chloroxybacteria) bas thylakoids in pairs; tbe remaining green plants (Cbloro- pbyta, Euglenophyta, Bryophyta and Tracheophyta) bave tbylakoids in stacks of three or more. In all, it appears tbat the light harvesting chlorophyll a,b- protein-complex is a hydrophobic, integral membrane protein which is exposed to both the N and P sides of the membrane (Andersson, Anderson and Ryrie, 1982). These green plants are characterized by substantial lateral heterogeneity in thylakoid membrane composition (Anderson, 1982a,b) with appressed regions of the membrane having the reaction centre and directly associated antennae pigment complexes, the light harvesting pigment—protein complexes, and a portion of the cytochrome h-f complexes. The non-appressed regions have the remaining cytochrome b-f connplexes, the reaction centres and directly associated antennae pigment complexes and the CF(,-CFi ATP synthetase complexes. These differences in chemistry and structure could have important influences on the rate and efficiency of photosynthetic energy transformations. If one accepts the 'Z' scheme for photosynthesis (cf. Arnon, Tsujimoto and Tang, 1981; Pirt, 1981), the lateral separation of photoreaction two from photoreaction one, and from the ATP synthetase, by an average of more than 100 nm might constrain the maximum rate of energy conversion per unit thylakoid membrane. Limitations could be introduced by the need for substantial lateral migrations of oxidized and reduced plastoquinone in the lipid part of the m.embrane, oxidized and reduced plastocyanin (or soluble cytochrome c in some algae) in the P phase, and of buffered H"*" in the N and P phases (Anderson, r982a,b; Haraux and de Kouchkovsky, 1982). It is widely accepted (Witt, 1979; cf. Groen et al., 1982) that the oxidation of reduced plastoquinone limits the rate of light saturated and COj saturated photosynthesis (cf. von Caemmerer and Farquhar, 1981) so the limitations i8o K. RICHARDSON et al. mentioned above relate to an already limiting process. The spatial separation of the light harvesting pigment-protein complexes from the photoreaction one complexes could mean less effective control of excitation energy transfer from the light harvesting pigment-protein complexes to one or other of the two photo- reactions. This switching of excitation energy is important at low photon flux densities in dealing with changes in spectral composition of light or with changing metabolic demands for ATP relative to NADPH, and at high photon flux densities in avoiding excess excitation energy transfer to photoreaction two, the primary site of damage in photoinhibition (Bennett, Steinback and Arntzen, 1980; Butler, 1978; Ried and Reinhardt, 1980; Sigfridson and Oquist, 1980; Fork and Oquist, 1981, Haworth, Kyle and Arntzen, 1982; Osmond, 1981; Zeinalov, 1982). A flnal possibility, not directly related to the lateral inhomogeneity of the thylakoids, is that the transmembrane light harvesting pigment-protein complex might act as an additional source of H"*" leakage or 'slippage' (Samuelsson, pers. comm.; see section on Energetic Considerations). These gloomy prognostications do not, however, accord with the facts. The maximum photosynthetic rate on a unit pigment or unit thylakoid basis is not lower for green (chlorophyll b) plants (see Mishkind and Mauzerall, 1980); the same is true of the specific growth rate for photolithotrophic growth (Eppley, 1972). The maximum quantum yield (cf. Pirt, 1981) is very similar for representatives of all three pigment groups (Kok, 1960; Welschmeyer and Lorenzen, 1981), and the green algae do not seem to suffer more from photoinhibition than do other algae (see section on genotypic adaptation). Furthermore, Barber (1982; cf. Briantais, Vernotte and Maison, 1982) cites evidence that the (reversible) phosphorylation of the light harvesting pigment-protein complex is related to a partial unstacking of thylakoid membranes, witb association of the phosphorylated pigment complex with photoreaction one complexes. Thus, there seems to be a molecular and structural basis for the observed physiological responses of green plants to varying photon flux densities and light qualities. We are, however, still left with the greater capacity for photolithotrophic growth at low photon flux densities in non-green than in green algae. We shall explore the generality of this conclusion by comparing a range of pbotosynthetic organisms which share some, or most, of the photosynthetic characteristics of green microalgae.

Comparison of the photosynthetic characteristics of green algae and other phototrophs Looking first phylogenetically ' backwards' from the eukaryotic microalgae, we consider the serial endosymbiotic hypothesis for the origin of eukaryotic photosynthesizers (Gibbs, 1981; Margulis, 1981; Whatiey and Whatley, 1981). According to this hypothesis, the plastids of the Rhodophyta originated from cyanobacterial cells; the plastids of the Chlorophyta from Prochhron-like Proch- lorophyta (Chloroxybacteria) (cf. Seewald and Stackenbrandt,, 1982; Van Valen, 1982); but the 'Prochromon' paradigm of the chromophyte (chlorophyll c containing organisms) plastids has not (yet) been discovered. We note that it is likely that euglenophyte and dinophyte plastids may have originated from endosymbiosis of, respectively, chlorophyte and chromophyte plastids (Gibbs, 1981; Whatley and Whatley, 1981). We thus believe that the differences in thylakoid organization now found in the 'green', 'red' and 'brown' lines of eukaryotic algae were derived from prokaryotic, oxygen evolving ancestors. The evolutionary scheme proposed by Olson (1970, 1978, 1981) for the derivation of oxygen evolving photosynthesis from the bacterial-type seems Adaptation of unicellular algae i8i generally acceptable. All extant hacteriochlorophyll-containing photosynthetic hacteria (i.e. the Chlorobiaceae and Chloroflexaceae in the Chlorobineae and the Chromatiaceae and the Rhodospirillaceae, hut excluding the Halobacteriaceae whose light energy transduction is based on carotenoid-protein complexes: Henderson, 1977), have a clear distinction between reaction centres and antennae pigments. Furtherimore, they have antennae pigments which are in the same protein complex as the single type of reaction centres as well as antennae pigments in protein complexes which function only in light harvesting and in the transfer of excitation energy to reaction centre complexes (Olson, 1980; Drews and Oelze, 1981; Feick, Fitzpatrick and Fuller, 1982). It is thus likely that the earliest oxygen evolvers would also have had a distinction between their light harvesting pigment-protein complexes. We feel that these early oxygen evolvers would have been relatively shade-tolerant. Our argument here requires that atmospheric oxygen is of photosynthetic origin (Margulis 1981) being the equivalent of the net storage of organic (photosynthetic) carbon in the earth's crust minus whatever oxygen has been consunrted in inorganic oxidation processes. In view of the inorganic (and later, biological) oxygen sinks available on the early earth, a rapid accumulation of atmospheric oxygen is unlikely, meaning that the early oxygen evolvers were not as well protected from solar u.v. irradiation as are extant aquatic organisms since with low atmospheric oxygen levels, the ozone screen could not have been very effective. The problem is exacerbated by the likelihood (Canuto et al., 1982) that the ratio of u.v. to visible radiation emitted by the sun was greater earlier in solar evolution than it is today. Accordingly, it seems likely that early phototrophs lived under a sufficient depth of water to screen out a substantial fraction of the u.v, radiation, a circumstance that would also substantially reduce the availability of photosynthetically active radiation. An important correlation among photosynthetic prokaryotes is that those which can grow at the lowest photon flux densities have peripheral light harvesting pigment-protein complexes (Raven and Beardall, 1982). Among the photosynthetic bacteria, the Chlorobineae have peripheral ('chlorosome') light harvesting pigment-protein complexes (Olson, 1980; Feick et al. 1982) while the Rhodospirillineae have integral light harvesting pigment-protein complexes (Drews and Oelze, 1981). At least the Chlorobiaceae in the Chlorobineae have the ability to grow at very low photon flux densities and have very large photosynthetic units; the