Control of lysogeny in marine bacteria: Studies with phiHSIC and natural populations

Viruses have an estimated global population size of 10 to the 31st, with a significant proportion found in the marine environment. Viral lysis of bacteria affects the flow of carbon in the marine microbial food web, but the effects of lysogeny on marine microbial ecology are largely unknown. In this thesis, factors that influence the control of lysogeny were studied in both the phiHSIC/Listonella pelagia phage-host system and in bacterioplankton populations in the Gulf of Mexico. Using macroarrays dotted with phiHSIC amplicons, viral gene expression over the course of a synchronous infection experiment was measured. Early, middle, late, and continually expressed genes were identified, and included open reading frames 45, 28, 18 and 17, respectively. Viral gene expression in cultures of the HSIC-1a pseudolysogen grown in low and normal salinity media was also analyzed. Overall, levels of viral gene expression were higher in the 39 ppt treatment as compared to the 11 ppt tre atment for most ORFs. In the 11 ppt treatment, free phage concentrations were one to two orders of magnitude lower than the 39 ppt treatment while intracellular phage concentrations were one-fold lower. Therefore, at low salinities, expression of phiHSIC genes is repressed resulting in a lysogenic-like state, while at 39 ppt, lytic interactions dominated. Few viral genes were highly expressed at low salinity, suggesting that repression of viral genes was controlled by host genes. Samples from the eutrophic Mississippi River Plume and the oligotrophic Gulf of Mexico were analyzed for lytic phage production and occurrence of lysogeny. Significant lytic viral production was only observed three stations, none of which were located within the MRP. This signifies that system productivity is not an accurate predictor of viral productivity. The lysogenic fraction was also inversely correlated to bacterial activity, which decreased with depth. These findings support the hypothesis that lysogeny is a survival mechanism for phages when host cell density is low or when conditions do not favor growth. A unifying theme from these experiments was that lytic processes dominated when bacterial growth conditions were optimal, while lysogeny was observed at unfavorable growth conditions or environmental stress (low salinity)