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Many experimental studies on detritus decomposition revealed a comparable microbial succession after the addition of a substrate pulse: from small. freely suspended single bacteria at the beginning, to more complex and larger growth forms during a later stage, accompanied by the appearance of bacterivorous protists. We examined in three model experiments with different organic carbon sources whether this shift in bacterial size structure is linked to the grazing impact of bacterivores. In short-term (8-10 d) microcosm experiments we added natural dissolved and particulate detritus (macrophyte leaves and leachate, dead phytoplankton cells) as an organic substrate source. By the use of size-fractionated inocula and eucaryotic inhibitors we obtained treatments without protists, in which bacteria developed without predation. These were compared, by measurements of bacterial activity and microscopical analysis of bacterial size structure, to incubations in which either cultured heterotrophic nanoflagellates or a natural protist assemblage was included in the inoculum. The presence of bacterial grazers resulted in a 50-90% reduction of bacterial biomass compared to grazer-free trials. The selective removal of freely suspended bacteria produced a very different relative composition of bacterial biomass: it became dominated bq large, grazing-resistant forms such as filaments and cells attached to particles or clustered in small aggregates. In grazer-free treatments, bacterial biomass was always dominated (> 80%) by free-living, single bacterial cells. The time course of the bacterial development suggested different underlying mechanisms for the appearance of predation resistant filamentous and of aggregated or attached bacteria. As bacterial aggregates developed in approximately similar amounts with and without grazers no specific growth stimulation by protists could be detected. In contrast, concentrations of filamentous bacteria were 2-10 times higher in treatments with protists, thus indicating a stimulation of this growth form during enhanced grazing pressure. Measurements of ectoenzymatic activity and H-3-leucine uptake indicated that microbial activity was also shifted to larger size fractions. In most cases more than 50% of bacterial activity in treatments with protists was associated with the size fraction > 10 mum whereas this value was < 2% without grazers. Grazing by protists also enhanced the specific activity of the bacterial assemblage which is in contrast to an assumed lower competitive ability of complex bacterial growth forms. The results imply that the selective force of bacterivory in nutrient-rich environments changes the structure and possibly the function of aquatic bacteria and their position in the food web. making protist-resistant bacteria more vulnerable to metazoan filter feeders and detritivores, acid possibly also subject to sedimentation
