Light Stimulation of Sulfide Oxidation in the Black Sea Anoxic Water Column

Elemental sulfur is a common product of hydrogen sulfide oxidation in the photic zone of meromictic basins due to the anoxigenic oxidation of hydrogen sulfide by photosynthetic bacteria. The photic zone in the Black Sea is limited to 50–60 m, which is much higher than the upper limit of the hydrogen sulfide zone interface, which is at a depth of 90–100 m in the center of the sea. In peripheral areas of the Black Sea, the depth of the redox interface reaches 150–170 m, where, as expected, photoautotrophic bacteria are rare and in an inactive state. A study of the distribution of elemental sulfur in the Black Sea anoxic zone showed that waters from depths of 180–300 m are light sensitive. This leads to a sharp increase in sulfur concentrations up to 11.3 µmol/kg with background values of 0.15–0.18 µmol/kg under strictly anaerobic conditions. It was found that such a significant increase in elemental sulfur is associated with the activity of photoautotrophic bacteria. The conditions for the existence of photoautotrophic bacteria at depths of 180–300 m in the Black Sea in the absence of light remain unclear.