Isolation of a putative sulfur comproportionating microorganism

Sulfur comproportionation, the coupled oxidation of sulfide and reduction of sulfate to elemental sulfur, is a heretofore undiscovered microbial catabolism that has been predicted based on thermodynamic calculations. Here, we report the isolation of an Acidithiobacillus thiooxidans strain that grew to cell densities of > 10⁷ cells mL^-1 in autotrophic sulfur comproportionation medium. It was isolated from extremely low pH snottite biofilms in the sulfidic karst at Frasassi, Italy. The organism's genome sequence revealed the presence of numerous genes involved in sulfur transformations that could be linked in a sulfur comproportionation pathway. Future investigations of this novel link in the biogeochemical sulfur cycle could include monitoring of the concentrations of sulfate, sulfide, and elemental sulfur during growth to learn whether they are consistent with stoichiometric ratios expected for sulfur comproportionation. In addition, stable and radioisotope incubations coupled with NanoSIMS, scintillation counting, and isotope ratio mass spectrometry should reveal that sulfate and sulfide are coupled in a single catabolic reaction to produce elemental sulfur.