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2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 16
Presentation Time: 5:15 PM

HYDROGEN ISOTOPE FRACTIONATION IN ANAEROBIC MICROBES


VALENTINE, David L.1, CHIDTHAISONG, Amnat2, SESSIONS, Alex3, RICE, Andrew4, REEBURGH, William S.5 and TYLER, Stanley C.5, (1)Geological Sciences, Univ of California, Santa Barbara, CA, (2)The Joint Graduate School of Energy & Environment, King Mongkut’s Univ of Technology Thonburi, Bankok, (3)Geological and Planetary Sciences, Caltech, Mail Code 100-23, Pasadena, CA 91125, (4)Joint Institute for the Study of the Atmosphere and Ocean, Department of Oceanography, Univ of Washington, Seattle, WA, (5)Earth System Science, Univ of California, Irvine, CA, valentine@geol.ucsb.edu

A series of laboratory studies were conducted to increase understanding of stable hydrogen (D/H) isotope fractionation arising from the anaerobic microbial processes of methanogenesis and homoactogenesis.

Studies of H2/CO2 methanogenesis were conducted in a flow-through system with the moderately-thermophilic Methanothermobacter marburgensis. The fractionation of hydrogen isotopes between source H2O and CH4 was found to range from 127 to 275‰. Fractionation was dependent on growth phase with greater fractionation associated with later growth stages. The maximum observed fractionation was 275‰, independent of the d2H-H2 supplied to the culture. Fractionation was positively correlated with temperature and/or metabolic rate. Results demonstrate significant variability in hydrogen isotope fractionation during methanogenesis from H2/CO2. The relatively-small fractionation associated with deuterium during H2/CO2 methanogenesis provides an explanation for the relatively-enriched deuterium content of biogenic natural gas originating from a variety of thermal environments.

Studies of H2/CO2 homoacetogenesis were conducted in a flow-through system with the mesophilic Sporomusa sp. Four cultures were grown, each in mineral medium of distinct water deuterium content. The deuterium content of membrane and cell wall lipids were analyzed and compared to the deuterium content of the growth medium. Results indicate only 60% of the lipid H comes from water, with the remainder coming from another source, possibly H2.

In each of the above studies the isotopic composition of H2 fed into the reactor was found to become more depleted as a result of microbial activity (net consumption of H2). These results are attributed to the uptake and subsequent release of H2 by the hydrogenase systems. Using a modeling approach we are able to constrain the overall efficiency of electron consumption by way of the hydrogenase systems in M. marburgensis and Sporomusa sp. to be less than 50%. In some cases the efficiency of the hydrogenase system is as low as 1%. This reversibility is likely to impact the isotopic composition of cellular H2 and possibly the isotopic content of biomarkers associated with H2 metabolism in nature.

Session No. 93

Geomicrobiology: Microbes, Minerals, and the Natural Environment I
Monday, 3 November 2003: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol. 35, No. 6, September 2003, p.248
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