Investigating Geochemical Niche-Separation in Methanotrophic Archaea at a Recently-Discovered, Shallow Cold Seep System

In 2012, a series of >500 previously undiscovered cold seeps were detected along the passive U.S. Atlantic Margin (USAM) at depths of 500-1500 meters. These seeps-rather than being fed by seabed fractures-result from dissociating methane hydrates at the upper limit of the gas hydrate stability zone (Skarke et al., 2014). Current and future episodes of bottom water warming therefore threaten to intensify methane seepage along the USAM, highlighting the need to understand the indigenous microbial communities mitigating methane emissions. In this study, we characterize microbial community composition, distribution, and relative activity at three of the USAM seeps using 16S rRNAand mcrAgene sequencing of DNA and RNA. We also analyze the coincident porewater geochemistry (CH₄, SO₄^2-, S^2-, NH₄⁺, NO₃^-, etc.) and investigate potential physicochemical drivers of the distribution of methanotrophic populations. We find that subgroups of the anaerobic methanotrophic (ANME) archaea are heterogeneously distributed across the sites, suggesting distinct geochemical niches. In particular, the relative abundance of ANME-1s versus ANME-2s appears to be driven by the concentration of methane in porewater-consistent with studies at other seep systems (Knittel & Boetius, 2009; Yanagawa et al., 2011). Ongoing manipulation experiments with seep sediment from Monterey Bay, CA aim to determine whether methane concentration alone can differentially affect ANME subgroup activity. Our combined observational and experimental results will help predict which subgroups are likely to colonize new and existing seeps, and thus influence future methane fluxes from the ocean.