Microbial processes of pelagic anaerobic methane cycling in oxygen minimum zones

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The overarching goal of this research is to understand how bacteria in marine oxygen minimum zones (OMZs) control interactions between the ocean methane and nitrogen cycles. OMZs constitute the largest pool of methane in the ocean water column, and also serve as sites where anaerobic microbes convert the essential element nitrogen from a form that can be used by organisms to a gaseous form (N2) that can be lost from the ocean. Recent studies, predominantly in freshwater environments, have discovered novel bacteria that link methane consumption to pathways of nitrogen loss. These researchers have recently shown that such bacteria also occur in OMZs. However, the contributions of these bacteria to ocean methane and nitrogen flux remain unknown. Here, the researchers will use a combination of genomics and biochemical measurements to characterize the metabolic potential and diversity of these bacteria in OMZs and to quantify their contribution to methane and nitrogen transformations. Meeting this goal is critical for constraining bulk fluxes of these chemicals in the open ocean and for predictive models of climate change, notably given the importance of methane as a potent greenhouse gas and the prediction that OMZs will expand with global warming. This project is also committed to enhancing knowledge of marine microbiology in the public arena and educational communities. The investigators will incorporate results and concepts from this research into instructional modules for use in the Summer Workshop in Marine Science (SWiMS) at Georgia Tech (https://swimsgatech.wordpress.com/), an annual workshop to train middle and high school educators in marine science using lectures and lab exercises. This project will also involve training of undergraduate students, a postdoc, and local high school teachers through the Georgia Tech GIFT Program. Through the combined research and educational activities, this research will determine how low-oxygen conditions in the ocean regulate pelagic microbial diversity and methane cycling, as well as inform, engage, and excite scientific and general audiences about marine microbiology and biogeochemistry. This research focuses specifically on bacteria conducting nitrite-dependent anaerobic methane oxidation (n-damo). This process has been described in bacteria of the NC10 division, in which a dismutation reaction generates both N2 and O2 gas, with the O2 used for intra-aerobic methane oxidation. Although NC10 bacteria have been described primarily from nitrite-rich freshwater and marine sediments, recent evidence indicates that NC10 are also present in anoxic OMZs. Given that OMZs contain substantial pools of the n-damo substrates nitrite and methane, it is hypothesized that OMZs harbor an anaerobic methane cycle coupled to nitrogen loss, and that this coupling is mediated by n-damo NC10 bacteria that occur as ubiquitous components in diverse OMZs. To test this hypothesis, the researchers will 1) quantify the contribution of n-damo to OMZ methane oxidation, N2 production, and oxygen production rates, 2) characterize the diversity and ecophysiology of OMZ NC10 isolates through enrichments and single-cell genomics, and 3) survey the abundance, diversity, and activity of NC10 bacteria across distinct OMZ systems.