A novel microbially diverse type of 1- to 5-cm-thick mat performing anaerobic oxidation of
methane (AOM) and covering several square metres of the seafloor was discovered in the Black Sea at 180 m
water depth. Contrary to other AOM-mat systems of the Black Sea these floating mats are not associated to free
gas and are not stabilized by authigenic
carbonates. However, supply of
methane is ensured by the horizontal orientation of the mats acting as a cover of
methane enriched fluids ascending from the underlying sediments. Thorough investigation of their community composition by
molecular microbiology and
lipid biomarkers,
metabolic activities and
elemental composition showed that the mats provide a clearly structured system with
extracellular polymeric substances (EPS) building the framework of the mats. The top black zone, showing high rates of AOM (15 mumol g(dw) (-1) day(-1)), was dominated by ANME-2, while the following equally active pink layer was dominated by ANME-1
Archaea. The lowest AOM activity (2 mumol g(dw) (-1) day(-1)) and cell numbers were found in the greyish middle part delimited towards the sediment by a second pink, ANME-1-dominated and sometimes a black outer layer (ANME-2). Our work clearly shows that the different
microbial populations are established along defined chemical gradients such as
methane,
sulfate or
sulfide.
