Light-aided digestion, grazing and growth in herbivorous protists

The effect of Light on digestion, grazing and growth rates of herbivorous p rotists was studied in a series of laboratory experiments. Relative to comp lete darkness, bright light (900 pmol photons m(-2) s(-1)) resulted in a 40 -fold increase in food vacuole loss rates (a proxy for digestion) in the he terotrophic dinoflagellate Noctiluca scintillans when fed phytoplankton pre y. However, light had no effect on vacuole loss rate when N, scintillans wa s fed heterotrophic (non-pigmented) prey. Ingestion rates of 2 ciliate spec ies feeding on phytoplankton were enhanced by factors of 2 to 7 in moderate light relative to darkness, and one of the ciliates (the tintinnid Coxliel la sp.) exhibited large light-dependent increases in population growth rate . At very low prey concentrations, the presence of light allowed modest gro wth (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 v s 0.02 d(-1)) in light versus dark treatments. Light-aided growth at both l ow (subsaturating) and high (saturating) prey concentrations indicates that light affected both the extent of digestive breakdown and the rate of dige stive throughput. A possible mechanism for the observed Light enhancement i s photooxidative breakdown of ingested organic matter in these nearly trans parent grazers. Photooxidation may be sensitized by chlorophylls and phaeop igments in ingested cells, and should be favored by the oxygen- and lipid-r ich environment of phytoplankton chloroplasts. Light-aided digestion, inges tion and growth of protist grazers has important ecological implications, i ncluding the possible systematic underestimation of rates of protist herbiv ory and growth in laboratory experiments, which are typically conducted in dim light or darkness. In aquatic ecosystems, the spatial and temporal coup ling of phytoplankton production and grazing losses by a single abiotic res ource -light-should lead to reduced temporal variation in oceanic phytoplan kton biomass, and may also influence prey selectivity by protists, the ocea n's dominant herbivorous grazers.