Aerotolerant methanogens use seaweed and seagrass metabolites to drive marine methane emissions

Methanogenesis is classically thought to be limited to strictly anoxic environments. While oxygenated oceans are a known methane source, it is argued that methanogenesis is driven by methylphosphonate-degrading bacteria or potentially is associated to zooplankton gut microbiomes rather than by methanogenic archaea. Here we show through in situ monitoring and ex situ manipulations that methane is rapidly produced by archaea in frequently oxygenated sandy sediments. By combining biogeochemical, metagenomic, and culture-based experiments, we show this activity is driven by aerotolerant methylotrophic methanogens ( Methanococcoides spp.) broadly distributed in surface layers of sandy sediments, providing evidence of a hidden process contributing to marine methane emissions. Moreover, we show that methane emissions are driven by methylated seaweed and seagrass metabolites, revealing an unexpected feedback loop between eutrophication-driven algal blooms and greenhouse gas emissions.