K. Simek et al., Ecological role and bacterial grazing of Halteria spp.: small freshwater oligotrichs as dominant pelagic ciliate bacterivores, AQUAT MIC E, 22(1), 2000, pp. 43-56
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 a
bundance, seasonal dynamics, and cell-specific uptake rates of different ci
liate taxa, as well as heterotrophic nanoflagellate bacterivory, we were ab
le to quantify bacterivory by individual ciliate species, total ciliates, a
nd 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 rese
rvoirs, 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 Halteri
a spp., peritrichs, and scuticociliates. We also examined food vacuole cont
ent in natural populations of Halteria;a spp. to estimate the proportion of
cells that had ingested algae. Our results and a review of previous report
s on the abundance of Halteria spp. suggest that small halteriids are ecolo
gically important bacterial consumers in meso- to eutrophic freshwater syst
ems due to: (1) efficient uptake of prey over a large size spectrum (approx
imately 0.4 to 5 pm), (2) high clearance rates on picoplankton-sized partic
les along with (3) high potential growth rate, and (4) lower vulnerability
to metazooplankton predation compared to other common pelagic ciliates. Cor
respondingly, we suggest a revised concept of planktonic ciliate bacterivor
y, where the principal role is attributed to small omnivorous filter-feedin
g oligotrichous ciliates.