EXPERIMENTALLY DETERMINED RATES OF METHANOGENESIS IN METHANE HYDRATE-BEARING SEDIMENTS

Hydrate modelers require data on in situ rates of methanogenesis and abiotic controls on these rates. For this work we estimated the in situ methane production rates in Hydrate Ridge (HR) sediments by coupling experimentally derived minimal rates of methanogenesis to methanogen biomass determinations for discrete locations in the sediment column. When starved in a biomass recycle reactor Methanoculleus submarinus produced ca. 0.017 fmol methane/cell/day. A real time polymerase chain reaction assay that targeted the methanogen-specific mcr gene indicated that 75% of the HR samples analyzed contained <100 methanogens/g of sediment. Samples with higher numbers of methanogens were either from sediments <10 meters below seafloor or from a few deeper locations that appeared to be associated with notable geological features such as the boundary of gas hydrate and free gas (known as the bottom-simulating reflector or BSR), and an ash-bearing zone with high fluid movement. Acetate concentrations in sample porewaters ranged from 3.17 to 2515 mM with the highest concentrations measured just above the BSR at a control site relative to HR. Locally high concentrations of acetate were evident at all BSRs, and often corresponded with gas hydrate presence. High acetate concentrations sometimes corresponded with low methanogen biomass. Acetate concentrations were typically lowest near the seafloor. Porewater hydrogen concentrations ranged from 16.45 to 1036 parts per million by volume (ppmv). Sometimes hydrogen and acetate concentrations were elevated concurrently. By combining methanogenesis rates for starved cells and the numbers of methanogens at selected depths we derived a maximum estimate of 1.7 x 10^-6 nmol methane produced/g sediment/day where methanogens could not be detected, a rate lower than previous methanogenic rate estimates in hydrate-bearing sediments. Where methanogen numbers are higher the rates may exceed 7.4 x 10^-2 nmol methane produced/g sediment/day. These data will improve models intended to predict the location of hydrates in marine sediments, the amount of biogenic gas that can accumulate in the sediments, and the potential influence that this microbially-mediated source of methane has on the global carbon cycle.