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 (ΔG_r) 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 ΔG_r, we predict a novel microbial metabolism—sulfur comproportionation (3H₂S + SO₄^2-+ 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 (ΔG_r<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 + SO₄^2-= S₂O₃^2-+ H₂O) and to sulfite (H₂S + 3SO₄^2-= 4SO₃^2-+ 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.