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We studied the effects of protist predation on the composition and morphological struc- ture of a marine bacterioplankton assemblage. A mixed bacterial assemblage (0.6 µm filtered North Sea inoculum) was cultivated with unamended seawater as medium in four 1-stage chemostats. After the establishment of a bacterial community, 2 of the chemostats received a 2 µm filtered seawater inoculum which enabled the development of heterotrophic nanoflagellates (HNF). The dynamics of bacterial and protist community composition in the chemostat cultures was followed by terminal- restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis (DGGE). These fingerprinting techniques revealed that continuous cultivation with unsupplemented seawater altered the bacterial community composition compared to the inoculum. Fluorescent in situ hybridisation (FISH) analysis showed that the bacterial assemblages in the chemostats were domi- nated by γ-Proteobacteria, particularly of the genus Alteromonas. Despite this shift, the overall rich- ness of bacteria in the chemostats remained relatively high with up to 30 different operational taxo- nomic units (OTUs). HNF exerted efficient grazing control on the bacterial communities during 3 wk of cultivation and reduced bacterial biomass to approximately 10% of that in the predator-free chemostats. Only a modest development of grazing-resistant bacteria occurred, which contrasts with previously published chemostat experiments with freshwater microbial communities. T-RFLP and DGGE analyses also revealed that protist grazing did not result in significant changes in bacterial community composition. However, both fingerprinting techniques showed a relatively high richness (up to 15 different OTUs) and an ongoing succession within the protist assemblage, which might be one explanation for only low feed-back effects and low appearance of grazing-resistance in the predation-controlled bacterial community.

Composition and population dynamics of planktonic bacteria and bacterivorous flagellates in seawater chemostat cultures