Drivers of methane-cycling archaeal abundances, community structure, and catabolic pathways in continental margin sediments

Mark Alexander Lever ^1* Longhui Deng ² Damian Bölsterli ³ Clemens Glombitza ⁴ Bo Barker Jørgensen ⁵ Hans Røy ⁵

• ¹ Marine Science Institute, College of Natural Sciences, The University of Texas at Austin, Port Aransas, Please select one, United States
• ² School of Oceanography, Shanghai Jiao Tong University, Shanghai, China
• ³ Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Sciences, ETH Zurich, Zurich, Zürich, Switzerland
• ⁴ Swiss Federal Nuclear Safety Inspectorate, Brugg, Switzerland
• ⁵ Section for Microbiology, Department of Bioscience, Faculty of Technical Sciences, Aarhus University, Aarhus, Capital Region of Denmark, Denmark

Marine sediments contain Earth's largest reservoir of methane, with most of this methane being produced and consumed in situ by methane-cycling archaea. While numerous studies have investigated communities of methane-cycling archaea in hydrocarbon seeps and sulfatemethane transition zones, little is known about how these archaea change from the seafloor downward in diffusion-dominated marine sediments. Focusing on four continental margin sites of the North Sea-Baltic Sea transition, we here investigate the in situ drivers of methane-cycling archaeal community structure and metabolism based on geochemical and stable carbonisotopic gradients, functional gene (mcrA) copy numbers and phylogenetic compositions, and thermodynamic calculations. We observe major changes in community structure that largely follow vertical gradients in sulfate concentrations and lateral gradients in organic carbon reactivity and content. While methane-cycling archaeal communities in bioturbated and sulfatic zones are dominated by known methyl-disproportionating Methanosarcinaceae and putatively CO2-reducing Methanomicrobiaceae, the communities change toward dominance of methane-oxidizing taxa (ANME-2a-b, ANME-2c, ANME-1a-b) in sulfate-methane transition zones (SMTZs). By contrast, the underlying methanogenesis zones are dominated by the physiologically uncharacterized ANME-1d, new genus-level groups of putatively CO2reducing Methanomicrobiaceae, and methyl-reducing Methanomassiliicoccales. Notably, mcrA copy numbers of several major taxa increase by 2 to 4 orders of magnitude from the sulfatic zone into the SMTZ or methanic zone, providing evidence of net population growth in subsurface sediment. We propose that burial-related geochemical changes cause methanecycling archaea in continental margin sediments to go through three successional stages (sulfatic, SMTZ, methanic). Herein, the onset of each new successional stage is characterized by a period of growth-and mortality-driven turnover in the dominant taxa.