MARINE ECOLOGY PROGRESS SERIES Vol. 86: 201-204. 1992 1Published September 10 Mar. Ecol. Prog. Ser.

NOTE

Light absorption by seagrass Posidonia oceanica leaves

Susana Enriquez, Susana Agusti, Carlos M. Duarte

Centro de Estudios Avanzados de Blanes, Cami de Santa Barbara sin, E-l7300 Blanes. Girona, Spain

ABSTRACT: We examined the importance of pigment content ships demonstrating a close relationship between and packaging within plant t~ssuesfor light absorption by light absorption and the amount and extent of pack- Posidonia oceanica leaves from different locations along the aging of pigments within the cells (More1 & Bricaud Spanish Mediterranean coast. Pigments of P. oceanica leaves absorbed 70.5 to 97.1 % of incident Light (at 675 nm); the light 1981, Agusti 1991a). The applicability of these rela- absorbed by pigments increased with increasing chlorophyll tionships for marine plants other than phytoplankton, density. The efficiency of chlorophyll a in absorbing light (i.e. however, has only been assessed to a limited extent the chlorophyll a-specific light absorption) also decreased sig- for some marine macroalgae (Ramus 1978). These nificantly (p < 0.001) with increasing pigment packaging (i.e. concentration) within the leaves, although the light absorbed studies have shown that the application of such per unit biomass increased with increasing pigment con- theories to describing light absorption by macroalgae centration. The positive trend between light absorption and is confounded by complex phenomena (e.g. the pack- chlorophyll density found for P. oceanica leaves was also age effect, heterogeneous absorption, and multiple found to apply to 12 other seagrass species from the Atlantic, scatter), largely derived from the varied structural Pacific, Caribbean, South China and Mediterranean Seas. The similarity between the patterns in light absorption by repertoire of the multicellular tissues of macroalgae seagrass leaves described here and those previously de- (Ramus 1990). Yet, these theories, based on physical scribed for phytoplankton suggests that these patterns reflect principles rather than properties specific to phyto- constraints on light absorption by phototrophic organisms plankton, may still hold for marine angiosperms, for which may apply to most aquatlc plants their photosynthetic tissues (i.e. blades) are much more uniform than those of macroalgae. Here we Seagrasses are important primary producers in most demonstrate the importance of chlorophyll a content tropical and subtropical areas and many temperate and its packaging within plant tissues for absorption littoral ecosystems worldwide, where they may play a of light by Posidonia oceanica leaves and provide dominant role in the carbon budget. The Medi- evidence of the applicability of these relationships to terranean seagrass Posidonia oceanica (L.)Delile is the other seagrasses. dominant seagrass in this mostly oligotrophic sea, Methods. Leaves of Mediterranean seagrasses (Posi- where its primary production can exceed that of phyto- donia oceanica, Cymodocea nodosa (Ucria.) Aschers., plankton. The local importance of the species is deter- Ruppia rnaritima L. and Zostera noltii Hornemann) mined by the area it covers which, in turn, is closely growing between 0.5 and 15 m depth were collected, dependent on the maximum depth it colonizes. Be- between April and October 1991, from 20 locations cause the depth limit of P. oceanica is closely linked, along the Spanish Mediterranean coast. Leaves of like that of other seagrasses, to light penetration Caribbean seagrasses (Thalassia testudinum Banks ex underwater (see review in Duarte 1991), the ability to Konig, and Halodule wrightii Aschers.) were collected absorb light is an important factor regulating the from different locations in the Mexican Caribbean; importance of P. oceanica in the carbon budget of leaves of Atlantic seagrasses (Zostera marina L., littoral Mediterranean ecosystems. Zostera noltii and Ruppia cirrhosa (Petagna) Grande) Extensive research on light absorption by phyto- were collected along the South Portuguese coast; and plankton has provided models and empirical relation- leaves of Pacific seagrasses (Enhalus acoroides (L.)