Chloroflexaceae (the other family in the Chlorobineae) have smaller photosynthetic units with a size range similar to that found in the Rhodospirillineae (Biebl and Pfenning, 1978; Olson, 1980; Feick et al., 1982). It may accordingly be argued that the possession of peripheral light harvesting pigment-protein complexes is a very necessary (but not suflficient) condition for the ability to grow photolithotrophically at very low photon flux densities. Looking phylogenetically 'forwards' from the eukaryotic microalgae, it behoves us to examine the photon flux density requirements for the photosynthesis and growth of higher ' green' plants. If there are higher green plants with as low a light compensation point as the Cyanobacteria, rhodophytes or ' chromophytes', then further investigation of the chemical and structural differences between the chlorophyll b containing and other oxygen evolving organisms with respect to the minimum photon flux density would he superfluous. Terrestial sporophytes of vascular plants may be disposed of relatively easily in this context, in that the large fraction of non-green cells implicit in the organization K. RICHARDSON et al. (non-green roots, non-green cells in sboots) implies substantial growth and maintenance respiration and thus sets a lower limit on the growth compensation point, granted the general constraints (not just those which may occur in chlorophyll b containing plants) on the ability of phototrophs to harvest and transform low photon flux densities. Such plants are not likely to capitalize (in evolutionary terms) on the intrinsic capacity of photosynthetic photosystems to use very lowphoton flux densities. Nevertheless, understoreyvascularplantsporophytes in tropical rain forests are able to grow at photon flux densities (in a 12 h photoperiod) of not more than 5 /iE m~^ s~* and can have light compensation points for leaf photosynthesis of 1-7 to 2-0/tEm~*s~^ (Bjorkman and Ludlow, 1972; Bjorkman, Ludlow and Morrow, 1972) i.e. at the lower end of the range for green microalgae. A more severe test is provided by plants or their organs with no or few non-green cells. The coenocytic marine green alga Oestrobium sp. grows beneath a layer of endosymbiotic dinophyte cells in the massive coral Favi at a photon flux density below \ fiE m~^ s.~^ (Halldal, 1968). Studies of photosynthesis of Oestrobium isolated from the coral are consistent with in situ data although the possibility that Oestrobium can grow in situ by (photo) heterotrophy cannot be excluded. Certain freshwater green plants (or their organs) with few non-green cells also have low light compensation points for photosynthesis. The gametophyte of the moss Drepanocladus sp. has a light compensation point of some 0-55 piE vnr^ s"^ (Priddle, 1980) while shade adapted leaves of the magnoliophyte Potamogeton obtusifolius have a ligbt compensation point of 0-275 fiE m~* s~^ (Spence and Chrystal, 1970). In the face of these light compensation values, it is difficult to sustain the view that chlorophyll b containing plants are intrinsically less capable of adapting to low photon flux densities provided that an evolutionary incentive in the form of the absence of excessive respiratory requirements in non-green cells is provided. Further measurements of green plant performance at low photon flux densities are required before a detailed analysis of the differences in photosynthetic and respiratory characteristics of chlorophyll b containing plants relative to other plants is justified.

ECOLOGICAL CONSIDERATIONS The variety of responses or adaptations to changes in photon flux density observed in microalgae suggest that these organisms are suited to occupy a variety of habitats and that seasonal changes in the available light habitats may be related to seasonal algal succession patterns. In a water column that is being mixed continuously, only one light habitat exists and optimal performance in such an environment requires tolerance of both high and low photon flux densities. Such would be so in temperate marine waters during winter and early spring. It is not surprising, then, that during these periods diatom species with low values for Io—/tmin ^nd high values for Id-/"max (or lo —Pmax) dominate; see Table 3. As the season progresses, a more stable water column develops and it seems likely that a humber of light environments may be created. In a stratified water column, one can envisage warm water and high photon flux densities in the upper region, cold water with very low photon flux densities in the lower part of the column and an intermediate region around the pycnociine. Because of the variety of light environments, we can predict greater algal species diversity in a stratified than in a mixed water column. Adaptation of unicellular algae 183 The occurrence of large subsurface chlorophyll maxima during summer months in temperate waters (often consisting of dinoflagellates; e.g. Holligan and Harbour, 1976) tends to support the hypothesis that a stable low light environment is being created in the water column. The traditional picture of seasonal primary production patterns in temperate waters is one of a spring bloom followed by a period of nutrient depletion and algal inactivity followed again by an autunnn bloom of phytoplankton (Cushing, 1959;Heinrich, 1962). This, however, assumes that what is happening in the surface waters is indicative of the entire water column. If there are a number of different light ' habitats' within a stratified water column which can be exploited by different species, this may not be so.. Of course, other factors as well as light (such as nutrient distributions) may also vary within a stratified water column and although light may be an underlying factor at all times, we do not mean to suggest that it is the only factor controlling phytoplankton distributions. Many phytoplankton that prefer low photon Hux densities possess the ability to regulate their own light environnnent, at least to some extent. Perhaps the most obvious method of controlling the light environment is for an alga to occur only in environments where exposure to low photon flux densities is ensured. There are well known examples of algal populations under ice cover (e.g. Wright, 1964; Bunt, 1967; Muller-Haeckel, 1981). Some workers have suggested that such algae may be supplementing photosynthetic carbon assimilation with heterotrophic or photoheterotrophic utilization of organic carbon sources. It now seems clear, however, that many algal populations can survive under ice cover photoautotro- phically (e.g. Boylen and Brock, 1974). However, low-light preferring microalgae also occur in regions where they may experience exposure to high photon flux densities. Van Liere and Walsby (1982) discuss two mechanisms by which blue-green algae can reduce the light energy that they receive. The first is through light shielding by pigments (either intra or extra-cellular) or by refractile structures such as gas vacuoles or calcified sheaths. The second method blue-green algae use to control their light environment is to actually alter their topographic position with respect to the light sources. This may be effected either by gliding movements or by regulation of cellular buoyancy. A most dramatic example of the latter occurs in Anabaena flos-aquae which uses gas vacuoles for buoyancy (Walsby, 1969). At low light, this organism produces enough gas vacuoles to make it buoyant. At high light, these gas vacuoles collapse and the cycle repeats itself. Their collapse at high irradiance can be correlated with a rise in cellular turgor pressure (Dinsdale and Walsby, 1972) which results from the increased photosynthetic activity in that light environment. Representatives of other algal classes also exhibit the ability to position themselves with respect to the light environment. Some have flagella and are phototactic (also see section on Genotypic Adaptation). For other, non-motile species light regulation takes on different forms. The hot spring diatom, Achnanthes exigua, exhibits growth inhibition at about 150 /lE m~^ s~^ but photon flux densities may reach ten times that value. The apparent solution to this problem for this alga is to associate with a thick algal mat which may serve as a light shield (Fairchild and Sheridan, 1974). In addition to actual relocation of an entire cell with respect to the light environment, a number of algal species have been shown to