Light-Aided Digestion, Grazing and Growth in Herbivorous Protists
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AQUATIC MICROBIAL ECOLOGY Vol. 23: 253–261, 2001 Published February 28 Aquat Microb Ecol Light-aided digestion, grazing and growth in herbivorous protists Suzanne L. Strom* Shannon Point Marine Center, Western Washington University, 1900 Shannon Point Rd., Anacortes, Washington 98221, USA ABSTRACT: The effect of light on digestion, grazing and growth rates of herbivorous protists was studied in a series of laboratory experiments. Relative to complete darkness, bright light (900 µmol photons m–2 s–1) resulted in a 40-fold increase in food vacuole loss rates (a proxy for digestion) in the heterotrophic dinoflagellate Noctiluca scintillans when fed phytoplankton prey. However, light had no effect on vacuole loss rate when N. scintillans was fed heterotrophic (non-pigmented) prey. Inges- tion rates of 2 ciliate species feeding on phytoplankton were enhanced by factors of 2 to 7 in moder- ate light relative to darkness, and one of the ciliates (the tintinnid Coxliella sp.) exhibited large light- dependent increases in population growth rate. At very low prey concentrations, the presence of light allowed modest growth (0.12 d–1) in populations that otherwise died rapidly (–0.46 d–1); at high prey concentrations, growth rate was nearly 20 times higher (0.36 vs 0.02 d–1) in light versus dark treat- ments. Light-aided growth at both low (subsaturating) and high (saturating) prey concentrations indi- cates that light affected both the extent of digestive breakdown and the rate of digestive throughput. A possible mechanism for the observed light enhancement is photooxidative breakdown of ingested organic matter in these nearly transparent grazers. Photooxidation may be sensitized by chlorophylls and phaeopigments in ingested cells, and should be favored by the oxygen- and lipid-rich environ- ment of phytoplankton chloroplasts. Light-aided digestion, ingestion and growth of protist grazers has important ecological implications, including the possible systematic underestimation of rates of protist herbivory and growth in laboratory experiments, which are typically conducted in dim light or darkness. In aquatic ecosystems, the spatial and temporal coupling of phytoplankton production and grazing losses by a single abiotic resource — light — should lead to reduced temporal variation in oceanic phytoplankton biomass, and may also influence prey selectivity by protists, the ocean’s dominant herbivorous grazers. KEY WORDS: Grazing · Growth · Digestion · Ciliate · Dinoflagellate · Herbivory · Light · Active oxygen Resale or republication not permitted without written consent of the publisher INTRODUCTION 1995). The variation of light with time of day, latitude, and season drives profound variation in primary The role of light in regulating phytoplankton photo- production over these temporal scales (Banse 1992, synthesis is probably one of the most intensively stud- Longhurst 1998). Light, along with nutrient supply, is ied aspects of biological oceanography. The degree of thought to constitute the principal ‘bottom-up’ control light penetration into ocean waters and its spectral on phytoplankton biomass and growth rate in the sea. composition at depth define the zone in which photo- Over the last 15 yr or so, protist grazers have been synthesis can occur (Kirk 1994) and establish a depth shown to consume a large fraction of primary produc- gradient of habitat types for different phytoplankton tion in both coastal and oceanic regions (reviewed by species (e.g. Campbell & Vaulot 1993, Moore et al. Lynn & Montagnes 1991, Sherr & Sherr 1994). Protist grazing is generally believed to be the major sink for phytoplankton production in open ocean surface *E-mail: [email protected] waters (Banse 1992). In conjunction with their impor- © Inter-Research 2001 254 Aquat Microb Ecol 23: 253–261, 2001 tance as grazers, the dissolved waste products of graz- gellate Prorocentrum micans (pigmented, UTEX1993) ing protists are the dominant source of recycled nutri- or the tintinnid Coxliella sp. (non-pigmented). For the ents in ocean surface waters (Johannes 1965, Caron & P. micans experiment, N. scintillans was fed P. micans Goldman 1990), while protist biomass in turn provides exclusively for 5 d, then concentrated using reverse fil- food for organisms at higher trophic levels (Stoecker & tration (100 µm mesh), resuspended, and reconcen- Capuzzo 1990, Gifford & Dagg 1991, Fessenden & trated to remove nearly all prey from the medium. For Cowles 1994). While considerably less well studied the Coxliella sp. experiment, N. scintillans was starved than ‘bottom-up’ processes, protist herbivory appears for 5 d, then fed the tintinnid Coxliella sp. for 2 d. Prey to be the primary ‘top-down’ control on phytoplankton was then removed from the N. scintillans culture as biomass and growth rate in the sea. above. Coxliella sp. in turn was reared on a mixture of Aside from behavioral phenomena such as diel ver- microalgae, then starved for 1 d (prey removed by tical migration and phototaxis, light is thought to affect sieving) before