Predation-mediated shifts in size distribution of microbial biomass and activity during detritus decomposition

Many experimental studies on detritus decomposition revealed a comparable m icrobial succession after the addition of a substrate pulse: from small. fr eely suspended single bacteria at the beginning, to more complex and larger growth forms during a later stage, accompanied by the appearance of bacter ivorous protists. We examined in three model experiments with different org anic carbon sources whether this shift in bacterial size structure is linke d to the grazing impact of bacterivores. In short-term (8-10 d) microcosm e xperiments we added natural dissolved and particulate detritus (macrophyte leaves and leachate, dead phytoplankton cells) as an organic substrate sour ce. By the use of size-fractionated inocula and eucaryotic inhibitors we ob tained treatments without protists, in which bacteria developed without pre dation. These were compared, by measurements of bacterial activity and micr oscopical analysis of bacterial size structure, to incubations in which eit her 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. Th e selective removal of freely suspended bacteria produced a very different relative composition of bacterial biomass: it became dominated bq large, gr azing-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 t ime course of the bacterial development suggested different underlying mech anisms for the appearance of predation resistant filamentous and of aggrega ted or attached bacteria. As bacterial aggregates developed in approximatel y similar amounts with and without grazers no specific growth stimulation b y protists could be detected. In contrast, concentrations of filamentous ba cteria were 2-10 times higher in treatments with protists, thus indicating a stimulation of this growth form during enhanced grazing pressure. Measure ments of ectoenzymatic activity and H-3-leucine uptake indicated that micro bial 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 poss ibly the function of aquatic bacteria and their position in the food web. m aking protist-resistant bacteria more vulnerable to metazoan filter feeders and detritivores, acid possibly also subject to sedimentation.