Ciliate grazing on <Emphasis Type="Italic">Nitrosomonas europaea</Emphasis> and <Emphasis Type="Italic">Nitrospira</Emphasis><Emphasis Type="Italic">moscoviensis</Emphasis>: Is selectivity a factor for the nitrogen cycle in natural aquatic systems? |
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Authors: | Elke Neubacher Mario Prast Ernst-Josef Cleven Ulrike-Gabriele Berninger |
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Institution: | (1) Department of Organismic Biology, University of Salzburg, Hellbrunnerstrasse 34, 5020 Salzburg, Austria;(2) Department of Limnology, University of Münster, Munster, Germany |
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Abstract: | Ciliated protists are important predators of bacteria in many aquatic habitats, including sediments. Since, many biochemical
transformations within the nitrogen cycle are performed by bacteria, ciliates could have an indirect impact on the nitrogen
cycle through selective grazing on nitrogen-transforming bacteria. As a case study, we examined ciliate grazing on nitrifying
bacteria of the genera Nitrosomonas and Nitrospira. All experiments were designed as in vitro-experiments with cultures of different bacteria and ciliate species. The nitrifying
bacteria used in our experiments were Nitrosomonas europaea Winogradsky 1892] and Nitrospira moscoviensis Ehrich 2001]. The ciliates comprised of four species that are known as efficient bacterivores and common members of the
protist community in aquatic systems: Paramecium aurelia Müller 1773], Euplotes
octocarinatus Carter 1972], Tetrahymena pyriformis Ehrenberg 1830] and Cyclidium glaucoma Müller 1786]. Our experimental approach, using a combination of DAPI and FISH staining, was successful in allowing the observation
of ingestion of specific bacteria and their detection within ciliate food vacuoles. However, the ciliates in this study showed
no significant selective grazing. No food preferences for a any bacterial taxon or any size class or morphotype were detected.
Correlation with time between ciliate abundance and bacterial abundance or biovolume, using log transformed growth rates of
ciliates and bacteria, showed no significant results. On the bacterial side, neither an active defence mechanism of the nitrifying
bacteria against ciliate grazing, such as changes in morphology, nor competition for resources were observed. These results
suggest that in our in vitro-experiments grazing by ciliates has no influence on abundance and growth of nitrifying bacteria
and nitrification. |
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Keywords: | Nitrifying bacteria Nitrogen cycle Bacterivorous ciliates Selective grazing Fluorescence in situ hybridization (FISH) |
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