HR: 1340h
AN: B43B-0153    [Abstracts]
TI: Phospholipid-Derived Fatty Acids and Their Stable and Radiocarbon Isotope Values as Indicators of Bacterial Methane Oxidation at a Thermogenic Methane Seep
AU: * Mills, C T
EM: chmills@mines.edu
AF: Department of Chemistry and Geochemistry, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401 United States
AU: Dias, R F
EM: rfdias@odu.edu
AF: Department of Chemistry and Biochemistry, Old Dominion University, 4541 Hampton Boulevard, Norfolk, VA 23529 United States
AU: Slater, G F
EM: gslater@mcmaster.ca
AF: School of Geography and Geology, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4K1 Canada
AU: Reddy, C M
EM: creddy@whoi.edu
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, MS#4, Woods Hole, MA 02543 United States
AU: Mandernack, K W
EM: kmandern@mines.edu
AF: Department of Chemistry and Geochemistry, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401 United States
AB: The importance of aerobic methanotrophy as a filter for biogenic methane emissions is well documented for environments such as natural wetlands, landfills, and rice paddies, but less is known about methane oxidation in soils overlying thermogenic methane seeps. We are utilizing phospholipid-derived fatty acids (PLFAs) extracted from soils overlying a high-rate, coal-bed methane seep in Southwestern Colorado to investigate the location and extent of bacterial methane consumption within the soil column. PLFAs have been widely used as indicators of both quantities and types of viable bacterial populations. Two specific PLFAs, 16:1$\omega$8 and 18:1$\omega$8, appear to be unique to type I and type II methanotrophs, respectively. We have detected higher abundances of these methanotroph biomarkers in surface soils ($\sim$0-30 cm) under wetter soil conditions and near the water table ($\sim$150 cm) under drier conditions. Maximum concentrations of both type I and type II methanotroph PLFA biomarkers were greatest in the shallow soils during wetter conditions with the type I maximum located just above the type II maximum. This is consistent with pure culture studies that have shown type I methanotrophs to prefer higher oxygen, lower methane conditions and type II methanotrophs to prefer lower oxygen, higher methane conditions. Soil gas methane concentrations during this wetter period were approximately 30$%$ at 20 cm depth and 80$%$ at 100 cm depth. During a drier period a type II methanotroph biomarker maximum was observed near the water table but no type I maximum was observed. Soil gas methane concentrations at this time were less than 1$%$ at 20 cm and 25$%$ at 100 cm. These data suggest that methanotrophs may consume a significant fraction of the methane as it rises through the soil column. Greater saturation of soil pore spaces during wetter conditions may inhibit atmospheric oxygen diffusion into deeper soils forcing methanotrophs to reside in the shallow soils and resulting in larger amounts of methane being released to the atmosphere. Both stable and radiocarbon values of detected PLFAs suggest that methane accounts for a significant fraction of the microbial carbon source in soils overlying the methane seep, especially in the deeper soils where organic matter carbon sources are more scarce.
DE: 1030 Geochemical cycles (0330)
DE: 1055 Organic geochemistry
DE: 0315 Biosphere/atmosphere interactions
DE: 0330 Geochemical cycles
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting