Methane hydrate found in marine sediments is thought to contain gigaton quantities of methane and is considered an important potential fuel source and climate-forcing agent. Much of the methane in hydrates is biogenic, so models that predict the presence and distribution of hydrates require accurate rates of in situ methanogenesis. We estimated the in situ methanogenesis 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. Quantitative PCR (QPCR) directed at the methyl coenzyme M reductase subunit A gene (mcrA) indicated that 75% of the HR sediments analyzed contained <1,000 methanogens/g. The highest numbers of methanogens were found mostly from sediments <10 m below seafloor. By considering methanogenesis rates for starved methanogens (adjusted to account for in situ temperatures) and the numbers of methanogens at selected depths, we derived an upper estimate of <4.25 fmol methane produced/g sediment/day for the samples with fewer methanogens than the QPCR method could detect. The actual rates could vary depending on the real number of methanogens and various seafloor parameters that influence microbial activity. However, our calculated rate is lower than rates previously reported for such sediments and close to the rate derived using geochemical modeling of the sediments. These data will help to improve models that predict microbial gas generation in marine sediments and determine the potential influence of this source of methane on the global carbon cycle.Subseafloor sediments near continental margins are often rich in dissolved methane as well as methane hydrates where pressures and temperatures are sufficient to maintain gases in this solid form (35,53). Because this methane, whether dissolved in sediment pore waters or present as a hydrate, occupies a large volume globally and is inherently unstable, these formations are of considerable interest as a potential source of energy, a mechanism for climate change, and a factor in seafloor stability (21,34).Much of the methane in subseafloor hydrate formations and the surrounding sediments is biogenic (33), and therefore, the conceptual and computational models describing hydrate occurrence, distribution, and abundance benefit from the knowledge of accurate in situ methane formation rates. With information about the primary biological methane supply, models may be able to provide estimates of the timescale for hydrate accumulation (64) and the transport of methane into the hydrate stability zone, to confine where in the sediments methane production can occur (23), and to simulate the formation of hydrate deposits (17).Determining accurate rates for microbial activities in subsurface environments, such as those that contain hydrates, is difficult. Subsurface microbial activities are believed to occur at exceedingly low ...

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