Consumption of picoplankton‐size particles by marine ciliates: Effects of physiological state of the ciliate and particle quality

Ingestion of picoplankton‐size particles was studied in two marine ciliates, a typical grazer of pica‐ and nanoplankton, the oligotrich Strombidium sulcatum , and a bacteriovorous scuticociliate, Uronema sp. In laboratory experiments, both logarithmic‐ (food‐unlimited) and stationary‐phase (food‐limited) populations were presented with particles of different sizes and surface properties: plain microspheres, protein‐adsorbing carboxylate microspheres, fluorescently labelled heterotrophic bacteria, and cyanobacteria. In both log‐ and stationary‐phase populations, Strombidium clearance rates varied linearly as function of prey size based on the ingestion of plain microspheres. In contrast, Uronema clearance rates were invariant with particle size for log‐phase populations but increased with particle size in stationary‐phase cells. Discrimination among prey with different surface properties was also dissimilar in the two ciliates. For Strombidium , log‐phase populations cleared 1‐µm‐diameter plain microspheres at higher rates than those with carboxylate surfaces. The pattern was repeated by stationary‐phase cells for both 0.5 and 1‐µm‐diameter microspheres. In Uronema , there was no significant difference in clearance rates with surface property in log‐phase populations, similar to the findings with regard to particle size. Only in stationary‐phase populations of Uronema was some selection evident, as 0.97‐µm carboxylate microspheres were cleared at rates higher than were 0.95 µm plain microspheres. Overall, selection was higher in stationary‐phase ciliates, and Strombidium seemed to feed more size‐selectively among picoplankton particles than did Uronema .