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Abstract

The Black Sea is the largest anoxic water basin on Earth and its stratified water column comprises an upper oxic, middle suboxic and a lower permanently anoxic, sulfidic zone. The abundance of sulfate‐reducing bacteria (SRB) in water samples was determined by quantifying the copy number of the dsrA gene coding for the alpha subunit of the dissimilatory (bi)sulfite reductase using real‐time polymerase chain reaction. The dsrA gene was detected throughout the whole suboxic and anoxic zones. The maximum dsrA copy numbers were 5 × 102 and 6.3 × 102 copies ml−1 at 95 m in the suboxic and at 150 m in the upper anoxic zone, respectively. The proportion of SRB to total Bacteria was 0.1% in the oxic, 0.8–1.9% in the suboxic and 1.2–4.7% in the anoxic zone. A phylogenetic analysis of 16S rDNA clones showed that most clones from the anoxic zone formed a coherent cluster within the Desulfonema–Desulfosarcina group. A similar depth profile as for dsrA copy numbers was obtained for the concentration of non‐isoprenoidal dialkyl glycerol diethers (DGDs), which are most likely SRB‐specific lipid biomarkers. Three different DGDs were found to be major components of the total lipid fractions from the anoxic zone. The DGDs were depleted in 13C relative to the δ13C values of dissolved CO2 (δ13CCO2) by 14–19‰. Their δ13C values [δ13CDGD(II–III)] co‐varied with depth showing the least 13C‐depleted values in the top of the sulfidic, anoxic zone and the most 13C‐depleted values in the deep anoxic waters at 1500 m. This co‐variation provides evidence for CO2 incorporation by the DGD(II–III)‐producing SRB, while the 1:2 relationship between δ13CCO2 and δ13CDGD(II–III) indicates the use of an additional organic carbon source.