being added to the N. scintillans cul- rates of zooplankton activity only indirectly, through ture. Epifluorescence microscopy showed few or no the abundance and physiological condition of phyto- algal cells inside individual Coxliella sp. after 1 d with- plankton prey. Long-term study of planktonic protist out food. grazers and their interaction with phytoplankton For each experiment, Noctiluca scintillans (1 to 2 pigments suggested that light might interact with cells ml–1), preconditioned as above, were distributed the digestion of pigmented prey in these consumers, into 300 ml polycarbonate bottles filled with ciliate many of which are transparent to visible light. Such medium. Triplicate bottles were prepared for each of an interaction was intimated in the work of Klein et 3 light treatments (9 per experiment). All bottles were al. (1986), who found that light enhanced the degra- held in the same water bath (temperature 14.5°C) and dation of alloxanthin and chlorophyll c by the protist illuminated continuously from above and below for 2 d. grazer Oxyrrhis marina. The goal of this study was Irradiance levels in the bottles were 0, 450, and to determine whether light exerts a direct effect on 900 µmol photons m–2 s–1. Subsamples were removed rates of digestion, feeding and growth in heterotrophic at the beginning and end of the incubation period and protists. preserved with acid Lugol's (final conc. 1.5%). Food vacuole dimensions (length and width) were measured using a video camera mounted on an inverted micro- METHODS AND MATERIALS scope, and Bioscan Optimas image analysis software. Food vacuole volumes were calculated assuming each Protist culture. Heterotrophic protists were isolated vacuole was a prolate spheroid; there were typically from local waters (northern Puget Sound, Washington, 4 to 6 vacuoles per cell (range 2 to 32). All food vac- USA) and grown in ‘ciliate medium’ (0.2 µm filtered, uoles in the first 30 N. scintillans cells encountered autoclaved seawater with dilute trace metal addition) were measured for each sample, and an average total according to Gifford (1985). Cultures were kept in dim vacuole volume per individual was then calculated for light (<10 µmol photons m–2 s–1) on a 14:10 h light:dark the sample. cycle at either 12 or 15°C, and fed mixed diets of phyto- Food vacuole loss rates for each bottle were esti- plankton (Strom & Morello 1998). None of the protist mated from the change in average vacuole volume per isolates were mixotrophic in the sense of possessing individual between the initial and final (2 d) sampling their own chloroplasts or retaining chloroplasts from times. The rate of decrease in food vacuole volume was ingested prey. Phytoplankton were obtained from cul- considered a measure of digestion rate. Vacuole loss ture collections and grown in f/2 without added Si; rates do not account for qualitative changes in ingested they were maintained at 15°C on a 14:10 h light:dark prey, which were also observed to vary among treat- cycle (irradiance range 70 to 100 µmol photons m–2 s–1). ments, but do provide a measure of throughput rate, Light for experiments was provided by ‘cool white’ flu- analogous to a gut passage time for a metazoan grazer. orescent bulbs, and all light intensities were measured Grazing experiments. In separate experiments, the with a Biospherical Instruments QSL 100 4π PAR sen- effect of light on the ingestion rate of 2 ciliate species sor. In general, grazer species for experiments were (Coxliella sp. and Strombidinopsis acuminatum) was chosen to represent common coastal heterotrophic measured. Neither of the ciliates retains chloroplasts. protist species that are transparent and feed on phyto- Both experiments were conducted at 13°C, and bottles plankton. Phytoplankton prey were chosen to repre- were incubated on a bottle roller (3 to 4 rpm) to ensure sent a range of taxa and thus pigment composition. that non-motile fluorescently labeled algae (FLA) Digestion experiments. In separate experiments, the remained in suspension. For the tintinnid Coxliella sp., heterotrophic dinoflagellate Noctiluca scintillans (non- a stock culture was sieved to remove remnant mainte- bioluminescent strain) was fed the autotrophic dinofla- nance prey cells and individuals were resuspended in Strom: Effect of light on herbivorous protists 255 ciliate medium (100 ml in triplicate polycarbonate To initiate the experiment, Coxliella sp. cells were re- bottles) for an experimental concentration of 5 to 6 cil- moved from residual maintenance prey by sieving; cili- iates ml–1. The autotrophic dinoflagellate Gymnodin- ates were resuspended into fresh sterile filtered seawa- ium simplex (UBC119a) was then added to a concen- ter (300 ml per bottle) in 12 polycarbonate bottles (3 per tration of 560 cells ml–1. After 3 h acclimation to treatment). Emiliania huxleyi cells (concentration de- experimental irradiance levels (0 and 100 µmol pho- termined by haemocytometer count at the beginning of tons m–2 s–1), fluorescently labeled (DTAF) G. simplex each light period) were added at high or low densities were added at a tracer level of 79 cells ml–1 (14% of as described above.