Relationships between sulfide-oxidizing bacterial mats and their carbon sources in northern Gulf of Mexico cold seeps

Chemical and isotopic characterization of microbial mats collected from methane and petroleum seeps in the Gulf of Mexico suggest microbial mat color is related to carbon utilization. A survey of white and orange-colored microbial mats collected in the northern Gulf of Mexico from the Brine Pool NR-1 (BP) and a hydrate mound site in Green Canyon 234 (GC 234) during Johnson Sea-Link submersible dives was conducted to explore the bacterial utilization of different carbon sources. Sedimentary organic matter at BP is predominantly derived from biogenic methane whereas organic material available in sediments at GC 234 originates from a mixture of methane and non-methane hydrocarbons. Interstitial sulfide concentrations were significantly elevated in porewater fluids below white mats collected from BP (1.2 ± 0.2 mM) and below orange mats at GC 234 (6.1 ± 0.7 mM). Sulfate reduction rates, ranging from 0.4 to 3.8 μmol SO4 cm−3 d−1, indicate rapid sulfate consumption below GC 234 orange mats. A model based on carbon isotopes was used to delineate possible inorganic and organic carbon sources to BP white mats and GC 234 orange mats. The results suggest the white mats are chemoautotrophic and that the orange mats may employ either heterotrophy or autotrophy.