| Viruses are an important component of the marine microbial food web, as they are capable of contributing to a significant fraction of the mortality of heterotrophic bacterioplankton. To better understand the ecological roles of viruses in the ocean and their possible influences upon biogeochemical cycles, I studied the fates of viruses in relation to other components of the microbial food web.; The fates of viruses were studied by examining loss of infectivity, biochemical degradation, the effects of viral enrichment on bacterial mortality, and virus production. Spatio-temporal analysis of surface seawater of Santa Monica Bay over five years demonstrated significantly higher viral and bacterial abundances during the rainy season, with nearly constant virus to bacterial ratios of about 10. Loss of infectivity was studied with the use of eight laboratory cultured host/virus systems. The decay of infectivity of these viruses was assessed in seawater, and was partitioned according to singular causative agents of decay, such as ultraviolet light, heat labile material such as extracellular enzymes, and/or particles for adsorption. Virus isolates native to Santa Monica Bay consistently degraded more slowly in full sunlight than bacteriophages isolated from the North Sea, and although sunlight was an important contributing factor to virus decay, decay due to particles and dissolved substances in seawater was also significant. Rates of biochemical degradation of radiolabelled material demonstrated that viruses degrade more slowly in oligotrophic waters than in eutrophic waters, that biochemical degradation of whole virus populations created much bioreactive low molecular weight material, but also that bacterial uptake of this material was difficult to assess. Turnover times of the viral populations as measured by biochemical degradation were on the order of 1-2 d.; New methods, using the stain SYBR Green I, were used to determine virus numbers and to estimate virus production and removal rates. These studies indicated turnover times of virus populations of 1-2 d and ca. 40% of the total bacterial mortality attributed to viruses even in oligotrophic offshore seawater. Rates of virus degradation, production, and removal are useful input for future models that include virus mediated processes. |
