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Applied and Environmental Microbiology, April 2005, p. 2106-2112, Vol. 71, No. 4
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.4.2106-2112.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

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

Department of Marine Sciences and Savannah River Ecology Laboratory, University of Georgia,¹ Savannah River Technology Center, Aiken, South Carolina,⁴ Center for Biomarker Analysis, The University of Tennessee, Knoxville, Tennessee,² Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Bristol, United Kingdom,³ Department of Geological Sciences, University of Missouri, Columbia, Missouri,⁵ Geochemical and Environmental Research Group, Texas A&M University, College Station, Texas⁶

Received 10 May 2004/ Accepted 29 October 2004

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 16S 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 16:17c/t (67%), 18:17c (17%), and 16:0 (8%), which are consistent with lipid profiles of known sulfur-oxidizing bacteria, including Beggiatoa. These results are supported by the 16S 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 ¹³C 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 ¹³C 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.

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