| Summary: | Our knowledge of microbial life residing in the oceans- photic and aphotic- and within the subseafloor sediment has grown exponentially within the last few decades. This is partly because of advances in next-generation sequencing technology, which has provided an opportunity to address previously unanswerable questions regarding microbial diversity and biogeography (Hamady & Knight, 2009; Petrosino, Highlander, Luna, Gibbs, & Versalovic, 2009). By utilizing a next-generation sequencing approach, I determined microbial community compositions and assessed their response to environmental and geographic variation within and between five different oceanic regimes (i) the South Pacific Gyre (SPG), (ii) the Eastern and Central Equatorial Pacific (EQP), (iii) the North Pacific Gyre (NPG), (iv) the Bering Sea (U1343), and (v) the Indian Ocean (NGHP-1-14). My first manuscript, " The bacterial and archaeal biogeography of the deep chlorophyll maximum of the South Pacific Gyre", examines the prokaryotic community composition at a continuous and biologically significant horizon, the deep chlorophyll maximum (DCM), across Earth's largest oceanographic province, the SPG. Our results demonstrate that bacterial and archaeal tag-sequence assemblages of the DCM are strikingly similar throughout the SPG, in terms of the presence and abundance of the most dominant bacterial taxa. Comparison of our SPG bacterial results to samples from the NPG and the relatively nutrient- and chlorophyll-rich EQP shows that DCM assemblages of the SPG, NPG and EQP are statistically distinct from each other, although they have many abundant tags in common. This distinctness is influenced by environmental conditions, as the communities of the two gyres (SPG and NPG) resemble each other more closely than either resembles the EQP community (which lives geographically between them). My second manuscript, "Vertical changes in bacterial diversity and community composition from seasurface to subseafloor", investigates the degree of connectivity between bacterial communities that reside in deep-sea sediment to those that reside throughout the water column at three Pacific Ocean (EQP and NPG) stations. In this study, my collaborators and I investigate a series of ecological gradients through examination into the vertical structure and richness of marine microbes and the how they are influenced by geographic location, light, oxygen concentration and depth. We provide the first pyrosequencing results to (i) address a possible mechanism by which deeply buried sedimentary communities develop deep beneath the seafloor and (ii) assess the degree to which the organisms in those communities are related to communities in the overlying ocean. My third manuscript, "Bacterial diversity, sediment age and organic respiration in the marine sedimentary environment", investigates the drivers of microbial diversity and taxonomic richness in deep subseafloor sediment of four geographically distant sites in the Pacific and Indian oceans (U1343, NGHP-1-14, EQP1 and EQP8). To accomplish this goal, my collaborators and I took samples for molecular analysis and interstitial water from a wide range of sediment depths (up to 404 meters below seafloor) and sediment age (up to 5.5 Ma). Our study of these samples demonstrates that abundance-weighted bacterial community composition shifts in response to availability of dissolved metabolic reactants (e.g. oxygen, sulfate, methane). Our study also demonstrates taxonomic richness declines exponentially with sediment age and generally matches the canonical expectation for changing rates of organic oxidation in subseafloor sediment over time (Jorgensen, 1978; Middelburg, 1989; Westrich & Berner, 1984).
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