Differential feeding by marine flagellates on growing versus starving, and on motile versus nonmotile, bacterial prey

Previous work has demonstrated selective feeding by bacterivorous protists on the basis of prey size. Other prey characteristics may also affect protist grazing rates. In this study, we investigated relative uptake rates of fluorescently labeled bacterial (FLB) cells made from growing (G-FLB) vs starving bacteria (S-FLB), and of live-stained motile vs non-motile bacteria by marine phagotrophic flagellates. We also determined relative growth rates and digestion times for flagellates fed growing or starved bacteria. A natural assemblage of marine flagellates and a flagellate isolate, Cafeteria sp., showed lower (p < 0.001) clearance rates on S-FLB than on G-FLB, which could be ascribed to sizeselective grazing by the flagellates. Moreover, flagellates showed longer (p < 0.001) prey digestion tmes when feedng on S-FLB than on G-FLB. The carbon-based gross growth efficiency of flagellates fed starved bacteria was nearly twice that of flagellates fed growing bacteria: 38.5% and 21.5%, respectively. Uptake rates by flagellates of several live-stained bacteria (LSB) of either motile or nonmotlle strains were compared with uptake rates of heat-killed FLB made from the same bacterial cultures. Clearance rates were significantly higher for LSB compared to FLB only for motile strains; no differences in uptake rates were observed for non-motile bacteria. Although FLB may be a good tracer for non-motile bacteria, use of FLB may lead to underestimation of actual grazing rates if motile bacteria comprise a significant fraction of the total bacterial assemblage. Our results suggest (1) grazing on starved bacteria results in a more efficient transfer of biomass to higher trophic levels than does grazing on growing bacteria; (2) use of motile LSB in prey uptake experiments should yield higher estimates of bacterivory compared to rates estimated using heat-killed or non-motile labeled prey; and (3) quahtative factors affecting predator-prey dynamics may be important in regulating elemental fluxes in microbial food webs.