Inorganic carbon fixation by sulfate-reducing bacteria in the Black Sea water column

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 × 10 ² 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 (δ ¹³C_CO2) by 14-19‰. Their δ¹³C values [δ¹³C_DGD(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 δ¹³C_CO2 and δ¹³C _DGD(II-III) indicates the use of an additional organic carbon source.