Grazing of protozoa and its effect on populations of aquatic bacteria

Abstract

Predation by bacterivorous protists in aquatic habitats can influence the morphological structure, taxonomic composition and physiological status of bacterial communities. The protistan grazing can result in bacterial responses at the community and the species level. At the community level, grazing-induced morphological shifts have been observed, which were directed towards either larger or smaller bacterial sizes or in both directions. Morphological changes have been accompanied by changes in taxonomic community structure and bacterial activity. Responses at the species level vary from species to species. Some taxa have shown a pronounced morphological plasticity and demonstrated complete or partial shifts in size distribution to larger growth forms (filaments, microcolonies). However, other taxa with weak plasticity have shown no ability to reduce grazing mortality through changes in size. The impact of protistan grazing on bacterial communities is based on the complex interplay of several parameters. These include grazing selectivity (by size and other features), differences in sensitivity of bacterial species to grazing, differences in responses of single bacterial populations to grazing (size and physiology), as well as the direct and indirect influence of grazing on bacterial growth conditions (substrate supply) and bacterial competition (elimination of competitors).

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

“…However, depending on which are the dominant protistan bacterivores in a given freshwater system, bacteria in a certain size range are thought to be protected against HNF grazing (usually between 3 and 5 µm long when small bacterivorous HNF dominate [16,19]). Large filaments, other large complex growth forms, bacterial aggregates, and particle-bound bacteria are thus largely resistant to HNF predation [16]. However, these bacterial forms were not likely resistant to the predation by the dominating groups of ciliates (mainly halteriids and vorticellids) and rotifers (400-600 individuals L −1 , bdelloids and Brachionus spp.)…”

Marine bacterial community structure resilience to changes in protist predation under phytoplankton bloom conditions

“…The physiological state of bacterial cells has also been suggested as a key factor in grazing selectivity (Del Giorgio and Gasol, 2008), and preferential grazing of the more active cells within a community by protist grazers has been repeatedly observed (Del Giorgio et al, 1996;Pernthaler et al, 1997;Simek et al, 1997;Tadonléké et al, 2005;Sintes and Del Giorgio, 2014). This is probably related to the general positive relation between cell size and activity in marine bacteria (Gasol et al, 1995;Hahn and Höfle, 2001;Matz and Jürgens, 2001;Matz et al, 2002;Corno and Jürgens, 2006), suggesting that larger bacterioplankton cells are also usually the most active ones. Moreover, there could be a concentration-dependent component, where grazing might be different on abundant versus non-abundant populations .…”

Protists have divergent effects on bacterial diversity along a productivity gradient

“…Bacterial communities undergo morphological and physiological changes when subjected to protist predators by forming predation-resistant filaments (Hahn et al 2000;Jurgens & Sala 2000;Hahn & Hofle 2001) or by increasing cell size (Corno & Jurgens 2006), but there has been less experimental effort focused on the ecological effects of protist predators on bacterial community diversity. Studies have tended to either involve intraspecific morphological diversity (Meyer & Kassen 2007;Friman et al 2008) simplified, constructed bacterial communities (Jiang & Morin 2005;Jiang & Krumins 2006;Meyer & Kassen 2007;Friman et al 2008), or have been observational studies of natural environments (Hahn & Hofle 2001).…”