Date of Award


Degree Type


Degree Name

Doctor of Philosophy



Major Professor

Steven W. Wilhelm

Committee Members

Alison Buchan, Annette S. Engel, Karen G. Lloyd, Erik R. Zinser


A majority of Earth’s surface is covered by the marine ecosystem. This ecosystem is characterized by the numerical dominance of microbes, both eukaryotic and prokaryotic and their infecting viruses. As the basis of the food web in marine systems these members of the microbial community are vital to the function of the planet. Despite their noted importance, the microbial communities in many oceanic regions are largely uncharacterized due to geographic isolation, including the largest oceanic basin on the planet, the North Pacific Ocean. Basin scale studies of the North Pacific have primarily focused on characterization of abundance and diversity of bacteria and viruses. Interactions between these microbial constituents have been largely unexplored on a basin scale. As such, this body of work aims to generate hypotheses on the influence that biotic interactions have on microbial community structure and evolution on a basin scale. First, the role biotic interactions responsible for shaping virus abundances and production rates in the North Pacific Ocean were examined. We found that virus abundance and production rates correlated to bacterial taxa that are enriched in particle associated communities, indicating that particles may serve as sites of increased viral activity in the North Pacific Ocean. Secondly, we utilized network analysis to explore inferred bacterial co-occurrences within the North Pacific Basin. We found that diverse taxa participate in at least one significant co-occurrence with another microbe within this basin. Further, network topology parameters, such as high connectance and low modularity, proposed to be hallmarks of community stability differed regionally, indicating that the North Pacific Subtropical Gyre may be more robust to environmental changes. Finally, we developed an assay to screen for Synechococcus WH7803 transposon mutants that are resistant to a co-occurring cyanomyovirus, to identify genes necessary for lytic viral infection. Characterization of these genes will allow for a more holistic understanding of the interactions between this cyanobacterium and its infecting viruses in the natural environment. These findings highlight the importance of considering interactions between microbial entities in the marine system in understanding the functioning of microbial communities in the North Pacific basin.


Chapter 1, Contrasting seasonal drivers of virus abundance and production in the North Pacific Ocean was previously published in PLoS One.

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