45 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments INTRODUCTION Methanogens in marine sediments produce up to 85-300 Tg of the potent greenhouse gas methane per year, which represents 7-25% of global methane production (Dean et al., 2018; Knittel & Boetius, 2009). However, anaerobic methane-oxidizing (ANME) archaea consume more than 90% of the in situ generated methane (Knittel & Boetius, 2009). While coastal zones cover only circa 15% of the total ocean surface area, they account for more than 75% of global marine methane emissions (Bakker et al., 2014; Bange et al., 1994) as an indirect result of high nutrient inputs and burial rates of organic matter (Wallenius et al., 2021). Recent estimates suggest that 5-28 Tg of methane per year is emitted from coastal waters to the atmosphere (Rosentreter et al., 2021) . Eutrophication – excess nutrient input – can further disrupt the balance between microbial methane production and consumption in the (near) future. For instance, eutrophication can cause seawater oxygen depletion due to aerobic microbial respiration coupled to degradation of fresh labile organic matter inputs from increased primary productivity, particularly in enclosed basins with a shallow water depth (Middelburg & Levin, 2009). Such oxygen loss is anthropogenically induced in coastal ecosystems (Conley et al., 2011; Wallenius et al., 2021) and could lead to decreased aerobic methane oxidation in the water column, increasing net methane emissions (Venetz et al., 2023; Żygadłowska et al., 2023). Moreover, eutrophication alters sediment geochemistry, such as the vertical compression of the typical marine sedimentary redox zonation (Schulz & Zabel, 2006), that could lead to the release of sulfide into bottom waters (Slomp, 2013), hence the development of euxinia. Additionally, high inputs of organic matter, following eutrophication, provide increased substrates for methanogenesis (Wallenius et al., 2021). While the combination of these processes is expected to increase methane production in sediments, it remains largely unknown how such processes impact anaerobic methane removal and benthic methane release into the water column. This makes it urgent to mechanistically understand coastal sediment methane dynamics in order to build predictive biogeochemical models of future changes (Lenstra et al., 2023) develop appropriate management strategies, and accelerate the pace of climate action. 2
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