Changes in bacterial community composition, dynamics and viral mortality rates associated with enhanced flagellate grazing in a meso-eutrophic reservoir. Appl Environ Microbiol 67

Bacterioplankton from a meso-eutrophic dam reservoir was size fractionated to reduce (<0.8-mm treatment) or enhance (<5-mm treatment) protistan grazing and then incubated in situ for 96 h in dialysis bags. Time course samples were taken from the bags and the reservoir to estimate bacterial abundance, mean cell volume, production, protistan grazing, viral abundance, and frequency of visibly infected cells. Shifts in bacterial community composition (BCC) were examined by denaturing gradient gel electrophoresis (DGGE), cloning and sequencing of 16S rDNA genes from the different treatments, and fluorescence in situ hybridization (FISH) with previously employed and newly designed oligonucleotide probes. Changes in bacterioplankton character-istics were clearly linked to changes in mortality rates. In the reservoir, where bacterial production about equaled protist grazing and viral mortality, community characteristics were nearly invariant. In the “grazer-free ” (0.8-mm-filtered) treatment, subject only to a relatively low mortality rate (;17 % day21) from viral lysis, bacteria increased markedly in concentration. While the mean bacterial cell volume was invariant, DGGE indicated a shift in BCC and FISH revealed an increase in the proportion of one lineage within the beta proteobacteria. In the grazing-enhanced treatment (5-mm filtrate), grazing mortality was;200 % and viral lysi