Estimates of biogenic methane production rates in deep marine sediments at Hydrate Ridge, Cascadia margin

Abstract

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.

FIG. 1.

(a) Representative QPCR plot comparing fluorescence to cycle number for Methanocaldococcus jannaschii standards and selected samples obtained from HR. (b) Melt curve analysis for the same standards and samples showing the comparison of changes in fluorescence (representative of denatured DNA due to sequence variability) over time as temperature was increased. Standards are amplified from sediments spiked with known concentrations of M. jannaschii cells and are expressed in terms of methanogen cells/g sediment. The analysis of 17 samples from depths of 1.0 to 5.0 mbsf are depicted, and eight of those samples yielded enough fluorescence in the QPCR reactions to be considered positive for the presence of the mcrA gene.

FIG. 2.

Numbers of methanogens in HR sediments plotted against depth (mbsf). Methanogens were enumerated by QPCR using primers directed at the mcrA gene in DNA extracted from the sediments. Symbols shown are means of triplicate QPCR runs performed on a single DNA extract of sediment samples acquired from Ocean Drilling Program sites 1244, 1245, and 1251. Two such extractions were conducted on each sample. The y axis, at 10³ methanogens/g, represents the limit of detection for the QPCR assay. Symbols do not distinguish between samples obtained from the different cores. Dashed lines represent the approximate numbers of total cells in the sediments for sites 1244 and 1245 (line A) and site 1251 (line B) (28).

FIG. 3.

Methanogen numbers (cells per g sediment) as determined by QPCR to detect the mcrA gene at three of the six sites on and near HR. A plan view (A) and a cross section derived by geophysical three-dimensional seismic survey (55) (B) show sites 1245, 1244 (both on the flank of HR), and 1251 (at the foot of HR), from which samples were acquired. Methanogen numbers for each of these three sites are plotted relative to depth in mbsf (C). One sample from site 1245 and two samples from site 1251 yielded higher mcrA gene numbers than all of the other samples, and the values for these genes are shown (converted to methanogen numbers). For each sample, three replicate QPCR analyses were performed for each of two subsamples. The depths of the sulfate-methane interface (SMI), the BSR, and the presence of horizon A (HA; only at site 1245) are marked at the appropriate depths and shown in red. At each location, the zone of hydrate stability extends from the seafloor to the depth of the BSR. Seismic reflections A, B, B′, Y, Y′, U, and AC in panel B are stratigraphic anomolies (55).

FIG. 4.

Results of BRR experiments to determine maintenance levels of activity of methanogens. (A) Methanoculleus submarinus cell numbers in samples obtained from the BRR are plotted against time of reactor operation. Stable cell numbers after 500 h suggest that the cells in the BRR have maintenance levels of activity. (B) Methane accumulation during incubation of BRR-starved M. submarinus cells in serum bottles. Specific maintenance level activity of M. submarinus was determined from these rates and enumerations of the cells in the BRR.