Potential for metal-coupled methane oxidation by Candidatus Methanocomedenaceae in coastal sediments

Anaerobic methanotrophic (ANME) archaea are important players in the microbial methane cycle, mitigating methane emissions from anoxic environments. ANME are found ubiquitously in methane-rich sediments, where they can couple anaerobic methane oxidation (AOM) to different electron acceptors such as sulfate, metal oxides, and natural organic matter (NOM). However, we still lack understanding of the geochemical niches and preferred metabolic pathways of most ANME subclades. Here, we investigated the genomic potential and ecophysiology of ANME-2a with respect to metal-dependent AOM in brackish metal-rich coastal sediments. We assembled several high-quality ANME MAGs from subclades with high strain heterogeneity and analyzed the genomic potential for metal-AOM. Additionally, we monitored long-term enrichments with various electron acceptors from the same sediments. Ultimately, we recovered 8 novel genomes of ANME-2a that clustered with an uncharacterized genus with only 2 representatives in public databases for which we propose the name Candidatus Methanoborealis. The analysis of the MAGs showed two different clusters within this genus; one comprising of MAGs from the Baltic Sea that showed high potential for extracellular electron transfer (EET) required for metal-AOM, and another cluster form more diverse environments with less EET potential. The Baltic Sea Ca. Methanoborealis were the only canonical methanotrophs in the incubations during active methane oxidation and metal reduction. Our results contribute to the understanding of the phylogenomic and metabolic diversity in ANME subclades, which will help to further characterize novel ANME lineages from complex sediment samples.