Chemosynthetic bacterial mats at cold hydrocarbon seeps, Gulf of Mexico continental slope

Abstract

White and pigmented filamentous bacterial mats dominated by several undescribed species of Beggiatoa were sampled during research submersible dives to cold hydrocarbon seep sites on the upper continental slope off Louisiana (130–550 m). Mats occur at the interface between reducing sediments and the oxygenated water column. They are localized at sea floor features related to seepage of biogenic methane and crude oil, but there is little evidence that the organisms utilize the hydrocarbons directly. Granules of elemental sulfur (S 0) are visible within cells of Beggiatoa, and mat material is characterized by high contents of S 0 (up to 193,940 ppm). The Beggiatoa biomass is isotopically light ( δ 13 C = −27.9‰ PDB ). Our geochemical data suggest that the Beggiatoa species are part of a complex bacterial assemblage in cold seep sediments. They oxidize H 2S derived from the bacterial sulfate reduction that accompanies bacterial hydrocarbon oxidation when O 2 is depleted in sediments, and fix isotopically light carbon from CO 2 that is the result of bacterial hydrocarbon oxidation. Beggiatoa mats appear to retard loss of hydrocarbons to the water column by physically retaining fluids in sediments, a function that could enhance production by other bacteria of the H 2S and CO 2 needed by Beggiatoa.