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In order to examine the role of bacterivorous protists during degradation and conversion of dissolved organic carbon (DOC), we performed a batch enrichment experiment with freshwater bacterioplankton and macrophyte leachate as a natural DOC source. Bacterial growth, activity and morphological structure, as well as the decrease in DOC concentration were compared in treatments, which consisted of bacteria only, and treatments that also contained bacterivorous protists (either cultured heterotrophic nanoflagellates or a natural protist community). Development of heterotrophic nanoflagellates (HNF) caused a reduction in bacterial biovolume to 12 ?18 % compared to the protist-free controls. Overall DOC decomposition rates were similar between the treatments, and around 60 % of the initial DOC concentration was utilized within one week. A major difference occurred, however, in the size-activity structure of the bacterial community. With grazers, the contribution of filamentous bacteria and bacterial aggregates to total bacterial biovolume increased strongly, and a major part of bacterial activity was concentrated in the size fraction > 10µm. Without grazers, > 90 % of the activity was found in the size fraction< 10µm. The bacterial communities in grazing treatments maintained a higher specific activity, as was evident from respiration and ectoenzymatic activities. The results of our model system suggest that protist grazing and resulting organic matter recycling simultaneously maintain a highly active bacterial community during detritus decomposition and trigger an increase in microbial size fractions.