THE ANAEROBIC OXIDATION OF METHANE - A REEF-FORMING PROCESS IN THE BLACK SEA

Below a stable chemocline located at a water depth of about a 100 m, the Black Sea contains the largest marine anoxic water body worldwide. Moreover, within waters and sediments, methane is present in very high concentrations and acts as an important substrate for phylogenetically different anaerobic and aerobic microorganisms. Even though the upper part of the water column is still 2 to 5-fold supersaturated compared to atmospheric methane concentrations, the anaerobic oxidation of methane (AOM) and the aerobic methanotrophy form a microbial barrier suggested to consume >98% of the methane released to the Black Sea. In total, net methane oxidation is by far highest in the large anoxic water body, but high aerobic methane oxidation rates also occur within the oxic-anoxic transition zone indicated (i) by a steep upward decrease in methane concentration and (ii) enrichments in ¹³C of the dissolved methane, and (iii) a strong increase of biomarkers specific for methanotrophic bacteria. The specific geo- and hydrological setting of permanently anoxic waters overlying methane emanating sediments, lead to the formation of unique AOM-performing microbial reefs growing at the sea floor of the Black Sea. These build-ups thrive at active methane gas seeps at a water depth of 230 m, emerge up to 4 m from the sediment, and are composed of structured cm- to dm-thick microbial mats internally stabilized by carbonate precipitates. Lipid biomarker and molecular biological approaches reveal that, among others, the microbial consortium mainly consists of densely aggregated archaea (ANME-1 cluster) and sulfate-reducing bacteria of the Desulfosarcina/Desulfococcus-group. In this presentation we show that the recent anoxic Black Sea offers a chance for tracking anaerobic methane carbon cycling in Earth history. Comparison with fossil seep deposits reveal that the microbial recycling of methane carbon has been an important subsystem for the turnover of buried organic carbon at least down to the late Jurassic. However, the deep branching of AOM-performing microbes in the phylogenetic tree of life suggests AOM as an important process throughout the whole life history and in fact, analyses of genomic radiations among prokaryotic evolution hint to an age of anaerobic methanotrophy of more than 3 billion years.