Coastal methane emissions driven by aerotolerant methanogens using seaweed and seagrass metabolites.

Methanogenesis is thought to be limited to strictly anoxic environments. While oxygenated oceans are a known methane source, it is argued that methane production is driven by methylphosphonate-degrading bacteria and potentially other sources rather than by methanogenic archaea. Here we develop in situ monitoring and ex situ manipulation experiments, combined with biogeochemical, metagenomic and culture-based experiments, to show that methane is rapidly produced by archaea in frequently oxygenated sandy sediments. We show that methane emissions from sandy sediments are not inhibited by repeated oxygen exposure and suggest the activity is driven by aerotolerant methylotrophic methanogens (primarily Methanosarcinaceae) broadly distributed in the surface layers of sandy sediments. Moreover, we show that methane emissions are driven by methylated seaweed and seagrass metabolites, revealing a feedback loop between primary production and greenhouse gas emissions.