Anaerobic oxidation of methane above gas hydrates at Hydrate Ridge, NE Pacific Ocean

At Hydrate Ridge (HR), Cascadia convergent margin, surface sediments contain massive gas hydrates formed from methane that ascends together with fluids along faults from deeper reservoirs. Anaerobic oxidation of methane (AOM), mediated by a microbial consortium of archaea and sulfate-reducing bacteria, generates high concentrations of hydrogen sulfide in the surface sediments. The production of sulfide supports chemosynthetic communities that gain energy from sulfide oxidation. Depending on fluid flow, the surface communities are dominated either by the filamentous sulfur bacteriaBeggiatoa(high advective flow), the clamCalyptogena(low advective flow), or the bivalveAcharax(diffusive flow). We analyzed surface sediments (0 to 10 cm) populated by chemosynthetic communities for AOM, sulfate reduction (SR) and the distribution of the microbial consortium mediating AOM. Highest AOM rates were found at theBeggiatoafield with an average rate of 99 mmol m–2d–1integrated over 0 to 10 cm. These rates are among the highest AOM rates ever observed in methane-bearing marine sediments. At theCalyptogenafield, AOM rates were lower (56 mmol m–2d–1). At theAcharaxfield, methane oxidation was extremely low (2.1 mmol m–2d–1) and was probably due to aerobic oxidation of methane. SR was fueled largely by methane at flow-impacted sites, but exceeded AOM in some cases, most likely due to sediment heterogeneity. At theAcharaxfield, SR was decoupled from methane oxidation and showed low activity. Aggregates of the AOM consortium were abundant at the fluid-impacted sites (between 5.1 × 1012and 7.9 × 1012aggregates m–2) but showed low numbers at theAcharaxfield (0.4 × 1012aggregates m–2). A transport-reaction model was applied to estimate AOM atBeggiatoafields. The model agreed with the measured depth-integrated AOM rates and the vertical distribution. AOM represents an important methane sink in the surface sediments of HR, consuming between 50 and 100% of the methane transported by advection.