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

Abstract

ABSTRACT:We conducted extensive studies on bacterivory and bacterial production over several seasons in 2 reservoirs: the meso-eutrophic Ř ímov 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 Ř ímov 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 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 µm), (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.

Changes in the Epilimnetic Bacterial Community Composition, Production, and Protist-Induced Mortality along the Longitudinal Axis of a Highly Eutrophic Reservoir

“…To measure protozoan grazing upon bacterioplankton, we used fluorescently labeled bacterioplankton (FLB; for more details see Šimek et al [37]). For grazing experiments, 250-500 ml of replicated samples were dispensed into acid soaked and rinsed 1-liter flasks and incubated at in situ temperature.…”

“…Thirtymilliliter subsamples for protozoan enumeration and tracer ingestion determinations were taken at 0, 5, 10, 15, 20, and 30 min after tracer addition and fixed by adding 0.5% of alkaline Lugol's solution, immediately followed by 2% borate-buffered formaldehyde (final concentration) and several drops of 3% sodium thiosulfate to clear the Lugol's color [34]. We determined ciliate grazing rates in time series from 5-15 min subsamples and flagellate grazing rates in subsamples from 10-30 min, respectively (for details see Šimek et al [37] and Comerma et al [10]). Samples from zero time were also inspected to avoid a potential bias of our data due to attachment of noningested tracers on protozoan surfaces.…”

“…In April, ciliates showed a conspicuous abundance peak at station 4 (144 cells ml −1 ), and two peaks in July, at stations 7 and 4 (89 and 87 cells ml −1 ). The above-described succession resulted also in a different longitudinal pattern of protistan bacte- Total heterotrophic nanoflagellate (HNF) and ciliate abundance, bacterial production and total protistan bacterivory (the details on cell-specific and community grazing rate and composition of both protistan groups in April and July have been discussed elsewhere [10,37]). Note that there are different scales for the data from April and July.…”

“…A typical longitudinal succession of planktonic foods web structures in the Sau Reservoir, i.e., dominated by bacteria, heterotrophic nanoflagellates, ciliates, and rotifers, has been recently documented [10]. Moreover, detailed studies on protistan succession and bacterivory along the longitudinal axis of the reservoir showed distinct peaks of heterotrophic nanoflagellates and ciliate abundance, and consequently spatially changing roles of these protistan groups as bacterivores [10,37].…”

“…Canyon-shaped reservoirs represent highly specific, remarkably spatially heterogeneous systems. Selected aspects of them, i.e., hydrodynamics, chemical processes, and zoo-, phytoplankton, and protistan dynamics, have been already reported [10,18,33,35,37]. However, the dominating biogeochemical processes occurring in the upper parts of canyon-shaped reservoirs, mediated mainly by microbial activities and interactions [43], are not yet fully understood.…”