Abstract
Methane, which contributes to global warming, is one of the microbial end-products of anaerobic organic matter remineralization in anoxic sedimentary environments. Estuary sediment with high primary production is known as the most active methane source because the enhanced microbial activity ultimately releases metabolic products such as methane and nutrients to the overlying water. To date, microbial methane production was thought to be phylogenetically limited to the archaeal phylum Euryarchaeota . However, advances in metagenomics-based studies have revealed that unsuspected lineages of archaea may contain key enzymes for methanogenesis, which expanded our view of methane metabolism. In this study, brackish water sediments of the Onga River, northern Kyushu, Japan were examined to assess the diversity of methanogenic archaea, through comparative phylogenetic analysis of 16S rRNA genes and functional genes necessary for methane production. Members of Methanosarcinales were only detected in the 16S rRNA gene amplicons, while the taxonomic classification of the functional genes showed the existence of diverse archaeal lineages, particularly hydrogenotrophic Methanomicrobiales . Furthermore, yet-uncultivated methanogens such as Verstraetearchaeota and Methanofastidiosales were also detected, indicating the importance of hydrogen-dependent methane generation from methanol, methylated amines, and dimethyl sulfide. Estuary methane production might be conducted by more phylogenetically widespread archaea than previously thought. Therefore, clone library analysis for the functional genes showed the potential to assess the diversity of methanogens not identified by the 16S rRNA gene amplicon analysis, which may improve our understanding of methane origins in estuary systems.
https://ror.org/03mfefw72 
