Changes in bacterial community composition and dynamics and viral mortality rates associated with enhanced flagellate grazing in a mesoeutrophic reservoir

Bacterioplankton from a meso-eutrophic dam reservoir was size fractionated to reduce (<0.8-mum treatment) or enhance (<5-mum treatment) protistan graz ing and then incubated in situ for 96 h in dialysis bags. Time course sampl es were taken from the bags and the reservoir to estimate bacterial abundan ce, mean cell volume, production, protistan grazing, viral abundance, and f requency of visibly infected cells. Shifts in bacterial community compositi on (BCC) were examined by denaturing gradient gel electrophoresis (DGGE), c loning and sequencing of 16S rDNA genes from the different treatments, and fluorescence in situ hybridization (PISH) with previously employed and newl y designed oligonucleotide probes, Changes in bacterioplankton characterist ics were clearly linked to changes in mortality rates. In the reservoir, wh ere bacterial production about equaled protist grazing and viral mortality, community characteristics were nearly invariant, In the "grazer-free" (0.8 -mum-filtered) treatment, subject only to a relatively low mortality rate ( similar to 17% day(-1)) from viral lysis, bacteria increased markedly in co ncentration. While the mean bacterial cell volume was invariant, DGGE indic ated 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-mum filtrate), grazing mortality was similar to 200% and viral lysis resu lted in mortality of 30% of daily production. Cell concentrations declined, and grazing-resistant flocs and filaments eventually dominated the biomass , together accounting for > 80% of the total bacteria by the end of the exp eriment. Once again, BCC changed strongly and a significant fraction of the large filaments was detected using a FISH probe targeted to members of the Flectobacillus lineage, Shifts of BCC were also reflected in DGGE patterns and in the increases in the relative importance of both beta proteobacteri a and members of the Cytophaga-Flavobacterium cluster, which consistently f ormed different parts of the bacterial flocs. Viral concentrations and freq uencies of infected cells were highly significantly correlated with grazing rates, suggesting that protistan grazing may stimulate viral activity.