Effects of predation pressure on bacterial abundance, diversity, and size-structure distribution in an oligotrophic system

We designed microcosm experiments to study the response of bacterial communities to altered predation pressure, in phosphorus-limited conditions. Different-sized predators were removed through filtration, yielding the following treatments: bacteria only (no predation, NP; <1 μm filtrate); small-sized predators (10P, <10μm filtrate) and small- to medium-sized predators present (50P, <50 μm filtrate). Natural control (NC) included predators of all sizes. Thus we compared the relative impact of differential predation on abundance, biovolume, community composition and size-structure distribution of bacterial assemblages subject to grazing by different-sized predators. The relative diversity of microbial communities was estimated by a fingerprinting based approach for both prey and predators. The results showed that the presence of grazers preserved the Shannon diversity of the bacterial community and shifted the size-structure distribution towards grazing- resistant forms. Absence of predation promoted competition for resources and resulted in a constant reduction of the relative diversity of the bacterial community. The change in the size-structure distribution of the bacterial communities in the treatments was accompanied by alterations in the relative operational taxonomic unit (OTU) composition of the eukaryotic and bacterial communities. Bacterial OTUs grouped in two distinct fractions linked to their size-structure distribution, in dependence to the presence of the predators: Small and Edible cells were favoured by low grazing pressure whereas Filaments and Aggregates were stimulated by predator presence. Eukaryotic OTUs successful at high grazing activity resulted as rather different than OTUs successful at lower degree of grazing. Under high protistan grazing pressure, there was a clear shift in bacterial community composition regarding both size-structure distribution and genotypes. Nevertheless, diversity was preserved. The opposite situation characterized the predator-free bacterial communities; a clear and constant reduction of the community diversity was indicated, confirming that moderate top-down control is fundamental to the shaping and preservation of natural bacterial communities, even in oligotrophic systems.