Ecological role and bacterial grazing of Halteria spp.:: small freshwater oligotrichs as dominant pelagic ciliate bacterivores

We conducted extensive studies on bacterivory and bacterial production over several seasons in 2 reservoirs: the meso-eutrophic Rimov Reservoir in the Czech Republic and the highly eutrophic Sau Reservoir in Spain. Based on abundance, seasonal dynamics, and cell-specific uptake rates of different ciliate taxa, as well as heterotrophic nanoflagellate bacterivory, we were able to quantify bacterivory by individual ciliate species, total ciliates, and aggregated protists in these systems. With increasing trophic status, a higher portion of bacterial production was consumed by protists, and there was a greater importance of ciliate grazing, accounting for 40 and 50 % of the total protistan bacterivory in the epilimnion of the Rimov and Sau reservoirs, respectively. Increases were attributable to the oligotrichs of the genus Halteria that often numerically dominate freshwater pelagic ciliate communities. In both reservoirs, the most important ciliate bacterivores in order of importance were: oligotrichs, primarily the bacterivorous Halteria spp., peritrichs, and scuticociliates. We also examined food vacuole content in natural populations of Halteria;a spp. to estimate the proportion of cells that had ingested algae. Our results and a review of previous reports on the abundance of Halteria spp. suggest that small halteriids are ecologically important bacterial consumers in meso- to eutrophic freshwater systems due to: (1) efficient uptake of prey over a large size spectrum (approximately 0.4 to 5 pm), (2) high clearance rates on picoplankton-sized particles along with (3) high potential growth rate, and (4) lower vulnerability to metazooplankton predation compared to other common pelagic ciliates. Correspondingly, we suggest a revised concept of planktonic ciliate bacterivory, where the principal role is attributed to small omnivorous filter-feeding oligotrichous ciliates.