BIOGEOCHEMICAL SIGNATURES AND COMMUNITY STRUCTURE OF A MICROBIAL (BEGGIATOA) MAT IN THE GAS HYDRATE SYSTEM OF GULF OF MEXICO

Exposed gas hydrates and adjacent sediments in the Gulf of Mexico are often covered by patches of microbial mats. These mats are dominated by sulfide-oxidizing bacteria such as Beggiatoa, which play important roles in carbon and sulfur cycling linked to anaerobic methane oxidation and sulfate reduction underneath the mat layers. However, the community structure and major pathways for carbon cycling within the mats are not well understood. Our approach for achieving such an understanding is an integration of lipid biomarkers, stable sulfur and carbon isotopes, and molecular DNA. During a cruise in summer 2002, an orange mat was collected using a suction device equipped on the Johnson-Sea-Link submersible at the site GC 234. An effort was made to avoid collecting sediments from beneath the mat. One portion of the mat material was used for lipid and isotope analyses and the other for 16S rRNA. The phospholipids fatty acids (PLFA) in the mat are dominated by 16:1w7 (67%), 18:1w7 (17%), and 16:0 (8%). These are different from PLFA profiles in the underlying sediments of the same site, which have considerable amount of branched fatty acids (43-58%) likely derived from sulfate-reducing bacteria. Preliminary PCR products show the presence of both bacteria and archaea. Experiments are ongoing to determine the sulfur isotopes of elemental sulfur within the cells of sulfide-oxidizing bacteria and carbon isotopes of bacterial fatty acids as well as archaeal biomarkers. Species diversities of the bacteria and archaea are also being determined using DGGE and clone libraries. The results should provide valuable information on the ecological function of microbial mats in the gas hydrate system. For example, the sulfur isotopes may provide insight into the cycling of sulfur between sulfide oxidation in the mats and sulfate reduction underneath the mats. Lipid biomarkers and isotopic signatures of the mat community may allow distinction of organic contribution from the mat biomass to the underlying sediments, where microbial biomasses are driven by sulfate-reducing bacteria and archea through anaerobic oxidation of methane and other hydrocarbons.