Light-aided digestion, grazing and growth in herbivorous protists

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-2s-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. Ingestion rates of 2 ciliate species feeding on phytoplankton were enhanced by factors of 2 to 7 in moderate 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 treatments. Light-aided growth at both low (subsaturating) and high (saturating) prey concentrations indicates that light a ffected 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 environment 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.