Multiple microbial guilds mediate soil methane cycling along a wetland salinity gradient - Supplemental Tables

Estuarine wetlands harbor considerable carbon stocks, but rising sea levels could affect their ability to sequester soil carbon as well as their potential to emit methane (CH₄). While sulfate loading from seawater intrusion may reduce CH₄production due to the higher energy yield of microbial sulfate reduction, existing studies suggest other factors are likely at play. Our study of 11 wetland complexes spanning a natural salinity and productivity gradient across the San Francisco Bay and Delta found that while CH₄fluxes generally declined with salinity, they were highest in oligohaline wetlands (ca. 3 ppt salinity). Methanogens and methanogenesis genes were weakly correlated with CH₄fluxes, but alone did not explain the highest rates observed. Taxonomic and functional gene data suggested that other microbial guilds that influence carbon and nitrogen cycling need to be accounted for to better predict CH₄fluxes at landscape scales. Higher methane production occurring near the freshwater boundary with slight salinization (and sulfate incursion) might result from increased sulfate-reducing fermenter and syntrophic populations, which can produce substrates used by methanogens. Moreover, higher salinities can solubilize ionically bound ammonium abundant in the lower salinity wetland soils examined here, which could inhibit methanotrophs and potentially contribute to greater CH₄ fluxes observed in oligohaline sediments.