O Inter-Research 1992 202 Mar. Ecol. Prog. Ser. 86: 201-204, 1992

Royle, Thalassia hernprichii (Ehrenb.) Aschers., Halo- area (mm2 mg-l) and then assuming a density of 1.0 phila ovalis (R. Br.) Hook, Syringodiurn isoetifoliurn (confirmed in the laboratory) for the leaf material. (Aschers.) Dandy, and Halodule uninervis (Forsk.) Results and discussion. The content of chlorophyll a Aschers.) were collected in the South China Sea, along in Posidonia oceanica leaves ranged widely in denslty the Philippine coast. Light absorption was measured (0.070 to 0.408 pg chl a mm-') and packaging (0.95 to on fresh leaves within a few hours of collection. 8.91 pg chl a mg DW-l). Absorption values (at 675 nm) Those plants collected away (>300 km) from our ranged between 0.53 and 1.54 O.D. (loglounits), which laboratory were preserved in a solution of 1.5 % corresponds to an absorption of 70.5 to 97.1 % of the formaldehyde in filtered seawater, buffered to a pH of incident light (at 675 nm) by chlorophyll a. We found 7.5 with Tris, to prevent pigment degradation (Stramski the observed differences in the extent of light ab- 1990). We found no significant reduction in absorbance sorbed by pigments among leaves to be linearly re- (t-test, p > 0.20) between fresh leaves and leaves pre- lated to their chlorophyll density (Fig. l), as described served for 4 d, and no significant effect of the fixation by the regression equation: procedure on light absorption or chlorophyll a concen- tration (ANOVA, p > 0.20), indicating that the preser- Absorption (675)(O.D.) = vation method worked efficiently. We failed, however, 0.42 + 2.17 chl a density (yg chl a to obtain measurements of light absorption by the cylindrical leaves of species of the genus Syringodiurn sp, from the Caribbean and South China Sea, which The finding that light absorption increases linearly have large air channels (den Hartog 1970). These with increasing chlorophyll density of Posidonia leaves deteriorated during transportation due to failure oceanica leaves resembles the similar linear scaling of the preserving agent to fully penetrate inside the between light absorption and chlorophyll content of air-filled channels. phytoplankton cells (Agusti 1991a). The efficiency of Light absorption was measured on intermediate chlorophyll a in absorbing light (i.e. the chlorophyll a- leaves (i.e. leaf number 2, > 50 d old for Posidonia specific light absorption) also decreased significantly oceanica), sufficiently old to have acquired the full (p < 0.001), as reported for phytoplankton (e.g. More1 pigmentation, but not old enough to support a signifi- & Bricaud 1981), with increasing pigment packaging cant epiphytic cover. Light absorption was measured within the leaves. This relationship, albeit weak at 675 nm, which corresponds to a peak in absorption (r = -0.36), indicates that pigments become increas- by chlorophyll a, by fitting leaf fragments mounted in ingly self-shaded as their packaging within the plant microscope slides into a Shimadzu UV-2100 dual- tissues increases. We found no significant relationship beam scanning spectrophotometer. To avoid scattering (p > 0.10) between light absorption and leaf thickness. artifacts leading to overestimation of light absorption, Although the light absorbed per unit chlorophyll a pro- we placed the leaves close to the end of the photo- duced appears to decrease as the internal chlorophyll a multiplier tube and both reference and measuring concentration increases, we found that, also similar to beam were diffused by an opal glass unit placed in phytoplankton (Agusti 1991a, b), the light absorbed front of the photomultiplier (Shibata 1959). In addition, per unit leaf biomass increases linearly with increasing we corrected our measurements to exclude non- pigment packaging within the plant tissue (Fig. 2) as pigment absorption and residual scattering by sub- described by the regression equation: tracting absorbance measurements at 750 nm from those at 675 nm (Duysens 1956). The percent light absorbed by pigments was calculated from measured optical densities (O.D.) using the equation (Clayton 1973):

% light absorbed = 100 (1 -

We referred the absorption measurements to the chlorophyll a density (i.e. chlorophyll a per absorption cross-section) of the leaves (Ramus 1990), which was measured spectrophotometrically on the same pieces used to measure light absorption, following pigment Chlorophylla density (pg mm -2 ) extraction with 80 % acetone in a homogenized sus- pension (Dennison 1990). We calculated the thickness Fig. 1. Posidonia oceanica. Relationship between light of the leaves examined by measuring their specific absorption (at 675 nm) and chlorophyll density of leaves Enriquez et al.: Light absorption by seagrass 203

Specific absorption (O.D. mg-' DW) = - 4.46 + 2.76 chl a (pg chl a mg-' DW)

R2 = 0.49; n = 80; SEslope= 0.32; F= 75.3, p < 0.001. %M This equation indicates that increasing chlorophyll a concentration, within the ranges measured here, results in increased light absorption per unit leaf weight. Be- cause the quantum yield (i.e.carbon fixed per unit light 4001 absorbed) is believed to be rather constant, increasing absorption per unit weight should increase photo- synthetic and growth rates of light-limited plants. Because most seagrass species have blade-like m 0 0-' leaves (den Hartog 1967), like those of Posidonia c 0 2 4 6 8 10 .m- oceanica, the results obtained are expected to hold VO) P. Chlorophyll a concentration(pg mg DW - 1) for most other species. This contention was sup- V1 ported by the observation that light absorption by leaves of 0th" seagrass sPecies1 collected in the ~i~,2, posidonia oceanjca, ~~l~t~~n~hi~between light Atlantic, Pacific, Caribbean, and Mediterranean Sea absorption per unit weight and chlorophyll concentration (Table l), followed patterns similar to those found for of leaves

Table 1 Light absorption (as optical density and percent light absorbed by pigments), chlorophyll a density, and thickness (mean f SE and range) for leaves of different seagrass species

