Fig. 2.

Predicted metabolite concentrations and isotopic fractionation in a model sulfate reducer. Shown are intracellular concentrations of sulfate (${\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{in}}$, row A), APS ([APS], row B), PPi ([PPi], row C), and sulfite ($\left[\mathrm{S}{\mathrm{O}}_3^{2-}\right]$, row D) and the net isotopic fractionation between the substrate sulfate and product sulfide (${}^{34}\epsilon$, row E) as functions of extracellular sulfate (${\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}$, horizontal axis) and sulfide concentrations ([H₂S], vertical axis). All concentrations are shown on logarithmic scales. Intracellular metabolite levels are calculated from Eqs. S22–S25, whereas isotopic fractionation is calculated by application of Eqs. 2 and 5. Regions where calculated PPi concentrations (and associated fractionations) are physiologically unlikely are shown as gray shaded fields (SI Materials and Methods) (rows C and E).