position chloroplasts within the cell differently depending on the available light energy. In general, at high photon flux densities, chloroplasts may appear in the least irradiated parts 184 K,. RICHARDSON et al. of the cells and vice versa (see Nultsch, 1974). Kiefer (1973) reported the chloroplasts in a marine diatom to contract and aggregate upon exposure to high photon flux densities. Oceanographers and phytoplankton ecologists have long considered the region of the sea in which net photosynthesis can occur as roughly that above the depth to which 1 % of surface illumination can penetrate. It is vital not to pass this myth along to future generations of aquatic ecologists. We have drawn attention here to phytoplankton species with light compensation points in the region of 0-2fiEm~^s~^ which is approximately equal to 10"* full sunlight and have identified a number of species that apparently prefer low photon fiux densities. This is not merely a laboratory phenomenon. Field studies have shown primary production to occur down to the '0-01 % level' (Holm-Hansen et aL, 1977) and <0-l % level (Jeffrey, 1981). The variety of genotypic and phenotypic responses to changes in photon fiux density discussed greatly complicate the task of modelling primary production patterns. When phytoplankton can respond to changes in the light environment in a variety of ways, the situation inevitably arises when apparently similar hydrographic systems populated by different phytopiankton populations will behave entirely dissimilarly with respect to primary production. An example of such a situation can be taken from the works of Rivkin et al., (1982) and Heath, Richardson and Spencer (in preparation). The former study describes the dinoflagellate Gonyaulax polyedra surviving at depth in the water column only at the expense of cell carbon. Up welling to the surface provides a more favourable environment for photosynthesis. In contrast. Heath et al. (in preparation) found a natural population of Rhizoselenia delicatula to grow better at a depth of 17 m (where the incident photon flux density' never exceeded 30 /tE m~^ s~^ during the day) than when artificially upwelled to 7 m. Vollenweider (1970) deals with the theoretical effect of two types of adaptation mechanisms on the overall photosynthetic output per unit surface area of a water column. There are, however, clearly more than two different adaptation responses to changes in photon fluence rate that phytoplankton may exhibit and few attempts have actually been made to include different pbotosynthetic responses in primary production modelling. It is clear that position, at least in a stable water column, does affect the photosynthetic activity of phytoplankton. Morel (1978) calculated the efficiency of photosynthesis of deep water phytoplankton to be five to ten times higher than that for the surface phytoplankton. Other workers have also reported that photosynthesis expressed on a unit chlorophyll basis may not remain constant throughout the water column (Platt and Jassby, 1976; MacCaul and Platt, 1977; Prezelin and Sweeney, 1977). Finally, laboratory studies (Welschmeyer and Lorenzen,, 1981) have indicated that the in vivo chlorophyll a absorption coefficient may differ between different algal species. Until a more tborough understanding of tbe photosynthetic adaptations that occur in response to changes in the light environment is gained, modellers cannot possibly accurately represent the primary production occurring in tbe sea. It is boped tbat the discussion in this section on Ecological Considerations, in conjunction with that in the earlier sections, points out the crucial importance of laboratory studies in evaluating the response of unicellular microalgae to tbe light climate in limnological or marine environments. Wbile being in no way dismissive of P vs I curves obtained for natural phytoplankton populations on research cruises Adaptation of unicellular algae 185 or of the models derived from them, we feel that accurate estimation of the photosynthetic production occurring in aquatic environments requires further laboratory studies of genotypic and phenotypic responses of continuous cultures of a range of microalgae to photon flux density and the biochemical and biophysical analysis of the differences which such studies reveal.

ACKNOWLEDGEMENTS We wish to thank the S.E.R.C. for support under research grants GR/A/6989.6 and GR/B/8146.5 and Dr M. L. Reed for critical appraisal of the manuscript.

REFERENCES AIBA, S. & OGAWA, T. (1977). Assessment of growth yield of a blue-green alga, Spirulina platemis in axenic and continuous culture. Journal of General Microbiology, 102, 179-182. ANDERSON, J. M. (1982a). Distribution of cytochromes of spinach chloroplasts between the appressed tnentibranes of grana stacks and strotna-exposed thylakoid region. P.E.B.S. Letters, 138, 62-66. ANDERSON, J. M. (1982b). The role of chlorophyll-protein complexes in the function and structure of chloroplast thylakoids. Molecular and Cellular Biochemistry (In press). ANDERSON, J. M.. BAKBETT, J. & THOBNE, S. W. (1982). Chlorophyll-protein complexes of photosynthetic eukaryotes and prokaryotes: properties and functional organisation. In: Photosynthesis {Proceedings oj the Fifth International Congress on Photosynthesis, vol III (Ed. by G. Akoyonoglou) pp. 301-315. International Science Services, Glenside, Pennsylvania. ANDEBSSON.B., ANDERSON, J. M. &RyRiE, I. J. (1982). Transbilayer organisation of the chlorophyll-proteins of spinach thylakoid European jfournal of Biochemistry, 123, 465—472. ARNON, D. I.. TsujiMOTO, H. Y. & TANG, G. M-S. (1981). Proton transport in photo-oxidation of waterr a new perspective on photosynthesis. Proceedings of the National Academy of Science, 78, 2942-2945. BARBER, J. (1982). Influence of surface changes on thylakoid structure and function. Annual Review of Plant Physiology, 33, 261-295. BEARDALL, J. (1976). Studies on the mechanism of photosynthesis in marine phytoplankton. Unpublished Ph.D. thesis. University of London. lSl pp. BEARDALL, J. & MORRIS. I. (1976). The concept of light intensity adaptation in marine phytoplankton: some experiments with Phaeodactytum tricornutum. Marine Biology, 37. 377-387. BELAY, A. (1981). An experimental investigation of inhibition of phytopiankton photosynthesis at iake surfaces. Nezv Phytologist, 89, 61-74. BENNETT, J., STEINBACK, K. E. & ABNTZEN. C. J. (1980). Chloroplast phosphoproteins: regulation of excitation energy by phosphoryiation of thylakoid polypeptides. Proceedings of the National Academy of Science, 77, 5253-5257. BlEBL, H. & PFENNING, N. (1978). Growth yields of green sulfur bacteria in nnixed cultures with sulfur and sulfate reducing bacteria. Archives of Microbiology, 117, 9-16. BjOBKMAN, O. (1981). Responses to different quantum flux densities. In: Encyclopedia of Plant Physiology, Vol. 12A. Physiological Plant Ecology I. Responses to the Physical Environment. (Ed. by O. L. Lange. P. S. Nobel, C. B. Osmond & H. Ziegler). pp. 57-107. Springer-Verlag, Berlin. BJORKMAN, O. & LUDLOW. M. M. (1972). Characterisation of the light climate on the floor of a Queensland rainforest. Carnegie Institute Yearbook, 71, 85-94. BJORKMAN, O., LUDLOW, M. M. & MOBROW, P. A. (1972). Photosynthetic performance of rainforest species in their natural habitat and analysis of their gas exchange. Carnegie Institute Yearbook, 71, 94-102. BLASCO, D. (1978). Ohservations on the diel migration of marine dinoflagellates off the Baja California Coast. Marine Biology, 46, 41-47. BOGORAD, L. (1975). Phycobiiiproteins and complementary chromatic adaptation. Annual Revietv of Plant Physiology, 26, 36?M01. BoYLEN. C. W. & BROCK, T. D. (1974). A seasonal diatom in a frozen Wisconsin lake, jlournal of Phycology, 10, 210-213. BRAND, L. E. (1982). Genetic variahility and spatial patterns of genetic differentiation in the reproduction rates of the marine coccolithophores Emiliana huxleyi and Gelphyrocapsa oceanica. Limnology and Oceanography, 27(2), 236-245. BRAND, L. E. & GUILLARD, R. R. L. (1981). The effects of continuous light and light intensity on the reproduction rates of twenty-two species of marine phytoplankton. Journal of Experimental Marine Biology and Ecology, 50, 119-132. BRIANTAIS, J-M., VEENOTTE, C. & MAISON, B. (1982). Influence ofstackingon the distribution of light energy in the photosynthetic apparatus. Physiologie Vegetale, 20, 111-122. i86 K. RICHARDSON et al.

