Changes in Bacterial Community Composition and Dynamics and Viral Mortality Rates Associated with Enhanced Flagellate Grazing in a Mesoeutrophic Reservoir

Abstract

Bacterioplankton from a meso-eutrophic dam reservoir was size fractionated to reduce (<0.8-m treatment) or enhance (<5-m 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 characteristics 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 "grazerfree" (0.8-m-filtered) treatment, subject only to a relatively low mortality rate (ϳ17% day ؊1 ) 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-m filtrate), grazing mortality was ϳ200% and viral lysis resulted 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 experiment. 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 proteobacteria and members of the Cytophaga-Flavobacterium cluster, which consistently formed different parts of the bacterial flocs. Viral concentrations and frequencies of infected cells were highly significantly correlated with grazing rates, suggesting that protistan grazing may stimulate viral activity.

Seasonal patterns of viral, microbial and planktonic communities in Sidi Salem: a freshwater reservoir (North of Tunisia)

“…However, our study reported the lower abundances of picocyanobacteria than showed by Hennes and Simon (1995). In their study, they have reported in mesotrophic Lake Constance that the abundances of viruses ranged from (10 x4 7 r 10 6 part.mL x1 ), the similar results were also reported by Simek et al (2001) in meso-eutrophic Rimov reservoir (Table 6).…”

Disentangling the relative influence of bacterioplankton phylogeny and metabolism on lysogeny in reservoirs and lagoons

“…(ii) The intrinsic low activity of a bacterial group may only allow lysogeny. The Bacteroidetes have been repeatedly found to grow more slowly than other phylogenetic groups (Fuchs et al, 2000;Simek et al, 2001;Zubkov et al, 2001, this study). This metabolic feature may thus represent a signal for the establishment of lysogeny.…”

Trade-Offs between Competition and Defense Specialists among Unicellular Planktonic Organisms: the “Killing the Winner” Hypothesis Revisited

“…However, in agreement with KtW, the effect of virus-induced mortality was higher in resource-replete treatments, where prokaryotic and viral abundance, prokaryotic and viral production, and virus-induced lysis rates increased (equations 5 and 6). Also, grazing stimulated viral abundance, production, and virus-induced mortality, as found in other studies (72,85,87). Jardillier et al (45) studied the influence of nutrients, grazing, and viral lysis on the prokaryotic community composition in an oligomesotrophic lake and in a eutrophic lake.…”

“…There are two experimental studies, both conducted in freshwater environments, that manipulated grazing pressure as the sole experimental factor (85,87). Both studies found that prokaryotic and viral activity increased in the presence of grazers and suggest that this stimulation is due to grazer-mediated resource enrichment.…”