Effects of protozoan grazing within river biofilms under semi-natural conditions

Biofilms play an important role in the material flux of many aquatic ecosystems, but little is known about the mechanisms controlling their community structure under natural conditions. In the present study, we focused on the effects of ciliates on the quantity and taxonomic composition of heterotrophic flagellates (HF), and the effects of HF on the quantity and life forms (single cells vs. microcolonies) of bacteria in the early phase of biofilm colonization. For this purpose, we established semi-natural biofilms in flow cells connected to the river Rhine at Cologne, Germany. Using filter car- tridges, we size-fractionated the potamoplankton, which is the source of the biofilm community, thus establishing biofilms containing (1) only bacteria (1.2 µm filter), (2) HF and bacteria (8 or 5 µm filter), or (3) ciliates, HF and bacteria (20 µm filter). The presence of ciliates negatively influenced the abun- dance of biofilm-dwelling HF and selectively altered the taxonomic composition of the HF commu- nity. The presence of HF resulted in a significant reduction in the abundance of single bacterial cells, but enhanced the abundance of bacterial microcolonies. Furthermore, the presence of ciliates stimu- lated the abundance of single-cell bacteria (probably due to an HF-mediated trophic cascade), but had no effect on bacterial microcolonies. Taken together, the results of this study show the impor- tance of protozoan grazing in shaping the species composition and morphology of early river biofilms under semi-natural conditions.