HR: 10:20h
AN: B42B-01    [Abstracts]
TI: Estimates of Biogenic Methane Production Rates in Deep Marine Sediments
AU: * Colwell, F S
EM: fxc@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, Biotechnology Department P.O. Box 1625, Idaho Falls, ID 83415-2203 United States
AU: Boyd, S
EM: boydss@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, Biotechnology Department P.O. Box 1625, Idaho Falls, ID 83415-2203 United States
AU: Delwiche, M E
EM: mde1@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, Biotechnology Department P.O. Box 1625, Idaho Falls, ID 83415-2203 United States
AU: Reed, D W
EM: reeddw@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, Biotechnology Department P.O. Box 1625, Idaho Falls, ID 83415-2203 United States
AB: Much of the methane in natural gas hydrates in marine sediments is made by methanogens. Current models used to predict hydrate distribution and concentration in these sediments require estimates of microbial methane production rates. However, accurate estimates are difficult to achieve because of the bias introduced by sampling and because methanogen activities in these sediments are low and not easily detected. To derive useful methane production rates for marine sediments we have measured the methanogen biomass in samples taken from different depths in Hydrate Ridge (HR) sediments off the coast of Oregon and, separately, the minimal rates of activity for a methanogen in a laboratory reactor. For methanogen biomass, we used a polymerase chain reaction assay in real time to target the methanogen-specific mcr gene. Using this method we found that a majority of the samples collected from boreholes at HR show no evidence of methanogens (detection limit: less than 100 methanogens per g of sediment). Most of the samples with detectable numbers of methanogens were from shallow sediments (less than 10 meters below seafloor [mbsf]) although a few samples with apparently high numbers of methanogens (greater than 10,000 methanogens per g) were from as deep as 230 mbsf and were associated with notable geological features (e.g., the bottom-simulating reflector and an ash-bearing zone with high fluid movement). Laboratory studies with {\it Methanoculleus submarinus} (isolated from a hydrate zone at the Nankai Trough) maintained in a biomass recycle reactor showed that when this methanogen is merely surviving, as is likely the case in deep marine sediments, it produces approximately 0.06 fmol methane per cell per day. This is far lower than rates reported for methanogens in other environments. By combining this estimate of specific methanogenic rates and an extrapolation from the numbers of methanogens at selected depths in the sediment column at HR sites we have derived a maximum estimate of 6 x 10$^{-6}$ nmol methane produced per g sediment per day for samples in which methanogens could not be detected. Rates are likely lower than this if methanogens are not actually present in these samples. Where methanogen numbers are higher in the HR samples rates may be 6 x 10$^{-4}$ nmol methane produced per g sediment per day or higher. Previous reports of higher methanogenic rates in hydrate-bearing sediments (e.g., up to 10$^{3}$ nmol methane produced per g sediment per day in Blake Ridge sediments) may indicate that those samples contain more methanogenic biomass and activity. Our revised estimates of in situ methanogenesis rates will help to improve models intended to predict the location and distribution of hydrates in marine sediments.
DE: 4805 Biogeochemical cycles (1615)
DE: 4806 Carbon cycling
DE: 4820 Gases
DE: 4840 Microbiology
DE: 3022 Marine sediments--processes and transport
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting