SREL Reprint #2825




Lipid Biomarkers and Carbon Isotope Signatures of a Microbial (Beggiatoa) Mat Associated with Gas Hydrates in the Gulf of Mexico

Chuanlun L. Zhang1, Zhiyong Huang1, James Cantu2, Richard D. Pancost3, Robin L. Brigmon4,
Timothy W. Lyons5, and Roger Sassen6

1Department of Marine Sciences and Savannah River Ecology Laboratory, University of Georgia
2Center for Biomarker Analysis, The University of Tennessee, Knoxville, Tennessee
3Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Bristol, United Kingdom
4Savannah River Technology Center, Aiken, South Carolina
5Department of Geological Sciences, University of Missouri, Columbia, Missouri
6Geochemical and Environmental Research Group, Texas A&M University, College Station, Texas

Abstract: White and orange mats are ubiquitous on surface sediments associated with gas hydrates and cold seeps in the Gulf of Mexico. The goal of this study was to determine the predominant pathways for carbon cycling within an orange mat in Green Canyon (GC) block GC 234 in the Gulf of Mexico. Our approach incorporated laser-scanning confocal microscopy, lipid biomarkers, stable carbon isotopes, and 168 rRNA gene sequencing. Confocal microscopy showed the predominance of filamentous microorganisms (4 to 5 µm in diameter) in the mat sample, which are characteristic of Beggiatoa. The phospholipid fatty acids extracted from the mat sample were dominated by 16:1ω7c/t (67%), 18:1ω7c (17%), and 16:0 (8%), which are consistent with lipid profiles of known sulfur-oxidizing bacteria, including Beggiatoa. These results are supported by the 168 rRNA gene analysis of the mat material, which yielded sequences that are all related to the vacuolated sulfur-oxidizing bacteria, including Beggiatoa, Thioploca, and Thiomargarita. The δ13C value of total biomass was -28.6%; those of individual fatty acids were -29.4 to -33.7%. These values suggested heterotrophic growth of Beggiatoa on organic substrates that may have δ13C values characteristic of crude oil or on their by-products from microbial degradation. This study demonstrated that integrating lipid biomarkers, stable isotopes, and molecular DNA could enhance our understanding of the metabolic functions of Beggiatoa mats in sulfide-rich marine sediments associated with gas hydrates in the Gulf of Mexico and other locations.

SREL Reprint #2825

Zhang, C. L., Z. Huang, J. Cantu, R. D. Pancost, R. L. Brigmon, T. W. Lyons, and R. Sassen. 2005. Lipid biomarkers and carbon isotope signatures of a microbial (Beggiatoa) mat associated with gas hydrates in the Gulf of Mexico. Applied and Environmental Microbiology 71:2106-2112.

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