Geochemical Control of Methanogenesis in Cape Lookout Bight, North Carolina

Hydrogen exerts thermodynamic control over the exclusion of methanogens by sulfate reducers in Cape Lookout Bight, NC, marine sediments. This has been demonstrated by previous in situ measurements, but has never been demonstrated in a batch incubation of unamended sediments and has never been combined with identification of the microorganisms involved in this process. We made triplicate anoxic incubations of sediments from the upper 3 cm of sediment over 122 days while taking weekly samples for DNA extraction, cell counts, and measurements of methane, sulfate, and hydrogen. The headspaces of the bottles were initially gassed with nitrogen and the third was subsequently gassed with methane, although the methane disappeared within the first two weeks and after that the incubation served as a third replicate. While sulfate was present, the hydrogen concentration was maintained below 2 nM. Hydrogen started to rise as sulfate concentrations fell below 3 mM, coinciding with a small increase in methane. Only when sulfate has been depleted and the hydrogen concentrations rise was methane continuously produced. Quantitative PCR (qPCR) suggests that Methanosarcinales and Methanomicrobiales increase when sulfate is depleted in all three incubations. 16s rRNA gene Miseq tag libraries support the increase of these methanogens as well as a novel archaeal group, Kazan 3A-21, and sulfur-oxidizing bacteria. qPCR and tag libraries show that the methanogen-like archaea, ANME-1, increase during early methanogenesis, but the values are near detection limits and are therefore noisy. The tag libraries suggest that sulfate-reducing bacteria maintain similar population levels throughout the sulfate reduction phase, decrease as sulfate is depleted, and then rebound during the methanogenic phase. This most likely signifies a switch from sulfate reduction to syntrophic fermentation of organic matter with methanogens. Total cell counts demonstrate a decline in cells with the decrease of sulfate until a recovery corresponding with production of methane. Our results suggest that competition for hydrogen influences what metabolic processes can occur in marine sediments and that a diversity of sulfate reducers and methanogens are involved in this competition.