Remarkable Capacity for Anaerobic Oxidation of Methane at High Methane Concentration

Anaerobic oxidation of methane (AOM), a central process in the carbon cycle of anoxic environments, moderates the release of methane from soils and sediments to water bodies and, ultimately, the atmosphere. The regulation of AOM in the environment remains poorly constrained. Here we quantified AOM and sulfate reduction (SR) rates in diverse deep seafloor samples at in situ pressure and methane concentration and discovered that, in some cases, AOM exceeded SR rates by more than four times when methane concentrations were above 5 mM. Methane concentration also affected other carbon-cycling processes (e.g., carbon assimilation) in addition to SR. These results illustrate that substantial amounts of methane may be oxidized independent of SR under in situ conditions, reshaping our view of the capacity and mechanism of AOM in methane-rich environments, including the deep biosphere, where sulfate availability is considered to limit AOM

Generation and Utilization of Volatile Fatty Acids and Alcohols in Hydrothermally Altered Sediments in the Guaymas Basin, Gulf of California

Volatile fatty acids (VFAs) and alcohols are key intermediates of anaerobic carbon metabolism, yet their biogeochemical cycling remains poorly constrained in hydrothermal systems. We investigated the abundance, stable carbon isotopic composition, and metabolic cycling of VFAs and alcohols to elucidate their generation and utilization pathways in hydrothermally influenced sediments (4 °C to 90 °C) from the Guaymas Basin. Acetate (up to 229 μM) and methanol (up to 37 μM) were abundant in porewaters. The δ13C values of acetate varied between −35.6‰ and −18.1‰. Carbon isotopic signatures, thermodynamic predictions, and experimental incubations suggested biological sources such as fermentation and acetogenesis for acetate. Acetate and methanol were predominantly consumed by nonmethanogenic processes (e.g., sulfate reduction), as reflected in high oxidation rates versus low methanogenesis rates, and further evidenced through inhibition experiments with molybdate. These results reveal an important role for VFAs and alcohols as energy sources for diverse chemoheterotrophs in organic-rich hydrothermally influenced sediments

Relative importance of methylotrophic methanogenesis in sediments of the Western Mediterranean Sea

Microbial production of methane is an important terminal metabolic process during organic matter degradation in marine sediments. It is generally acknowledged that hydrogenotrophic and acetoclastic methanogenesis constitute the dominant pathways of methane production; the importance of methanogenesis from methylated compounds remains poorly understood. We conducted various biogeochemical and molecular genetic analyses to characterize substrate availability, rates of methanogenesis, and methanogen community composition, and further evaluated the contribution of different substrates and pathways for methane production in deltaic surface and subsurface sediments of the Western Mediterranean Sea. Major substrates representing three methanogenic pathways, including H2, acetate, and methanol, trimethylamine (TMA), and dimethylsulfide (DMS), were detected in the pore waters and sediments, and exhibited variability over depth and between sites. In accompanying incubation experiments, methanogenesis rates from various 14C labeled substrates varied as well, suggesting that environmental factors, such as sulfate concentration and organic matter quality, could significantly influence the relative importance of individual pathway. In particular, methylotrophic and hydrogenotrophic methanogenesis contributed to the presence of micromolar methane concentrations in the sulfate reduction zone, with methanogenesis from methanol accounting for up to 98% of the total methane production in the topmost surface sediment. In the sulfate-depleted zone, hydrogenotrophic methanogenesis was the dominant methanogenic pathway (67–98%), and enhanced methane production from acetate was observed in organic-rich sediment (up to 31%). Methyl coenzyme M reductase gene (mcrA) analysis revealed that the composition of methanogenic communities was generally consistent with the distribution of methanogenic activity from different substrates. This study provides the first quantitative assessment of methylotrophic methanogenesis in marine sediments and has important implications for marine methane cycling. The occurrence of methylotrophic methanogenesis in surface sediments could fuel the anaerobic oxidation of methane (AOM) in the shallow sulfate reduction zone. Release of methane produced from methylotrophic methanogenesis could be a source of methane efflux to the water column, thus influencing the benthic methane budgets

Cold water coral mounds revealed

The discovery of mounds and reefs hosting cold-water coral ecosystems along the northeastern Atlantic continental margins has propelled a vigorous effort over the past decade to study the distribution of the mounds, surface sediments, the ecosystems they host, and their environments. This effort has involved swath bathymetry, remotely operated vehicle deployments, shallow coring, and seismic surveys