Graduate Thesis Or Dissertation


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  • Marine bacteria play vital roles in every niche of the ocean, from small-scale symbioses to large-scale productivity and the regulation of Earth’s climate. Recent advances in molecular tools now allow us to probe the genetic potential of entire microbial communities. The next step is linking these diverse communities to the critical functions they perform, in order to better understand how microbes regulate biogeochemical processes and predict how these processes may change as humans continue to alter the physical and chemical properties of the oceans. The three main body chapters of this dissertation use genomic tools and biogeochemical rate measurements to examine the ecology and environmental controls of two keystone groups of marine bacteria. Both Chapter II and Chapter III focus on marine diazotrophs, a group that converts dinitrogen gas into bioavailable nitrogen, thus helping to fuel productivity in oligotrophic, surface, open-ocean waters. Chapter II compares diazotrophic diversity and nitrogen fixation rates across three distinct ocean regions. This work indicates that diazotrophs are cosmopolitan in marine waters and that dominant diazotrophic groups have distinct biogeographical patterns, but that nitrogen fixation rates are restricted. Chapter III investigates the diversity and functional potential of microorganisms associated with colonies of the filamentous cyanobacterial diazotroph Trichodesmium. Molecular analyses revealed that the Trichodesmium colonies used in this study were inhabited by diverse assemblages of microorganisms which were distinct from the surrounding bacterioplankton. Furthermore, epibionts included non-Trichodesmium diazotrophs and organisms with the genetic potential to influence colony nutrient acquisition. Chapter IV focuses on a separate group of marine bacterioplankton, Vibrio spp., which have significance for environmental health and marine disease due to the pathogenicity of several species. This chapter tracks the abundance of oyster-pathogenic Vibrio spp. in select Oregon estuaries and a shellfish hatchery. Results indicate that local growth drives elevated abundances of the oyster-pathogen V. coralliilyticus in Netarts Bay, and suggest that this species is particularly resistant to hatchery cleaning procedures.
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