Shifts in bacterial community composition associated with different microzooplankton size fractions in a eutrophic reservoir

Using oligonucleotide probes for Eubacteria and four eubacterial subgroups, we monitored changes in bacterial community composition (BCC) with differe nt degrees of grazing pressure, as follows: unfiltered water (UNF, all bact erivores present); <20 mu m (bacteria, heterotrophic nanoflagellates [HNF], and small ciliates); <5 mu m (bacteria and HNF only); and <1 mu m or <0.8 mu m (bacteria only), incubated in dialysis bags. Experiments were conducte d in the Rimov Reservoir (South Bohemia) during the clear-water phase (expe riment I), a period of law protistan gazing pressure on reservoir bacteriop lankton, and during the late summer phytoplankton peak (experiment II), a p eriod of high protistan grazing pressure. In both experiments, there was a significant shift in BCC in the <5 mu m treatments, which came in the form of increased proportions of alpha subclass of the class Proteobacteria (ALF ) and Cytophaga/Flavobacterium group (C/F), corresponding with increased ba cterivory of the ungrazed HNF populations. Changes in BCC in other experime ntal treatments were related to preincubation protistan grazing pressure. I n experiment I, bacterioplankton were subjected to negligible protistan bac terivory in the reservoir and did not shaw a change in BCC in the predator- free treatment (<1 mu m), while BCC changed in treatments that yielded incr eases in protistan bacterivory compared with that in the ambient reservoir water. In experiment II, a significant shift in BCC was induced when bacter ioplankton that were subjected to heavy predation pressure in the reservoir were transferred into the predator-free treatment. Treatments that induced small changes in protistan grazing pressure (<20 mu m and UNF in experimen t II) did not show significant BCC shifts. Proportions of filamentous bacte ria (>4 mu m) increased in treatments that yielded large increases in bacte rivory. Filament formation also showed season-specific features; in experim ent I, all filaments belonged into ALF, whereas in experiment II, most of t hem hybridized with the C/F probe. We conclude that the sudden shifts that violate the established balance between bacterial production and the protis t-induced bacterial mortality led to the significant shifts in cell morphol ogy and BCC.