Fluorescently labeled virus probes show that natural virus populations can control the structure of marine microbial communities

Fluorescently stained viruses were used as probes to label, identify, and enumerate specific strains of bacteria and cyanobacteria in mixed microbial assemblages. Several marine virus isolates were fluorescently stained with YOYO-1 or POPO-1 (Molecular Probes, Inc.) and added to seawater samples that contained natural microbial communities. Cells to which the stained viruses adsorbed were easily distinguished from nonhost cells; typically, there was undetectable binding of stained viruses to natural microbial assemblages containing>106 bacteria ml21 but to which host cells were not added. Host cells that were added to natural seawater were quantified with 99 % 6 2 % (mean 6 range) efficiency with fluorescently labeled virus probes (FLVPs). A marine bacterial isolate (strain PWH3a), tentatively identified as Vibrio natriegens, was introduced into natural microbial communities that were either supplemented with nutrients or untreated, and changes in the abundance of the isolate were monitored with FLVPs. Simultaneously, the concentrations of viruses that infected strain PWH3a were monitored by plaque assay. Following the addition of PWH3a, the concentration of viruses infecting this strain increased from undetectable levels (<1 ml21) to 2.9 3 107 and 8.3 3 108 ml21 for the untreated and nutrient-enriched samples, respectively. The increase in viruses was associated with a collapse in populations of strain PWH3a from ca. 30 to 2 % and 43 to 0.01 % of the microbial communities in untreated and nutrient-enriched samples, respectively. These results clearly demonstrate that FLVPs can b