The ecology of marine bacteriophages and their contribution to bacterial mortality

Marine bacteria are integral to many biogeochemical processes and consume a major fraction of oceanic primary production. Understanding the links between bacteria and the rest of the marine food web requires knowledge of the fate of bacterial production and recently viruses have been recognized as an important source of bacterial mortality in the sea. Continual infection and lysis in a plankton community is expected to enhance respiration of organic matter by the bacterial community and decrease carbon transfer to higher trophic levels. Therefore, viral ecology is critical for understanding variability in carbon flow through the marine food web.; The objectives of the work presented in this dissertation were to measure virus production in seawater, estimate the percentages of bacterial production lysed by viruses, and determine possible causes for the variability in virus-induced mortality of bacteria. To accomplish these objectives, a radiolabeling method for estimating virus production was developed and employed in the field (Southern California Bight; Bering and Chukchi Seas). Seawater was incubated with {dollar}sp{lcub}32{rcub}{dollar}P-orthophosphate or {dollar}sp3{dollar}H-thymidine followed by purification steps to measure label incorporation into viral DNA. Electron microscopy (EM) of field and mesocosm samples was also used to count viruses and bacteria and to quantify viral infections of bacteria.; Viruses and viral infections of bacteria were observed throughout the study sites. Virus and bacteria concentrations were correlated and viruses were about ten times more abundant on average. Viral and bacterial production were also correlated and the percent contribution of viruses to bacterial mortality increased with increasing bacterial abundance and production. In highly productive neritic waters viruses accounted for 20 to 40% of bacterial mortality, comparable to flagellate grazing. In more oligotrophic waters, mortality due to viruses was typically around 10%. It is concluded that the relative impact of viruses on bacteria is most significant at times and places where bacteria are most abundant and active, such as consistently eutrophic coastal waters, but only episodically important in the open ocean, e.g., following phytoplankton blooms. However, because of the specificity of viral infections and non-lethal virus-host interactions, viruses may significantly influence bacterial communities even where total community mortality due to viruses is low.