FABRICS OF METHANE CARBONATES AND ASSOCIATED MICROBIAL COMMUNITIES IN BLACK SEA COLD SEEP ENVIRONMENTS

Cold seeps on the NW shelf and slope close to the Dniepr Canyon of the Black Sea were investigated during the GHOSTDABS expedition 2001. In anoxic waters, between 200 and 400m, we found different modes of carbonate precipitation, such as chimney-like buildups projecting up to 4 m into the water column, lenticular concretions abundantly forming within the sediment, and gas bubble-rich lithified sediments. Isotope analyses of the concretionary Mg-calcite yielded d13C values as low as –31‰ PDB, suggesting that the carbonate predominantly derives from the anaerobic oxidation of methane (AOM). The concretions are surrounded by more or less orange colored microbial mats which apparently mediate the formation of ca. 100µm sized aggregates of fibrous calcite that fuse together to form the concretions. Surrounding sediment and concretions are distinguishable by a strong UV-epifluorescence induced by large amounts of organic matter enclosed in the calcite aggregates. The conspicious angular arrangement of the crystallites appears to be controlled by the spatial organization of extracellular polymeric substances (EPS). Fluorescence in situ hybridization, TEM, and field emission electron microscopy reveal that the microbial mats harbour numerous types of microorganisms, associated in different kinds of consortia. Prominent members are large colonies of sulfate reducing bacteria(SRB) (DSS658 FISH-probe) including greigite-forming magnetotactic SRB, surrounded by sheeted, rod-shaped archaea (ANME-1). The metabolism of SRB apparently accounts for the observed, significant enrichment of the concretions in framboidal Fe-sulfides (greigite & pyrite). Organic extracts from mat samples and carbonates contain isoprene-based membrane lipids derived from archaea, and carbon skeletons of bacterial origin, both strongly depleted in 13C. These patterns illustrate an anabolic uptake of methane carbon by the source biota, further pointing to AOM by sulfate reduction as the relevant process controlling the formation of these methane carbonates.