MEASUREMENT AND SIMULATION OF PREY UPTAKE BY MARINE PLANKTONIC CILIATES FED PLASTIDIC AND APLASTIDIC NANOPLANKTON

A laboratory study was conducted to investigate the food preferences of two marine planktonic ciliates, especially the importance of aplastidic nanoplankton in their diets, and the influence of prey quantity and composition on prey uptake. Ingestion, growth, and conversion efficiencies of these ciliates were measured while they fed on single- and multiprey assemblages of aplastidic and photosynthetic (plastidic) nanoplankton. Both tintinnid and "naked" oligotrich ciliates ingested and grew on diets of aplastidic nanoplankton. In single-prey experiments with plastidic and aplastidic nanoplankton of similar size and C:N, ingestion of and growth on aplastidic nanoplankton could exceed those of ciliates fed phytoplankton. The highest growth rates, however, especially in tintinnid ciliates, generally occurred when they were fed nutritious phytoplankton. On multiprey diets, patterns of prey uptake depended on the total spectrum of available food. When ciliates were fed various concentrations of nutritionally acceptable plastidic and aplastidic nanoplankton, each prey was ingested according to relative abundance. However, when offered acceptable prey in combination with apparently nutritionally inadequate or less preferred prey, ciliates showed differential uptake of prey. The experimental data were fit to a feeding model derived from theory describing simple linear competitive inhibition of enzyme-catalyzed reactions. This was used in conjunction with an encounter model as a diagnostic tool to evaluate the possibility that the measured responses could be due to differences in encounter, capture, handling, and digestion among prey. Results indicated that encounter and capture rates and handling times did not differ sufficiently to account for observed patterns in prey uptake. Rather, "poor" prey apparently are ingested at lower rates either because they are actively rejected or digested at lower rates or with reduced efficiency.