ANME-2a drive methane oxidation in brackish coastal sediments via multiple pathways

Methane is a powerful greenhouse gas. Typically, a large fraction of the methane formed in coastal sediments is removed via anaerobic methane oxidation (AOM). Here, we demonstrate the potential for a range of AOM pathways in brackish coastal sediments by ANME-2a archaea. At our study site, geochemical profiles indicate that AOM is primarily restricted to a shallow, metal-oxide-rich sulfate-methane transition zone (SMTZ). ANME-2a were the sole methanotrophs detected, and metatranscriptomics showed the highest expression levels of the ANME-2a genes in the SMTZ. AOM activity was observed in sediment incubations with various electron acceptors, including sulfate, metal oxides, and the organic matter analogue graphene oxide. Highest potential rates were observed in sediments from below the SMTZ, pointing towards fast stimulation of the deeper methanotrophic community when alleviating the electron acceptor limitation. The variety of AOM pathways and persistence of methanotrophs below the SMTZ likely contribute to the resilience of the microbial methane filter in brackish coastal sediments.