BROWN, T. E, & RICHARDSON, F, L. (1968). The effect of growth environment on the physiology of algae: light intensity. Journal of Phycology, 4, 38-54. BUNT, J. S. (1967). Some characteristics of microaigae isolated from Antarctic sea ice.. In: Biology qf Antarctic Seas (Ed... by G. A. Llano iSt W, L. Schmitt). Antarctic Research Series, vol. 11. pp. 1-14. American Geophysical Union Publication No. 1597. BUTLER, W. L. (1978), Energy distribution in the photochemical apparatus of photosynthesis- Annual Review qf Plant Physiology, 29, 345-378. CANUTO, V. M.^ LEVINE, J. S., AUGUSTSSON, T. R. & IMHOFF, C. L, (1982). Ultraviolet radiation from the young sun and oxygen and ozone levels in the prebiological atmosphere. Nature, 296^ 816-820. CASTENHOLZ, R. W. (1964), The effect of daylength and light intensity on the growth of littoral marine diatoms in culture. Physiologia Plantarum, 17, 951-963. CHAN, A. T. (1978). ComparatJve physiological study of marine diatoms and dinoflagellates in relation to irradiance and cell size. I. Growth under continuous light, yournal of Phycology, 14^ 396—402. CusHiNG, D. H. (1959). The seasonal variation in oceanic production as a problem in population dynamics. Journal du Conseil, 24, 455-464. DELRIEU, M. J. (1981). Light intensity saturation properties of the S state transitions in Chlorella and chloroplasts. In: Photosynthesis II: Electron Transport and Photophosphorylation (Ed. by G. Akovo- noglou), pp. 395-404. Balaboau International Science Sen'ices^ Glenside, California. DiNSDALE, M. T- & WALSBY, A. E. (1972). The interrelations of cell turgor pressure, gas vacuolation and buoyancy in a blue-green alga.. Journal of Experimental Botany, 23., 561-570. DODGE, J. D. & HAHT-JONES, B. (1977),. The vertical and seasonal distribution of dinoflagellates in the North Sea, Botanica marina, 20, 307-311, DHEWS, G. & OELZE, J. (1981)- Organization and differentiation of membranes of phototrophic bacteria. Advances in Microbial Physiology, 22, 1-92. DHING, M. J- (1981). Chromatic adaptation of photosynthesis in benthic marine algae: an examiination of its ecological significance using a theoretical model- Limnology and Oceanography, 26(2), 271—284.. DuRBiN, E- G. (1974). Studies on the autecology of the marine diatom, Thalassiosira nordenskioldii Cleve. I. The influence of daylength, light intensity and temperature on growth. Journal of Pkycology, 10, 220-225. EPPLEY, R. W. (1972)- Temperature and phytoplankton growth in the sea. Fishery Bulletin, 70, 1063-1085. EAIRCHILD, E, & SHERIDAN, R. P. (1974)- A physiological investigation of the hot spring diatom, Acknantkes exigua Grun. Journal of Pkycology, 10, 1—4. FALKOWSKI, P. G. (1980)- Light-shade adaptation in marine phytoplankton. In: Primary Productivity in the Sea (Ed- by P- G. Ealkowski), pp, 99-119. Plenum Pres.s, New York. FALKOWSKI, P. G. & DUBINSKY, Z. (1981). Light-shade adaptation of Stylophora pistillata, a hermatypic coral from the Gulf of Eilat. Nature, 289. 172-175. FALKOWSKI, P. G. & OWENS,, T. G- (1978). Effects of light intensity on photosynthesis and dark respiration in six species of marine phytoplankton. Marine Biology, 45, 289-295- FALKOWSKI, P. G- & OwENEs', T. G. (1980). Light-shade adaptation. Two strategies in marine phytoplankters. Plant Physiology (Bethesda), 66, 592-595. FEICK, R. G., FITZPATRICK, M. & FULLER, R. C. (1982). Isolation and characterisation of cytoplasmic membranes and chlorosomesfrom the green bacterium, Ckloroflexus aurantiacus. Journal of Bacteriology, 150, 905-915. FLEISCHACKER, P, H. & SENGER, H. {1978). Adaptation of the photosynthetic apparatus of Scenedesmus obliquus to strong and weak light conditions- IL Differences in photocheniical reactions, the photosynthetic electron transport and photosynthetic units. Physiologia Plantarum, 43, 43-51. FORK, D. C. & OQUIST, G. (1981). The effect of dessication on excitation energy transfer at physiological temperatures between the two photosystems of the red alga Porphyra perforta. Zeitschrift fur Pfianzenpkysiologie, 104, 385-394. FORK, D-.,, OQUIST, G- & POWLES, S, B.. (1981). Photoinhibition in bean. A fluorescence analysis. Carnegie Institute Yearbook, 80, 52-57. FOY, R. H., GIBSON, C. E. & SMITH, R-V. (1976). The influence of daylength, light intensity and temperature on the growth rates of blue-green algae. British Phycological Journal, 11, 151-163. GALLAGHER, J. (1982). Physiological variation and electrophoretic banding patterns of genetically different seasonal populations of Skeletonema costatum (BaciUsLriophyceaie). Journal of Phycology, 18(1), 148-162. GANF, G. G. (1975). Photosynthesis production and irradiance —photosynthesis relationships of the phytoplankton from a shallow equatorial lake (Lake George, Uganda). Oecologia, 18, 165-183. GIBBS, S. P. (1981). Chloroplast endoplasmic reticulum: structure, function and significance. International Reviews of Cytology, 72, 49-99. GBIFFITHSJ D.J. (1973). Factors affecting the photosynthetic performance of laboratory cultures, of the diatom Phaeodactylum tricornutum. Marine Biology, 21, 91-97. GROEN, A. K., WANDERS, R. J. A., WESTERHOFF, M. V., VAN DER MEER, R. & TAGER, J. M. (1982). Quantification of the contribution of various steps to the control of mitochondrial respiration. Journal of Biological Chemistry, 257 2754-2757. Adaptation of unicellular algae 187