Species Origin n O.D. (675 nm) Chl a Thickness (W mm-2) (mm)

Posidonia oceanica E. Spain

Cymodocea nodosa E. Spain, S. Portugal

Enhalus acoroides Philippines

Halodule uninervis Philippines

Halodule wrightii Mexican Caribbean Halophila ovalis Philippines

Ruppia maritima E. Spain, S. Portugal

Ruppia cirrhosa S. Portugal

Thalassia hemprichii Philippines

Thalassia testudinum Mexican Caribbean

Zostera manna S. Portugal

Zostera noltij E. Spain Portugal 204 Mar. Ecol. Prog. Ser.

thickness were broader, and absorption by the thick, cylindrical leaves of Synngodium sp. may deviate from the patterns described here.

Acknowledgements. This research was funded by a project funded by the Ramon Areces Foundation, funds allocated by the Spanish International Cooperation Agency, and project CI1'-CT91-0952 (HSMU) of the EC. We thank Just Cebrian and Nuria Marba; Luis Fonseca and Esther Serrao; Margarita Gallegos and Martin Merino; and Mike Fortes and Edna Garz6n for assistance while sampling in the Mediterranean, Atlantic, Caribbean, and the Philippines, respectively. This Chlorophyll a density (kg mm -' ) paper benefited from the comments of 2 anonymous reviewers. Fig. 3. Relationship between light absorption (at 675 nm) and chlorophyll density of leaves of different seagrass species. LITERATURE CITED Symbols represents the mean for the leaves of each species measured (Table 1) Agusti, S. (1991aJ.Allometric scaling of light absorption and scattering by phytoplankton cells Can. J. Fish. Aquat. Sci. 48: 763-767 P. oceariica. The fraction of light absorbed by leaves Agusti, S. (199lb). Light environment within dense algal pop- of different seagrasses ranged between 50 % for the ulation~:cell size influences on self-shading. J. Plankton small Caribbean seagrass Halodule wrightii and Res. 13: 863-871 Clayton, R. K. (1973). Light and living matter, Vol. 1, The 90 % for the large seagrass P. oceanica, differences physical part. McGraw-Hill, New York in average light absorption by pigments at 675 nm den Hartog, C. (1967). The structural aspect in the ecology of being closely related (R2 = 0.81) to the average sea-grass communities. Helgolander wiss. Meeresunters. chlorophyll density of the leaves of the 13 species 15: 648-659 Dennison, W. C. (1990). Chlorophyll content. In: Phillips, R. examined here (Fig. 3). C., McRoy, P. (eds.) Seagrass research methods, UNESCO, The relationships described here (Figs. 1 to 3) indi- Paris, p. 83-85 cate a close correspondence between light absorption Duarte, C. M. (1991). Seagrass depth limits. Aquat. Bot. 40: patterns and pigment content of seagrasses and phyto- 363-377 plankton (cf. Morel & Bricaud 1981, Agusti 1991a). The Duysens, L. M. N. (1956). The flattening effect of the absorption spectra of suspensions as compared to that of similarity between the light absorption patterns by solutions. Biochem. Biophys. Acta 19: 1-12 seagrass leaves described here and those previously Morel, A., Bricaud, A. (1981). Theoretical results concerning described for phytoplankton suggests that these pat- llght absorpt~onin a discrete medium, and application to terns reflect physical constraints to light absorption by specific absorption of phytoplankton. Deep Sea Res. 28: 1375-1.393 phototrophic organisms which should apply across all Ramus, J. (1978). Seaweed anatomy and photosynthetic aquatic plants. performance: the ecological significance of light guides, We found no effect of blade thickness on light absorp- heterogeneous absorption and multiple scatter. J. Phycol. tion by seagrasses, contrary to the findings for phyto- 14: 352-362 plankton (Morel & Bricaud 1981, Agusti 1991a, b) and Ramus, J. (1990). A form-function analysis of photon capture for seaweeds. In: Lindstrom, S. C., Gabrielsen, P. W (eds.) macroalgae (Ramus 1978,1990),where light absorption Thirteenth International Seaweed Symposium. Hydro- is also dependent on plant thickness. The reason for this biologia 204/205: 65-7 l may be that seagrass leaves are relatively similar in Shibata, K. (1959). Spectrophotornetry of translucence bio- thickness (Table l), while phytoplankton cells and logical materials - opal glass transmission method Meth. biochern. Analys~s7: 77-109 macroalgal thalh range over 100-fold in thickness. Thus, Stramski, D. (1990).Artifacts in measuring absorption spectra a similarly important effect of thickness on light absorp- of phytoplankton collected on a fllter Limnol. Oceanogr. tion by seagrasses would be expected if the range in leaf 35: 1804-1809

This note was presented by G. W. Thayer, Beaufort, Manuscript first received: December 18, 1991 N. Carolina, USA Revised version accepted: August 7, 1992