Biogeochemical and microbial controls on manganese-driven oxidation of methane in coastal sediments

Atmospheric concentrations of methane, a key greenhouse gas, are steadily increasing, and are linked to anthropogenic and natural sources. Coastal systems account for the majority of marine methane emissions. In such systems, the rate of methane production in the sediment often exceeds its removal via aerobic and anaerobic oxidation, highlighting the need to understand the pathways of methane oxidation. While sulfate is the major electron acceptor for anaerobic methane oxidation in marine systems, other electron acceptors such as metal oxides may also be used. Definitive proof of methane oxidation coupled to manganese oxide reduction is not available for marine sediments. Furthermore, the microbes that can perform manganese driven methane oxidation in such environments are not yet identified. In the research presented in this PhD thesis, we explore the role of manganese oxides in methane oxidation in marine sediments from a geochemical and microbiological perspective. Our research showed that manganese oxides can drive methane oxidation in marine sediments, both via a direct coupling of the two reactions, and via a cryptic sulfur cycle driven by manganese oxide reduction. Furthermore, our work highlights the role of ANME-2ab archaea as key organisms for manganese driven methane oxidation. This knowledge fundamentally alters our understanding of how coastal ecosystems will regulate greenhouse gas emissions in a future of changing climate and ocean chemistry.