Another chemolithotrophic metabolism missing in nature: sulfur comproportionation

Abstract

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 favourable and quantify 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 (3H₂ S + ${\mathrm{SO}}_4^{2-}$ + 2H⁺ ⇌ 4S⁰ + 4H₂ O). 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^-1 . We suggest that this may be sufficient energy to support a chemolithotrophic metabolism currently missing from the literature. Other versions of this metabolism, comproportionation to thiosulfate (H₂ S + ${\mathrm{SO}}_4^{2-}$ ⇌ ${S_{2}O}_3^{2-}$ + H₂ O) and to sulfite (H₂ S + 3 ${\mathrm{SO}}_4^{2-}$ ⇌ 4 ${\mathrm{SO}}_3^{2-}$ + 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.

Figure 1

Values of ΔG _r for Reaction 1 calculated with Equation 3 as a function of temperature and pH. Values of ${\Delta G}_r^0$ were computed with the SUPCRT92 software package (Johnson et al. 1992). Activities of aqueous H₂S and ${\mathrm{SO}}_4^{2-}$ across the temperature and pH space represented here were calculated with equilibrium speciation among H₂S and HS⁻ given a total sulfide activity of 10⁻³, and among ${\mathrm{HSO}}_4^{-}$ and ${\mathrm{SO}}_4^{2-}$ given a total sulfate activity of 10⁻². [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

Values of ΔG _r for Reaction 1 calculated as in Fig. 1 and plotted as a function of activities of H₂S and ${\mathrm{SO}}_4^{2-}.$ A. 15°C and pH 2. B. 50°C and pH 2. C. 15°C and pH 5 D. 50°C and pH 5. The scale for all four panels is shown in D. [Color figure can be viewed at wileyonlinelibrary.com]