VARIABLE MICROBIAL CONSORTIA PERFORMING ANAEROBIC OXIDATION OF METHANE IN THE BLACK SEA

During a Black Sea research cruise we discovered massive microbial mats growing at the sea floor within the permanently anoxic water column. Huge microbial reefs thrive at active methane gas seeps (d¹³CH₄ –62.4 to –68.3‰ vs. VPDB) at a water depth of 230m and emerge up to 4m from the sediment. These structured build-ups are composed of cm- to dm-thick microbial mats which are internally stabilized by carbonate precipitates. The massive microbial mats are performing combined anaerobic oxidation of methane (AOM) and sulphate reduction (SR). Molecular biological approaches reveal that the consortium mainly consists of densely aggregated archaea (ANME-1 cluster) and sulfate-reducing bacteria (SRB: Desulfosarcina/Desulfococcus-group). Furthermore, the transfer of an active massive AOM-performing microbial mat into the laboratory was successful, which enabled us to investigate in vitro metabolic and biosynthetic activity. Strong ¹³C depletions of total biomass (about –72‰) and of carbonate carbon (about –30‰) indicate methane to be the dominant carbon source in the system. However, cross sections of the chimney like structures displayed concentrically arranged types of macroscopically different microbial mats - carbonate associated brownish green mat (ANME-2 dominated); massive pink mat; gray to black colored outer layer (mainly unknown Archaea). Lipid analyses revealed substantial differences between these three features regarding both, their lipid composition and the carbon isotope signature of individual compounds. To obtain information on the biochemical processes, we incubated living microbial mat material in vitro under strictly anoxic conditions with ¹³C-enriched methane as the sole organic substrate. The microbial mats reduced sulphate rapidly. Moreover, the incorporation of methane-derived ¹³C into the biomass could be observed for the bulk mat as well as for selected lipids. Distinct bacterial and archaeal biomarkers show significant differences in the uptake of ¹³C-carbon revealing the complexity of the microbial composition at that AOM-site and indicating different rates of biomass accumulation for the diverse microbiota.