Images of a chimney-like structure combined with results of molecular microbiological investigations. (A) Chimney-like structure growing in the anoxic waters of the Black Sea (230 m water depth). Predominating black parts of the gas- and water-filled globules are at the top (mat type I). (B) Part of a broken microbial structure with the massive pink-orange predominating mat (mat type II). The interior of the bioherms consists of gray-greenish porous carbonate. (C and D) Images obtained by transmission electron microscopy (TEM) from mat type I and mat type II, respectively. (E) Fluorescence in situ hybridization (FISH) of mat type I (probes: EelMS932 (ANME-2), fluorescence green; Desulfosarcina/Desulfococcus group DSS658cy3, fluorescence red). (F) FISH of mat type II (probes: ANME-1, fluorescence green; Desulfosarcina/Desulfococcus group DSS658cy3, fluorescence red). For technical details see Materials and Methods. Note the lesser abundance of SRB in the ANME-1 association compared with the ANME-2 association.