GitUMBACH, K. H,., LICHTENTHALER,, H. K. •& EHISMANN, K. H, (1978). Incorporation of ^^COg in photosynthetic'pigments of CA/ore/ZiOjbjjrenoifi'osa'. Planta, 140, 37-43'. HALLDAL, P. (1964). Ultraviolet action spectra of photosynthetic inhibition in a green and a red alga. Physiologia Plantarum, 17, 415-421, HALLDAL, P. (1967). Ultraviolet action spectra in algology: a review. Photochemistry and Photobiology, 445-460. HALLDAL, P. (1%S), Photosynthetic capacities and photosynthetic action spectra of the endozoic algae of the massive coral, Favia. Biological Buiietin, 134, 411-424. HALLDAL, P. Sc TAUBE, P. (1972). Ultraviolet action spectra and! photoreactivation in algae. Fhotophysiology, 7, 163-188. HARAUX, F. & DEKOUCHKOVSKY, Y. (1982). Further investigations of the lateral and transversal proton current at the thylakoid membrane level hy hydrogen-deuterium exchange. Biochemica et Biopkysica Acta , 679, 235-247. HARRIS, G. P, (1978). Photosynthesis, productivity and growth: the physiological ecoiog>' of phytoplankton. Ergebniesse der Limnologie (Arckiv fur Hydrobioiogie), 10, 1-17L HARHIS, G. P., HEANEY, S. I. & TALUNG, J, F. (1979). Physiological and environmental constraints in the ecology of the planktonic dinoflagellate, Ceratium hirundiella. Freshwater Biology, 9, 413—428. HASLE, G. R. (1950). Phototactic vertical migration in marine dinoflagellates. Oikos, 2, 162-175. HAWORTH,. P., KYLE, B. J. & ARNTZEN, C. J- (1982). A dennonstratioini of the physiological role of memhrane phosphoryiaition in chloroplasts, using the hipartite and tripartite models of photosynthesis. Biochemica et Biopkysica Acta, 6S0', 343-351. HEINRICHJ A. K. (1962). The life histories of plankton animals and seasonai cycles of plankton communities in the oceans. Jaurnal du Conseil., 27, 15-24. HENDERSON, R. (1977). The purple membrane oi Halobacterium halobium. Annual Review of Biophysics and Bioengineering, 6, 87-109. HERSEY, R. L. & SWIFT, E. (1976). Nitrate reductase activity of Ampidinium carterae and Cachonina niei (Dinophyceae) in batch culture: diel periodicity and effects of hght intensity and ammonia, Journal of Phycology, 12(1), 36-44. HoLLiGAN", P. M. & HARBOUH., D. S, (1976). The vertical distrihution and succession of phytoplankton in the Western English Channel in 1975 and 1976. Journal of the Marine Biological Association (U.K,.), SI, 1075-1093. HOLM-HANSEN, O., EL-SAYED, S.. Z.,, FRANCESCHINI, G. A. &: CUHEL, R. (1977). Primary production and the factors controlling plankton growth in the Antarctic Seas. In: Adaptations within Antarctic Ecosystems, 3rd Symposium on Antarctic Biology, Washington, 1974. HOLT, M. G. & SMAYDA, T. J. (1974). The effect of daytength and light intensity on the growth rate of the marine diatom Detonula confervacea (Cleve) Gran. Journal of Phycology, 10, 231-237. ILMAVIRTA, V. & HAKALA, I. (1972). Acrylic plastic and Jena glass bottles used in measuring phy to planktonic primary production in the 14-C method. Annales Botanici Fennici, 9, 77-84. INCOLL, L. D., LONG, S. P. & ASHMORE, M. R. (1981). SI Units in Publications in Plant Science. In: Commentaries in Plant Science, vol. 2 (Ed. by H. Smith), pp. 83-96. Pergamon Press, Oxford. JEFPBEY, S. W. (1981). Responses to light in aquatic plants. In: Encyclopedia of Plant Physiology, vol, 12A Physiological Plant Ecology L Responses to the Physical Environment (Ed, by O. L. Lange, P. S. Nobel, C, B, Osmond 8c H, Ziegler). Springer-Verlag, Berlin, JEFFREY, S. W. & VESK, M, (1977), Effect of hlue-green light on photosynthetic pigments and chloroplast structure in the marine diatom, Stephanopyxis turris. Journal of Phycology, 13, 271-279. JERLOV, N. G. (1968). Optical Oceanography Elsevier, New York. JERLOV, N. G. (1976). Optical Oceanography Eisevier, New York. JEWSON, D. H. (1976). The interaction of components controlling net phytoplankton photosynthesis in a well mixed lake (Loch Neagh, Northern Ireland). Freshwater Biology, 6, 551-576. jiTTs, M, R., MCALLISTER, C. D,, STEPHENS, K, & STRICKLAND, J. D, H, (1964). The cell division rates of some marine phytoplankters as a function of light and temperature. Journal of the Fisheries Research Board, Canada,. 21, 139-158. JiTTS, M, R., MOREL, R. & SAIJIO, Y. (1976). The relation of oceanic primary production to available photosynthetic irradiance, Australian Journal of Marine and Freshwater Research, 21, 441-454. JONES, L. W. & KOK, B, (1966a). Photoinhibition of chloroplast reactions. I. Kinetics and action spectra. Plant Physiology, 41, 1037-1043. JONES, L. W, & KOK, B. (1966b). Photoinhibition of chloroplast reactions. II. Multiple effects. Plant Physiology, 41, 1044-1049. J0RGENSEN, E. G. (1964). Adaptation to different light intensities in the diatom Cyclotella menegkiana Kutz. Physiologia Plantarum, 17, 136-145. JeHGENSEN, E. G. (1966). Photosynthetic activity during the life cycle of synchronous Skeletonema cells. Physiologia Plantarum, 19, 789-799. J0RGENSEN, E. G. (1969). The adaptation of planktonic algae, IV. Light adaptation in different species. Physiologia Plantarum, 22, 1307-1315. i88 K. RICHARDSON et al.

