Deciphering the microbial contributors to methane cycling in coastal wetlands

Although wetlands are increasingly recognized as important contributors to the global methane budget, the microorganisms and processes involved methane cycling are poorly characterized, particularly in coastal brackish and saline systems. Here, we investigated microbial and geochemical factors contributing to methane dynamics in three coastal wetlands with different salinities, dominant vegetation types, and soil chemical characteristics. These included a freshwater flotant marsh, a cypress swamp, and a mesohaline salt marsh. Specifically, we paired methane porewater concentrations, surface fluxes, geochemistry, and 16S rRNA gene sequencing to address how microbial community composition links to porewater concentrations and its potential effects on emissions. We found that porewater methane concentrations across sites were the highest in the swamp, followed by the salt marsh and the flotant marsh, and were explained by methanogen richness and abundance. While methane-cycling microbial communities were significantly structured by salinity, two microbial taxa (Methanosaeta and Methanomicrobiaceae) were present across all sites. Hydrogenotrophs were the most abundant methanogen functional group, with Methanomicrobiaceae and Methanobacterium discriminant among wetlands. In contrast, methanotroph functional types varied among wetlands. Type I dominated the freshwater flotant marsh, while the anaerobic methanotrophic archaea the saltwater marsh. These findings contribute to an enhanced understanding of the microbiological contributions to methane emissions from coastal wetlands.