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The direct effects of viral enrichments upon natural populations of marine viruses and bacteria were studied in seawater from Santa Monica Bay, CA. USA. Active virus concentrates, or control additions (ultrafiltered seawater or autoclaved virus concentrate) were added to 2 1 incubations of protist-free seawater, and the effects were monitored for about 3 d. At the beginning of the experiments, the virus numbers reflected the expected addition of intact virus particles as determined by transmission electron microscopy (TEM). Subsequently, the mean frequency of visibly infected bacteria [FVIB; % bacteria which were visibly infected with 5 or more virus-like particles) was greater in the enriched incubations than in the controls. In controls, the estimated percent of bacteria that were infected remained constant at about S to 10% of the total bacterial population, but with active enrichment, 10 to 35% of the total bacterial population was infected at a given time. Therefore, by increasing the concentration of active viruses in seawater incubations we were able to increase the amount of bacterial mortality attributed to virus infection. Even with the presumed increase in bacterial mortality, the net increases in bacterial abundance in the samples that were enriched with active virus concentrate were higher than those seen in the controls. The viral abundance in bottles that were enriched with the active virus concentrate was significantly higher than that in the controls in Expts 2 and 3 (p < 0.05), but by the end of the experiments, viral abundances in the enriched incubations approached control levels. In Expts 1 and 2, rates of DOP hydrolysis were higher in the samples enriched with the active virus concentrate, and may have been due to an increase in the incidence of viral lysis. However, overall analysis of DCAA, DFAA, and DOP hydrolysis were quite variable and difficult to interpret. Results indicate that viral enrichment increased the incidence of bacterial infection and consequently stimulated the growth of subpopulations of non-infected heterotrophic bacterioplankton.

Effects of viral enrichment on the mortality and growth of heterotrophic bacterioplankton