| Despite a growing awareness of their role in microbial food webs, there is little experimental or observational evidence of the role of viruses in shaping marine bacterial assemblages. This dissertation aimed to determine whether (1) viruses caused significant mortality of bacteria and whether mortality rate scaled with the production rate of host assemblages; (2) enhancement of viral pressure enhances host assemblage richness and diversity; and (3) whether a virus-pressure-host diversity relationship exists across water masses of different trophic status. To address these questions, two high-throughput fingerprinting approaches were used to examine all bacteria (automated rRNA intergenic spacer analysis; ARISA) and a subset of these with presumably low relative abundance, diazotrophs (nifH terminal restriction fragment length polymorphism; nifH TRFLP), in response to changes in viral pressure. Ambient viral pressure across several habitats, including sediments, was determined using a viral dilution technique. Viral pressure upon host communities was enhanced by concentrating native viruses and adding them back to seawater and sediment incubations. Viruses may cause significant mortality of bacteria across most habitats, which is not related to primary production rates, nor the same within similar water masses (i.e. patchy viral impacts upon bacterioplankton). Enhancement of viral pressure in 15 incubations in 3 ocean basins showed that this treatment had significant impacts upon measured parameters at most stations. However the impacts on parameters were not consistent among experiment locations. Moreover, enhancement of viral pressure had no discernible impact upon the relative dominance of common microbial fingerprint components in plankton, but significant negative impacts on common components matched with significant positive impacts on rarer components in sediment experiments. Correlation analysis of viral parameters and the abundance, productivity, relative richness and diversity of bacterioplankton and diazotrophs indicate that overall diversity of host assemblages may be related to their habitat and resource availability, but viruses have significant impacts upon their assemblages within habitats. The data in this dissertation suggest that viruses may have a more complicated role in shaping host assemblages in marine environments than previously believed, made more complex by the inclusion of a diverse dissolved organic matter pool, resistant bacteria, and lysogens. |
