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Geology; March 2002; v. 30; no. 3; p. 239-242; DOI: 10.1130/0091-7613(2002)030<0239:LACIEO>2.0.CO;2
© 2002 Geological Society of America
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Lipid and carbon isotopic evidence of methane-oxidizing and sulfate-reducing bacteria in association with gas hydrates from the Gulf of Mexico

Chuanlun L. Zhang*,1, Yiliang Li*,1, Judy D. Wall*,2, Lise Larsen*,2, Roger Sassen*,3, Yongsong Huang*,4, Yi Wang*,4, Aaron Peacock*,5, David C. White*,5, Juske Horita*,6 and David R. Cole*,6

1 Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA
2 Department of Biochemistry, University of Missouri, Columbia, Missouri 65211, USA
3 Geochemical & Environmental Research Group, Texas A&M University, College Station, Texas 77845, USA
4 Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
5 Institute for Applied Microbiology, University of Tennessee, Knoxville, Tennessee 37932, USA
6 Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

An integrated lipid biomarker–carbon isotope approach reveals new insight to microbial methane oxidation in the Gulf of Mexico gas-hydrate system. Hydrate-bearing and hydrate-free sediments were collected from the Gulf of Mexico slope using a research submersible. Phospholipid fatty acids consist mainly of C16–C18 compounds, which are largely derived from bacteria. The phospholipid fatty acids suggest that total biomass is enhanced 11–30-fold in gas-hydrate–bearing sediment compared to hydrate-free sediment. Lipid biomarkers indicative of sulfate-reducing bacteria are strongly depleted in 13C ({delta}13C = –48{per thousand} to –70{per thousand}) in the hydrate-bearing samples, suggesting that they are involved in the oxidation of methane ({delta}13C = –47{per thousand} for thermogenic methane and –70{per thousand} for biogenic methane). Isotopic properties of other biomarkers suggest that sulfur-oxidizing bacteria (Beggiatoa) may also contribute to the lipid pool in hydrate-bearing samples, which are characterized by less negative {delta}13C values (to –11.2{per thousand}). In the hydrate-free sample, fatty acid biomarkers have {delta}13C values of –27.6{per thousand} to –39.6{per thousand}, indicating that crude oil (average ~–27{per thousand}) or terrestrial organic carbon (average ~–20{per thousand}) are the likely carbon sources. Our results provide the first lipid biomarker–stable isotope evidence that sulfate- reducing bacteria play an important role in anaerobic methane oxidation in the Gulf of Mexico gas hydrates. The coupled activities of methane-oxidizing and sulfate-reducing organisms contribute to the development of ecosystems in deep-sea environments and result in sequestration of carbon as buried organic carbon and authigenic carbonates. These have implications for studying climate change based on carbon budgets.

Key Words: Gulf of Mexico • gas hydrates • anaerobic methane oxidation • sulfate-reducing bacteria • lipid biomarkers • carbon isotopes




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