JoRGENSEN, E. G. (1970). The adaptation of planktonic algae. V. Variation in the photosynthetic characteristics of Skeletonema costatum cells grown at low light intensity. Physiologia Plantarum, 23,11-17. KALFF, J. & WELCH, H. E. (1974). Phytoplankton production in Char Lake, a natural polar lake, and in Meretta Lake^ a polluted polar lake, Cornwalhs Island, Northwest Territories. Journal of the Fisheries Research Board, Canada., 31, 621-636. KAWAMURA, M., MIMURO, M. & FUJITA, Y. (1979). Quantitative relationship between two reaction centers in the photosynthetic system of blue-green algae. Plant and Cell Physiology, 20, 697-705. KiEFER, D. A. (1973). Chlorophyll a fluorescence in marine centric diatoms: responses of chloroplasts to light and nutrient stress. Marine Biology, 23, 39—46. KoK, B. (1960). The efficiency of photosynthesis. In: Encyclopedia of Plant Physiology, vol. 5/2 (Ed. by W. Ruhland), pp. 566-633. Springer-Verlag, Berlin. KmuGER, G. H. J. & ELOFF, J. N. (1977). The influence of light intensity on the growth of different Microcystis isolates. Journal of tke Limnological Society of South Africa, 3, 21-25.. KYLE, D. J., HAWORTH, P. Sc ARNTZEN, C. J. (1982). Thylakoid memhrane protein phosphorylation leads to a decrease in connectivity between photosystem two reaction centres. Biochemica et Biophysica Acta, 680, 336-342. LEHMAN, P. W. (1981). Comparison of chlorophyll a and carotenoid pigments as predictors of biomass. Marine Biology, 65, 237-244. LELETKIN, V. A., ZvALiNSKY, V. 1. & TiTLYANOV, E. A. (1980). Photosynthesis of zooxantkellae in corals from different depths. Fizioloziya Rastenii., 27(6), 1163-1171. LEY,, A. C. & MAUZERALL, D. C. (1982). Absolute absorption cross-sections for photosystem II and the minimum quantum requirement for photosynthesis in Chlorella vulgaris.. Biochemica et Biophysica Acta, 680, 95-106. LiCHTENTHALEB, H. K. & GRUMBACH, K. H. (1975). Observations on the turnover of thylakoids and their prenyl lipids in Hordeum vulgaris.. In: Proceedings of the 3rd International Congress on Photosynthesis (Ed, by M, Avron), pp. 2007-2016. Elsevier, New York. MACCAUL, W. A. (SC PLATT, T. (1977). Diel variations in the photosynthetic parameters of coastal marine phytoplankton. Limnology and Oceanography, 22, 723—731. MARGULIS, L. (1981). Symbiosis in Cell Evolution. Freeman and Company, San Francisco. MARRA, J. (1978). Effect of short-term variations in light intensity on photosynthesis of a marine phvtoplankter. A laboratorv simulation study. Marine Biology, 46, 203—208. MCALLISTER, C. D., SHAH, N. & STRICKLAND, J. D. H. (1964). Marine phytoplankton photosynthesis as a function of light intensity, a comparison of methods. ^OMrma/ of the Fisheries, Research Board, Canada, 21, 159-181. MELIS, A. & BROWN, J. A. (1980). Stoichiometry of system one and system two reaction centres and of plastocjuinone m different photosynthetic membranes. Proceedings of the National Academy of Sciences (Washington), 77, 4712-4716. MiSHKiND, M. & MAUZERALL, D. (1980). Kinetic evidence for a common photosynthetic step in diverse seaweeds. Marine Biology, 56, 261—265. MITCHELL, P. (1979). Direct chemiosmotic ligand conduction mechanisms in protonmottve comiplexes. In; Membrane Bioenergetics (Ed. by C. P. Lee, G. S, Schatz & L. Ernster), pp. 361-372. Add!ison-Wesley Publishing Co. Reading, Massachusetts. MoHEL, A. (1978). Available, usable and stored radiant energy in relation to marine photosynthesis. Deep Sea Research, 25, 673-688. MORGAN, K. C. & KALFF, J. (1979). Effect of light and temperature interaction on growth of Cryptomonas erosa (Cryptophyceae). J^owrna/ of Phycology, 15, 127—134. MORRIS, I. & GLOVER, H. E, (1974). Questions on the mechanism of temperature adaptation in marine phytoplankton. Marine Biology, 24, 147-154. MORRIS, I. & GLOVER, H. E. (1981). Physiology of photosynthesis by marine coccoid cyanobacteria — some ecological implications. Limnology and Oceanography, 26(5), 957-961. MUR, L. R., GONS, H. J. & VAN LIERE, L. (1977). Some experiments on the competition between green algae and blue-green bacteria in light limited environments. F.E.M.S. Microbiology Letters, I, 335—338. MYERS, J. (1970). Genetic and adaptive physiological characteristics observed in the Chlorellas. In: Prediction and Measurement of Photosynthetic Productivity Proceedings of IBP/PP Meeting, Tebon, 1969. pp. 447-454. PUDOC Wageningen. MULLER-HAECKEL, A. (1981). The low light adapted dinofiagellate Gonyaulax catenata. Sarsia, 66, 267-272, NELSON, D. M., D'ELIA, C. F, & GUILLAHD, R. R. L. (1979). Growth and competition of the marine diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana. II. Light limitation. Marine Biology, 50, 313-318- NoRDLi, E. (1957). Experimental studies on the ecology of Ceratia. Oikos, 8, 200-265. NuLTSCH, W. (1974). Movements. In: Algal Physiology and Biochemistry (Ed. by W, D. P. Stewart). Botanical Monographs, vol. 10, pp. 864—893. Blackwell Scientific Publications, Oxford. Adaptation of unicellular algae 189

OLSON, J, M. (1970). The evolution of photosynthesis. Science, 168, 438-446. OLSON, J. M. (1978). Precambrian evolution of photosynthetic and respiratory organisms. Evolutionary Biology, 11, 1-37. OLSON, J. M- (1980). Chlorophyll organisation in green photosynthetic bacteria. Biockemica et Biophysica Acta,S9A, 33-51. OiLSON, J. M. (1981), Evolutioii of photosynthetic reaction centres. Biosystems, 14, 89-94. OSMOND, C. B. (1981). Photo respiration and photoinhibition. Some implications for the energetics of photosynthesis. Biochimica et Biophysica Acta, 639, 77-98. OWENS, O. V. H. & SELIGER, H. H. (1975). Effect of light intensity on dinoflagellate growth and photosynthesis. In: Physiological Adaptations to the Environment (Ed. by F, J. Vernberg), pp. 355-364, Intext Educational Publishers, New York. PERRY, M. J., LAIBSEN, M. C. & ALBERTE, R. S. (1981). Photoadaptation in marine phytoplankton: responses of the photosynthetic unit. Marine Biology, 62, 91-101. PiRT, J. S. (1981), Extension of the limits of photosynthesis by novel solar reactors. Rivista di Biologia, 74, 27-47. PLATT, T. &|ASSBY, A. D. (1976), The relationship between photosynthesis and light for natural assemblages of coastal marine phytoplankton. Journal of Phycoiogy, 12, 421—430. PowLES, S. B. & CRITCHLEY, C. (1980). The efFect of light intensity during growth on photoinhibition of intact attached bean leaflets. Plant Physiology, 65, 1181-1187, PowLBs, S. B., OSMOND, C. B. & THORNE, S. W. (1979). Photoinhibition of intact leaves of C3 plants illuminated in the absence of both carbon dioxide and photorespiration. Plant Physiology, 64, 982-988. PHEZELIN, B. B. (1976), The role of peridinin-chlorophyll a-proteins in the photosynthetic light adaptation of the marine dinoflagellate, Glenodinium sp. Planta.