Oxidation state of sulfur in thiosulfate and implications for anaerobic energy metabolism

Disproportionation of thiosulfate is an important pathway in bacterial energy metabolism in anoxic sediments. It has been described as an inorganic fermentation process in which a part of the molecule is oxidized to sulfate, while the other stoichiometrically equivalent part is used as electron acceptor and reduced to sulfide. However, an intramolecular redox change is difficult to envisage because, according to the currently held view, the two sulfur atoms of thiosulfate exist in the oxidation state of sulfate (+6) and sulfide (−2) and do not change their respective oxidation states upon disproportionation. Our results based on XANES spectroscopy indicate that the two different sulfur atoms in thiosulfate have charge densities corresponding to +5 and −1 oxidation states which support a redox mechanism in the disproportionation of thiosulfate to sulfate and sulfide.