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Abstract
This dissertation explores pelagic (and occasionally benthic) methane dynamics in marine marginal environments. Continental margins are among the most biogeochemically active regions of the oceans. The diverse underlying geology, stark depth gradients, and nutrient inputs from terrestrial environments lead to dynamic chemical cycling and biodiverse communities. Sedimentation and burial of organic carbon along continental margins can lead to production of methane biogenically, thermogenically, or both. This methane is gradually released from benthic into pelagic environments, where it may be advected elsewhere, consumed by methanotrophs, or released into the atmosphere, where it acts as a powerful greenhouse gas. The pelagic methanotrophic community acts as the final barrier between subsurface-derived methane and the atmosphere. We measured methane concentrations and methane oxidation rates at a hydrothermal vent in Guaymas Basin, in a seasonal hypoxic zone in the northern Gulf of Mexico, in subtropical surface waters of the Gulf of Mexico and U.S. Atlantic margin, and at cold seeps in all three of the aforementioned basins (Gulf of California, Gulf of Mexico, U.S. Atlantic margin). Across this spectrum of habitats, we measured methane concentrations spanning six orders of magnitude and methane oxidation rates spanning eight, including the highest rates yet observed in the marine water column. We also investigated the microbial genomic potential of these habitats, explored potential methane production pathways, and investigated connections between the methane, nitrogen, and oxygen dynamics of these different environments. Expanding our knowledge of marine methane cycling is important not only because of methane's significance as a greenhouse gas, but also because methane is a labile carbon compound whose sources and sinks are well established, but poorly constrained, and seldom directly measured. Understanding variability in methane consumption rates across diverse environments is important for understanding methane climate feedbacks, as well as how marine methane cycling may evolve in our uncertain future of ocean warming and deoxygenation.