Community succession and hydrocarbon oxidation in marine microbial mats – an in-situ time series experiment. David L. Valentine, Department of Earth Science, University of California, Santa Barbara, CA 93106 Benthic microbial mats occur in cold-seep environments and have long been associated with the production of sulfide in the subsurface. These mats are composed of chains of filamentous bacteria capable of precipitating elemental sulfur as intracellular inclusions, and generate energy through oxidation of reduced sulfur compounds. A C D B We have conducted an in-situ time series experiment in which 16 modular surfaces (A) were placed on the sea floor overlying an active gas seep at a depth of 20 m. Mats grew rapidly on the surfaces (B) and one module was collected each week. Isotopic shifts (C) indicate succession from sulfide oxidizing communities to novel methane oxidizing communities (D), followed by oscillation between the two end member communities. Both CO2 and CH4 are implicated as important carbon sources to these microbial communities.