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Abstract
This dissertation explores the microbial ecology and biogeochemistry of carbon cycling in extreme marine environments using novel approaches. The Guaymas Basin is a young ocean basin where thermocatalyic alteration of organic matter and hydrothermal fluid flow impact biogeochemical regimes. Methane, a product of hydrothermalism, is actively metabolized in benthic habitats. In situ methane concentrations exhibited strong spatiotemporal variability. High methane concentrations (46 mmol L-1) were measured using Jumbo OsmoSamplers, underscoring the need to make in situ measurements to constrain their dynamics. Methane concentrations varied substantially over a 3-month time-series reflecting the dynamic nature of the system. Energy-laden hydrothermal fluids mix with cold bottom water to form buoyant plumes enriched with labile organic compounds. Heterotrophic metabolism of methane, methanol, and acetate was evaluated in a hydrothermal plume. Organic carbon assimilation rates were high, accounting for more than 50% of microbial uptake in the plume, underscoring the importance of heterotrophy in hydrothermally-impacted waters.
In pelagic environments impacted by hypoxia, pelagic organic carbon cycling is poorly documented. Microbially-mediated cycling of methanol and methane were examined across the Gulf of Mexico hypoxic zone; Ammonia Oxidizing Archaea (AOA) are abundant in these habitats. Oxidation of methane was low compared to previous measurements and methanol oxidation and assimilation were important. Constraining the metabolic capabilities and environmental regulation of AOA, among the most abundant microbes in the ocean, is necessary to document their role in ocean biogeochemistry. AOA oxidized methane and incorporated leucine, extending their metabolic portfolio and suggesting additional roles for the AOA in ocean carbon and nitrogen cycling. Collectively, these studies advance our understanding of microbial carbon cycling in several extreme environments.
In pelagic environments impacted by hypoxia, pelagic organic carbon cycling is poorly documented. Microbially-mediated cycling of methanol and methane were examined across the Gulf of Mexico hypoxic zone; Ammonia Oxidizing Archaea (AOA) are abundant in these habitats. Oxidation of methane was low compared to previous measurements and methanol oxidation and assimilation were important. Constraining the metabolic capabilities and environmental regulation of AOA, among the most abundant microbes in the ocean, is necessary to document their role in ocean biogeochemistry. AOA oxidized methane and incorporated leucine, extending their metabolic portfolio and suggesting additional roles for the AOA in ocean carbon and nitrogen cycling. Collectively, these studies advance our understanding of microbial carbon cycling in several extreme environments.