The feeding behavior of Spumella sp. as a function of particle size: Implications for bacterial size in pelagic systems

Direct observation was used to measure feeding rates of the flagellate Spumella on three sizes of bacteria plus 0.3 μm latex beads using video microscopy. Feeding rate was maximum on the intermediate-sized bacteria. Maximum ingestion rates (I_m) for the large- (0.53 μm³), intermediate- (0.08 μm³) and small-sized (0.02 μm³) bacteria and 0.014 μm³ latex beads were 11, 38 and 14 bacteria and 9 beads flagellate^-1 h^-1, respectively. The growth rates of Spumella sp. feeding on monoxenic cultures of the large- vs. the intermediate-sized bacteria were indistinguishable but Spumella sp. could not sustain its population density when feeding on the small bacterium as the sole food source. Our data are consistent with the hypothesis that Spumella sp., and possibly other flagellate protozoa, tend to feed selectively on larger prey. One consequence of this hypothesis is that differential grazing by bactivores may select for small bacteria in natural waters.