Evidence of cryptic methane cycling in sulfate-reducing sediments of the Santa Barbara Basin

Abstract

<p>             Methane in anoxic marine sediments comes primarily from microbial methanogenesis. Methanogenesis is facilitated by groups of obligate anaerobic archaea and is the last step in carbon remineralization according to the redox cascade. Before the methane is emitted into the water column and ultimately the atmosphere, where is acts as a potent greenhouse gas, a large portion (~90%) of the methane is consumed by anaerobic oxidation of methane (AOM). In anoxic marine sediments AOM is typically mediated by a consortium of methanotrophic archaea and sulfate-reducing bacteria to oxidize methane to inorganic carbon within a sediment layer classically known as the sulfate-methane transition zone (SMTZ). Organic matter in sediments above the SMTZ is consumed by organoclastic sulfate reduction, which thermodynamically outcompetes methanogenesis for hydrogen and acetate. However, methanogenesis can persist in sulfate-reducing environments with non-competitive substrates such as methylamines, which are produced from the microbial degradation of glycine betaine and dimethylsulfoniopropionate. Methanogenesis from methylamine can directly fuel AOM, now known as the “cryptic methane cycle”, in sulfate-reducing sediments. The cryptic methane cycle above the SMTZ is still poorly understood. Here we will present our preliminary research that shows evidence of cryptic methane cycling in sulfate-reducing sediments of the organic-rich Santa Barbara Basin (SBB).</p><p>            We sampled the top 10-15cm of sediments at 5 stations along a depth transect across the basin. Sediment samples were subjected to radioisotope incubations with <sup>14</sup>C-methane, <sup>14</sup>C-mono-methylamine, and <sup>35</sup>S-sulfate, gas chromatography, and porewater geochemical and metabolomic analysis. Porewater methane concentrations ranged from 3 to 13 µM. Metabolomic analysis of porewater using nuclear magnetic resonance for mono-methylamine concentrations found evidence of mono-methylamine presence below the quantification limit (< 3 µM). Results from the radiotracer incubations with <sup>14</sup>C-methane detected ex-situ AOM rates at all 5 stations, where the highest rates were found within the top 1 cm. Integrated AOM (0-11cm) activity decreased with station water depth. <sup>14</sup>C-mono-methylamine incubations revealed concurrent methanogenesis and AOM from mono-methylamine in the presence of sulfate reduction at all 5 stations. These results indicate evidence of potential cryptic methane cycling near the sediment-water interface in the Santa Barbara Basin.</p>