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 a five year period demonstrated that viral and bacterial abundances were significantly higher during the rainy seasons. However, the virus to bacterial ratios ranged from 10 to 15 throughout the entire year. Loss of infectivity was studied with the use of eight laboratory cultured host/virus systems, isolated from either the North Sea or from Santa Monica Bay. 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. In addition to studying rates of loss of infectivity, studies were done to determine the rates of biochemical degradation of radiolabelled viruses and viral lysis products in seawater. These experiments demonstrated that rates of degradation were significantly lower 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. Uptake of 33P (phosphate) labeled viruses and viral lysis products indicated that uptake of virus material may be important in nutrient limited waters Turnover times of the viral populations as measured by biochemical degradation experiments were on the order of 1-2 d.

A new method, using the nucleic acid stain SYBR Green I, was developed and makes enumeration of viruses in aquatic samples simple, cheap, fast, and possible in the field using epifluorescence microscopy. An extension of this method was also developed using fluorescently labeled viruses as tracers of virus mediated processes. With this method and calculations used previously for the isotope dilution technique, I have been able 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.