Fig. 3.

Model calibration to experimental data. (A) Net isotopic fractionation $\left({}^{34}\epsilon \right)$ by A. fulgidus grown at 80 °C at constant csSRR (6, 7) as a function of extracellular sulfate concentrations $\left({\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}\right)$. Orange contours show the value of ${}^{34}\epsilon$ for different values of csSRR in fmol H₂S⋅cell⁻¹⋅d⁻¹. The black curve shows ${}^{34}\epsilon$ for the harmonic mean of the csSRR values that were reported for some of the experiments (black squares). The csSRR values were not reported for other experiments (white squares), resulting in the scatter around the black curve. (B) Predicted ${}^{34}\epsilon$ as a function of csSRR for ${\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}$ between 10 μM and 100 mM. Experiments for which ${\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}$, csSRR, and ${}^{34}\epsilon$ were all reported are also shown, color coded by ${\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}$. (C) Predicted fractionation exponent $\left({}^{33}\lambda \right)$ as a function of ${}^{34}\epsilon$ for the same ${\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}$ as in B. Experiments for which minor isotope data exist are also shown (14). (D) Predicted ${}^{34}\epsilon \mathrm{-}{\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}$ relationship for Desulfovibrio strain DMSS-1 grown at ∼20 °C, 21–14 mM ${\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}$, and 2–7 mM sulfide (8–10). (E) Measured (white squares) and model (black curve) ${}^{34}\epsilon$ vs. csSRR for the experiments in D. These experiments were run in batch culture, so we assumed the average ${\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}$ and external sulfide concentrations for the interval over which ${}^{34}\epsilon$ vs. csSRRs were determined (SI Materials and Methods). (F) ${}^{33}\lambda$ vs. ${}^{34}\epsilon$ for the experiments in D. Error bars are 1σ reported in the experiments. (G–I) Same as D–F, but for D. vulgaris Hildenborough, grown at 25 °C and with precisely controlled ${\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]}_{\mathrm{out}}$ of 28 mM (11). Sulfide concentrations for the model curves were assumed to be 0.1 mM.