^. 130, 225-233. PREZELIN, B. B. (1981). Light reactions in photosynthesis. In: Physiological Bases of phytoplankton Ecology (Ed. by T. Platt), pp, 1-46. Canadian Bulletin of Fisheries and Aquatic Sciences;, no. 210, Ottowa. PREZELIN;, B- B. & MATLrCK, H. A. (1980). Time-course of photoadaptation in the photosynthesis-irradiance relationship of a dinoflagellate exhibiting photosynthetic periodicity. Marine Biology, 58, 85-96. PREZELIN, B. B. & SWEENEY, B. M. (1977). Characterization of photosynthetic rhythms in marine dinoflagellates. II. Photosynthesis-irradiance curves and iwi'itJO chlorophyll a fluorescence. Plant Physiology, Lancaster, 60, 384-387. PREZELIN, B, B. & SWEENEY, B. M. (1978). Photoadaptation of photosynthesis in Gonyaulax polyedra. Marine Biology, 48, 27-35. PREZELrN, B. B. & SWEENEY, B, M. (1979), Photoadaptation of photosynthesis in two bloom-formiing dinofiagellates. In: Toxic Dinoflagellate Blooms (Ed. hy D. Taylor & H, Seliger), pp. 101-106. Elsevier North Holland, Inc., PRIDDLE, J, (1980). The production ecology of benthic plants in some Antarctic lakes. II. Laboratory physiolog>' studies. J''ourna/ of Ecology, 68, 155-166. RADMER, R. J. & KoK, B. (1977), Light energy conversion in photosynthesis. In: Encyclopedia of Plant Physiology, New Series, voL 5 (Ed. by A. Trebst & M. Avron), pp. 125-135. Sp ringer-Verlag, Berlin. RAMUS, J. (1981). The capture and transduction of light energy. In: The Biology of Seaweeds Botanical Monographs, vol. 17 (Ed. by C. S. Lobban & M.J.Wynne), pp. 458^92. Blackwell Scientific Publications, Oxford. RAVEN, J. A. (1981). Light quality and solute transport. In: Plants and the Daylight Spectrum (Ed. by H. Smith), pp. 375-390. Academic Press, London. RAVEN, J. A. (1982). The energetics of freshwater-algae; energy requirements for biosynthesis and volume regulation. New Phytologist, 92, 1-20. RAVEN,J. A. &BEARDALL J.(1981). Respiration and Photorespiration. In :PAj'siWo^ica/^aseso/PMoN«n^fon Ecology (Ed. by T. Platt), pp. 55-82. Canadian Bulletin of Fisheries and Aquatic Sciences, no, 210, Ottawa. RAVEN, J. A. & BEARDALL, J. (1982). The lower limit of photon fluence rate for phototrophic growth: the significance of "slippage' reactions. Plant Cell and Environment, 5, 117-124. RHEE, G-Y. & GOTHAM, I, J. (1981). The eifect of environmental factors on phytoplankton growth: light and the interactions of light with nitrate limitation. Limnology and Oceanography, 26(4), 649-659. RICHARDSON, K. (1980). The role of dissolved organic matter in the nutrition and .survival of marine phytoplankton. Unpublished Ph.D. Thesis University of Wales. 178 pp. RrcHATtDSON, K. & FOGG, G. E. (1982). The role of dissolved organic material in the nutrition and survival of marine dinofiagellates. Phycologia, 21,, 17-26. RiED, A, & REINHARDT, B. (1980). Distribution of excitation energy between photosystem one and photosystem two in red algae. IIL Quantum requirements of the induction of a state 2 - state 1 transition. Biochemica et Biophysica Acta, 592, 76-86. RIPER, D. M., OWENS, T. G. & FALKOWSKI, P. G. (1979). Chlorophyll turnover in Skeletonema costatum, a marine planktonic diatom. Plant Physiology, 64, 49-54.

7 ANP 93 K. RICHARDSON et al.

RIVKIN, R. B., SELIGER, H. H., SWIFT, E. & BIGGLEY, W. H. (1982). Light-shade adaptation by the oceanic dinoflagellates Pyrocystis noctiluca and P. fusiformis. Marine Biology, 68(2), 181-192. RIVKIN, R. B., VOYTEK, M. A. & SELtGER, H. H. (1982). Phytoplankton division rates in light litnited environments: two adaptations. Science, 215, 1123-1125. RYTHER, J. H. (1956). Photosynthesis in the ocean as a function of light intensity. Limnology and Oceanography^ 1, 61—70. RYTHEH, J. H. & MENZEL, D. W. (1959). Light adaptation by marine phytoplankton. Limnology and Oceanography, 4, 492-497. SAMUELSSON, G. & RICHARDSON, K. (1982). Photoinhibition at low quantum flux densities in a marine dinoflagellate. Marine Biology, 70, 21-26. SiNCLAIH, J. & CousiNEAU, C. (1982). The deactivation of photosystem two in Chlorella as a second-order process. Biochemica et Biophysica Acta, 680, 258-264. ScHLESlNGER, D. A,, MOL6T, L. A. & SHUTER, B. J. (1981). Specific growth rates of freshwater algae in relation to cell size and light intensity. Journal of the Fisheries Board, Canada, 36, 1052-1058. SENGER, H. & FLEISHCHACKER, P. (1978). Adaptation of the photosjiithetic apparatus of Scenedesmus oMiquus to strong and weak light conditions. I. Differences in pigments, photosynthetic capcity, quantum yield and dark reactions. Physiologia Flantarum, 43, 35—42. SEEWALDT, E. & STACKENBRANDT, E. (1982). Partial sequence of 16 s ribosomal ENA and the phylogeny of Prochloron. Nature, 295, 618-620. SHEAR, H. & WALSBY, A. E. (1975). An investigation into the possible light-shielding role of gas vacuoies in a planktonic hlue-green alga. British Fhycological Journal, 10, 241-251. SHIMURA, S. & ICHlMURA, S-E. (1973). Selective transmission of light in ocean waters and its relation to phytoplankton photosynthesis. Journal of the Oceanographical Society of Japan, 29, 257-266. SlGFlDSSON, B. & OQUIST, G. (1980). Preferential distribution of excitation energy into photosystem one of dessicated samples of the lichen Cladonia impiexa and the isolated lichen alga Trebouxia pyriformis. Physiologica Plantarwn, 49, 329-335. SlRON%^AL, C. (1963). Chlorophyll metabolism and the leaf content in some other tetrapyrrole pigments. Photochemistry and Fkotobiology, 2, 207-221. SMAYDA, T. J. (1969). Experimental ohservations on the influence of temperature, light and salinity on cell division of the marine diatom,, Detonula confervacea (Cleve) Gran. Journal of Phycotogy, 5, 150-157. SMITH, R. C, BAKER, K. S., HOLM-HANSEN, O. & OLSEN, R. (1980). Photoinhibition of photosynthesis in natural waters. Photochemistry and Photobiology, 31, 585-592. SPENCE, D. H. N. & CHRYSTAL, J. (1970). Photosynthesis and zonation of freshwater macrophytes. II. Adaptability of species to deep and shallow water. Ne-w Phytoiogist, 69, 217-227. STADLEMANN, P., MOORE, J. E. & PICKET, E. (1974), Primary production in relation to temperature structure, biomass concentration and light conditions at an inshore and an offshore station in Lake Ontario. J^ourno/ of the Fisheries Research Board, Canada, 31, 1215-1232. STEEMANN-NIELSEN, E, & HANSEN, V. KH. (1959). Light adaptation in marine phytoplankton and its interrelationship with temperatiire. Physiologia Plantarum, 12, 353-370. STEEMANN-NIELSEN, E. & J0HGENSEN, E. G. (1962), The physiological background for using chlorophyll measurements in hydrobiology and a theory explaining daily variations in chlorophyll concentration. Archivfur Hydrobiologie, 58, 349-357, STEEMANN-NIELSEN, E. & JORGENSEN, E. G. (1968). The adaptation of planktonic algae. I. General part. Physiologia Plantarum, 21, 401^13. SWIFT, E. & MEUNIER, V. (1976). Effects of light intensity on division rate, stimulable biolutninescence, and cell size on the oceanic dinoflagellates Dissodinium lunula, Pyrocystis fusiformis, and P. noctiluca. Journal of Phycology, 12(1), 14-22. TAYLOR, F. J. R.