Biomarker and 16S rDNA evidence for anaerobic oxidation of methane and related carbonate precipitation in deep-sea mud volcanoes of the Sorokin Trough, Black Sea

Many mud volcanoes were recently discovered in the euxinic bottom waters of the Sorokin Trough (NE Black Sea). Three of them, i.e., NIOZ, Odessa, and Kazakov, were selected for a detailed biogeochemical investigation. Four methane-related carbonate crusts covered with microbial mats, and sediments (‘mud breccia’) from these mud volcanoes were collected during the 11th Training-Through-Research cruise (TTR-11) in 2001, the first finding of methanotrophic microbial mats associated with authigenic carbonates in the deep Black Sea. We measured the concentrations and δ13C values of methane and specific archaeal and bacterial lipids, and determined archaeal and bacterial 16S rRNA gene sequences. The δ13C of the microbial lipids reflected the carbon isotopic values of the methane, indicating that methane was the main carbon source for microorganisms inducing carbonate formation. Anaerobic oxidation of methane (AOM) in these settings was performed by archaea affiliated with the so-called ANME-1 group. None of the identified archaeal sequences were closely related to known methanogens. The combined 16S rRNA gene sequence and biomarker data revealed a distinct difference in archaeal assemblage between the carbonate crusts and mud breccias. Besides gene sequences of sulfate-reducing bacteria, DNA analysis of bacterial communities revealed a diversity of bacteria with apparent contrasting metabolic properties. The methane utilization via AOM processes was detected in the uppermost sediments where it subsequently induces authigenic carbonate precipitation most probably below seafloor. The results of integrated biomarker and 16S rRNA gene study reveal a crucial role of AOM processes in formation of authigenic carbonates in methane seep environments.