Environmental Controls on Microbe-Methane Dynamics in the Terrebonne-Timbalier Estuary, Louisiana

Current estimates of methane (CH₄) emissions from coastal salt marshes, produced primarily from soil microbial processes, are underrepresented in global carbon budgets and climate models. Limited representation results from difficulty gathering belowground CH₄ measurements, with associated soil and microbial constraints. The Terrebonne-Timbalier Estuary of Louisiana (USA) is an ideal setting to study landscape controls on microbial CH₄ dynamics. A Wardenaar Peat Profile Sampler (Eijkelkamp) was used to extract intact soil monoliths up to 1 m depth from approximately 63 km of coastline. The monoliths provided large sample volumes for up to 26 different yet coordinating analyses from the same depth increments. Analyses spanned from in situ soil physicochemistry to Rhizon-extracted porewater and dissolved gas measurements, as well as microbial community characterization. Sites with higher porewater salinity had higher sulfateconcentrations inversely and non-linearly correlated to porewater CH₄ concentrations,which supported the widely accepted paradigm that high sulfate concentrations suppresses methanogenesis.Significant correlations among porewater CH₄ to total nitrogen, ammonium, and acetate concentrations suggested a strong link between microbial CH₄ and N cycling. The total abundance of archaea ranged from 10^6.8 to 10^8.1 per gram of soil, with copies of the mcrA gene, used to estimate the abundance of putative methanogens, ranging from 10^4.9 to 10^7.3 per gram of soil. Salinity also significantly correlated to alpha diversity indices and the top five methanogenic operational taxonomic units (OTUs). Methanofastidiosales and Methanomassiliicoccales, both methylotrophic methanogens, had the most abundant and shared OTUs from multiple locations and soil depths and regardless of higher salinity or sulfate concentrations, thereby providing evidence that methylotrophic methanogens may be contributing to CH₄ cycling in the tidal marsh soils. This study adds important new taxonomic information about archaeal diversity from tidal marsh soils and provides a rich dataset to explore in more detail in the future. The combined data of porewater chemistry, soil characteristics, and microbiome data improve our understanding of “bottom-up” processes that affect spatiotemporal inventories of CH₄ and cycling. The results should also provide more robust data at the microbial scale for climate models that predict changes in CH₄ emissions over time from coastal marshes.