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UBC Theses and Dissertations

Influence of bacterial viruses on nitrogen cycling in the ocean Shelford, Emma Joyce


Current studies indicate that viruses of marine bacteria are biological carbon sinks, transforming bacterial carbon into dissolved organic matter, the majority of which is respired rather than incorporated back into biomass. In contrast, this dissertation focusses on viruses, not as a carbon sink but as a catalyst of nitrogen cycling, benefiting phytoplankton by liberating nitrogen from bacterial lysates that would otherwise be tied up in bacterial biomass. The results in this dissertation show that organic nitrogen released by viral lysis of heterotrophic marine bacteria is remineralised by uninfected bacteria, and the resulting ammonium taken up by phytoplankton. In an initial laboratory experiment, only a portion of the amino acids derived from heterotrophic bacterial lysates could be taken up by other heterotrophic bacteria within the duration of the experiment. Both D- and L-amino acids were taken up in proportion to their initial concentrations, demonstrating a lack of preference for the generally more labile L-amino acids. In a subsequent field experiment, reduction of the viral fraction in a marine microbial community resulted in reduced ammonium remineralisation and phytoplankton abundance, suggesting that remineralised nitrogen from bacterial metabolism of viral lysates contributes to phytoplankton growth. Another experiment added a marine bacterium labeled with 15N and infected with a lytic virus to microbial communities. This experiment directly demonstrated that remineralised nitrogen from bacterial lysates released through the action of viruses was a significant source of nitrogen for phytoplankton. In a final series of experiments, viruses were reduced from seawater from 22 field stations using bacterial concentration techniques to explore correlations between environmental factors and ammonium remineralisation from viral lysis. Viral mediated ammonium remineralisation changed with different chlorophyll a concentrations and salinities, suggesting potential predictive associations. These results show that liberated nitrogen from viral lysis of bacteria is readily degraded by heterotrophic marine bacteria and remineralised into ammonium for uptake by autotrophic organisms. The results in this dissertation demonstrate that viruses are key players in the cycling of nitrogen in marine systems and stress the need to incorporate viral mediated nutrient release into models of global biogeochemical cycling.

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