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Aquatic Microbial Ecology

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AME 22:43-56 (2000)  -  doi:10.3354/ame022043

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

Karel Simek1,2,*, Klaus Jürgens3, Jirí Nedoma1, Marta Comerma4, Joan Armengol4

1Hydrobiological Institute of the Academy of Sciences of the Czech Republic, and
2Faculty of Biological Sciences, University of South Bohemia, Na sádkách 7, 37005 Ceské Budejovice, Czech Republic
3Max Planck Institute of Limnology, PO Box 165, 24302 Plön, Germany
4Department of Ecology, University of Barcelona, Av. Diagonal 645, 08028 Barcelona, Spain

ABSTRACT: We conducted extensive studies on bacterivory and bacterial production over several seasons in 2 reservoirs: the meso-eutrophic Rímov 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 Rímov 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.

KEY WORDS: Halteria cf. grandinella · Oligotrichous ciliates · Feeding rates · Feeding ecology · Ciliate bacterivory · Reservoirs · Lakes

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