Another chemolithotrophic metabolism missing in nature--sulfur comproportionation
Chemotrophic microorganisms gain energy for cellular functions by catalyzing oxidation-reduction (redox) reactions that are out of equilibrium. Calculations of the Gibbs energy ({Delta}Gr) can identify whether a reaction is thermodynamically favorable, and the accompanying energy yield at the temperature, pressure, and chemical composition in the system of interest. Based on carefully calculated values of {Delta}Gr, we predict a novel microbial metabolism--sulfur comproportionation (3H2S + SO42- + 2H+ = 4S0 + 4H2O). We show that at elevated concentrations of sulfide and sulfate in acidic environments over a broad temperature range, this putative metabolism can be exergonic ({Delta}Gr<0), yielding [~]30-50 kJ/mol. We suggest that this may be sufficient energy to support a chemolithotrophic metabolism currently missing in nature. Other versions of this metabolism, to thiosulfate (H2S + SO42- = S2O32- + H2O) and to sulfite (H2S + 3SO42- = 4SO32- + 2H+), are only moderately exergonic or endergonic even at ideal geochemical conditions. Natural and impacted environments, including sulfidic karst systems, shallow-sea hydrothermal vents, sites of acid mine drainage, and acid-sulfate crater lakes, may be ideal hunting grounds for finding microbial sulfur comproportionators.