Abstract
Summary Molecular hydrogen (H 2 ) is the key intermediate in the anaerobic degradation of organic matter. Its removal by H 2 ‐oxidizing microorganisms is essential to keep anaerobic degradation energetically favourable. Sulfate‐reducing microorganisms (SRM) are known as the main H 2 scavengers in anoxic marine sediments. Although the community of marine SRM has been extensively studied, those consuming H 2 in situ are completely unknown. We combined metagenomics, PCR‐based clone libraries, single‐amplified genomes (SAGs) and metatranscriptomics to identify potentially H 2 ‐consuming SRM in anoxic coastal sediments. The vast majority of SRM‐related H 2 ase sequences were assigned to group 1b and 1c [NiFe]‐H 2 ases of the deltaproteobacterial order Desulfobacterales . Surprisingly, the same sequence types were similarly highly expressed in spring and summer, suggesting that these are stable and integral members of the H 2 ‐consuming community. Notably, one sequence cluster from the SRM group 1 consistently accounted for around half of all [NiFe]‐H 2 ase transcripts. Using SAGs, we could link this cluster with the 16S rRNA genes of the uncultured Sva0081‐group of the family Desulfobacteraceae . Sequencing of 16S rRNA gene amplicons and H 2 ase gene libraries suggested consistently high in situ abundance of the Sva0081 group also in other marine sediments. Together with other Desulfobacterales these likely are important H 2 ‐scavengers in marine sediments.