Morphological and compositional changes in a planktonic bacterial community in response to enhanced protozoan grazing

We analyzed changes in bacterioplankton morphology and composition during e nhanced protozoan grazing by image analysis and fluorescent in situ hybridi zation with group-specific rRNA-targeted oligonucleotide probes. Enclosure experiments were conducted in a small, fishless freshwater pond which was d ominated by the cladoceran Daphnia magna. The removal of metazooplankton en hanced protozoan grazing pressure and triggered a microbial succession from fast-growing small bacteria to larger grazing-resistant morphotype. These were mainly different types of filamentous bacteria which correlated in bio mass with the population development of heterotrophic nanoflagellates (HNF) . Small bacterial rods and cocci, which showed increased proportion after r emoval of Daphnia and doubling times of 6 to 11 h, belonged nearly exclusiv ely to the beta subdivision of the class Proteobacteria and the Cytophaga-F lavobacterium cluster. The majority of this newly produced bacterial biomas s was rapidly consumed by HNF. In contrast, the proportion of bacteria belo nging to the gamma and alpha subdivisions of the Proteobacteria increased t hroughout the experiment. The alpha subdivision consisted mainly of rods th at were 3 to 6 mu m in length, which probably exceeded the size range of ba cteria edible by protozoa. Initially, these organisms accounted for less th an 1% of total bacteria, but after 72 h they became the predominant group o f the bacterial assemblage. Other types of grazing-resistant, filamentous b acteria were also found within the beta subdivision of Proteobacteria and t he Cytophaga-Flavobacterium cluster. We conclude that the predation regimen is a major structuring force for the bacterial community composition in th is system. Protozoan grazing resulted in shifts of the morphological as wel l as the taxonomic composition of the bacterial assemblage. Grazing-resista nt filamentous bacteria can develop within different phylogenetic groups of bacteria, and formerly underepresented taxa might become a dominant group when protozoan predation is the major selective pressure.