Biogeochemical signatures and microbial activity of different cold-seep habitats along the Gulf of Mexico deep slope

Abstract

Microorganisms and the processes they mediate serve as the metabolic foundation of cold seeps. We characterized a suite of biogeochemical constituents and quantified rates of two key microbial processes, Sulfate Reduction (SR) and Anaerobic Oxidation of Methane (AOM), to assess variability between habitats at water depths exceeding 1000 m in the northern Gulf of Mexico. Rates of SR were highest in sediments beneath microbial mats, lower in brine-influenced and oil-influenced sediments, and lowest in animal habitats. Sediments collected near tubeworms had the highest SR rates for animal habitats. Rates of AOM generally were low, but higher rates were associated with brine-influenced, oil-influenced, tubeworm- and urchin-inhabited sediments. Rates of both SR and AOM were orders of magnitude lower at deep-slope sites compared to upper-slope sites examined previously. As observed at upper-slope sites, SR and AOM rates were often loosely coupled. At one site, AOM rates exceeded SR rates, suggesting that an alternate electron acceptor for AOM is possible. Extremely depleted δ13C values in methane illustrated the broad significance of biogenic methane production at deep-slope sites. Brine-influenced habitats were characterized by extremely high concentrations of ammonium and dissolved organic carbon, serving as important focused sources of these chemicals to adjacent environments.