(198O). Basic features of phytoplankton cells. In: Tke Physiological Ecology of Phytoplankton (Ed. by I. Morris), Studies in Ecology, vol. 7, pp. 3-56. Blackwell Scientific Publications. Oxford. THOMAS, W. H. (1966). Effects of temperature and illuminance on cell division rates of three species of tropical oceanic phytopiankton. Journal of Phycology, 2, 17-22. TYLER, M. A. & SELIGER, H. H. (1978). Annual subsurface transport of a red tide dinoflagellate to its bloom area: water circulation patterns and organism distributions in the Chesapeake Bay. Limnology and Oceanography, 23(2), 227-246. VAN L.IERE, L, & MuR, L. R. (1979). Growth kinetics of Oscillatmia agardUi Gomont in continuous culture, limited in its growth by light energy. Journal of General Microbiology, llS, 153—160. VAN LIKKE, L. & WALSY, A. E. (1982). Interactions of cyanobacteria with light. In: Biology of the Cyano- bacteria (Ed. by N. G. Carr and B. A. Whitton) (In press). VAN VALEN, L. M. (1982). Phylogenies in molecular evolution: Prochloron. Nature, 298, 493-494. VERITY, P. G. (19Bla). Effects of temperature, irradiance, and daylength on the marine diatom, Lepto- cylindricus danicus Cleve. I. Photosynthesis and cellular composition. Journal of Experimental Marine Biology and Ecology, 59, 79-91. VERITY, P. G. (1981b). Effects of temperature, irradiance, and daylength on the marine diatom, Lepto- Adaptation of unicellular algae igi

cylindricus danicus Cleve. O. Dark respiration. Journal of Experimental Marine Biology and Ecology, 159-169. VERITY, P. G. (1982a). Effects of temperature, irradiance, and daylength on the marine diatom, Lepto- cylindricus danicus Cleve. III. Dark respiration. Journal of Experimental Marine Biology and Ecology, 60, 197-208. VERITY, P. G, (1982b). Effects of temperature, irradiance, and daylength on the marine diatom, Lepto- cylindricus danicus Cleve, IV, Growth. Journal of Experimental Marine Biology and Ecology, 60, 209-222. VESK, M. & JEFFREY, S. W. (1977), Effect of blue-green light on photosynthetic pigments and chloroplast structure in unicellular marine algae from six classes. Journal of Phycology, 13, 280^288. VoLLENWEiDEE, R. A. (1970). Models for calculating integral photosynthesis and some implications regarding structural properties of the community metabolism of aquatic systems. In: Prediction and Measurement of Photosynthetic Productivity Proceedings of the IBP/PP Technical Meeting Trebon, 1969. Wageningen. VON CAEMMEHER, S. & FARQUHAR, G. D. (1981). Some relationships between the biochemistry of photosynthesis and the gas exchange of leaves. Planta, 153, 376-387. VINCENT, W. F. (1980). Mechanisms of rapid photosynthetic adaptation in natural phytoplankton communities. II. Capacity for non-cyclic electron transport. Journal of Phycology, 16, 368-377. VINCENT, W. F. (1979), Mechanisms of rapid photosynthetic adaptation in natural phytoplankton communities, I. Redistribution of excitation energy between photosystems I and O. Journal of Phycology, 15, 429-^34. VosKHESENSKAYA, N. P. iSt PoLYAKOV, M. A. (1975). Enhancement of COj-gas exchange in the leaves of Convolaria mayalis L.: its action spectrum and light saturation. Plant Science Letters, 5, 333-338. VosKKESENSKAYA, N. P., DBOZDOVA, I. S. & KRENDELEVA, T. E. (1977). Effect of light quality on the organisation of photosynthetic electron transport chain of pea seedlings. Plant Physiology, 59, 151—154. WALL, D, & BRIAND, F. (1979). Response of lake phytoplankton communities to in situ manipulations of light intensity and colour. Journal of Plankton Research, I, 103-111. WALLEN^ D. G, & GEEN, G. H, (1971a). Light quality in relation to growth, photosynthetic rates and carbon metabolism in two species of marine plankton algae. Marine Biology, 19, 34-43. WALLEN, D. G. & GEEN, G. H. (1971b). Light quality and concentration of proteins, RNA, DNA and photosynthetic pigments in two species of marine plankton algae. Marine Biology, 10, 44—5 L WALLBN, D. G. & GEEN, G. H. (1971C). The nature of the photosymthate in natural phytoplankton populations in relation to light quality. Marine Biology, 10, 157-168. WALSBY, A. E. (1969). The permeability of blue-green algal gas vacuole membrane to gas. Proceedings of the Royal Society, 173, 235-255. WELSCHMEYER, N. A. &LORENZEN, C. J. (1981). Chlorophyll-specific photosynthesis and quantum efficiency at subsatiirating light intensities, jf^owrwa/ of Phycology, 17, 283-293. WETHEV, D- S. & PoKTER, J. W. (1976). Sun and shade differences in productivity of reef corals. Nature^ 262, 281-282. WHATLEY, J. M. & WHATLEY, F. R. (1981). Chloroplast evolution. New Phytoiogist, S7, 233-247. WHEETLES, W- N. & NEUSHAL, M. (1981). The aquatic environment. In: Encyclopedia of Plant Physiology New Series, vol. 12A. (Ed. by O, Lange, P. S. Nobel, C.B.Osmond & H. Ziegler), pp. 228-247. Sp ringer-Verlag, Berlin. WHITE, A. W. (1974).. Growth of two facultatively heterotrophic marine centric diatoms. Journal of Phycology, 10, 292-300. WITT, H. T. (1979). Energy conversion in the functional membrane of photosynthesis. Analysis by light pulse and electrical pulse methods. The control of the electric field. Biockemica et Biophysica Acta,. 505, 355-427. WBIGHT, R. T. (1964). Dynamics of a phytoplankton community in an ice-covered lake Limnology and Oceanography, 9, 163-178. YENTSCH, C. S. (1974). Some aspects of the environmental physiology of marine phytoplankton: a second look. Oceanography and Marine Biology Annual Review, 12, 41-75. YENTSCH, C. S. & LEE, P. W. (1966). A study of photosynthetic light reactions and a new interpretation of sun and shade phytoplankton. Journal of Marine Research, 24, 319-337, ZEINALOV, YU. (1982). Existence of two different ways for oxygen evolution in photosynthesis and the photosynthetic unit concept. Photosynthetica, 16(1), 27-35.^

7-2