Sulfur Biogeochemistry—Past and Present

Microbially mediated sulfur-redox 27Figure 3. ∆G r(kJ/mol) of the S 0 -reduction and S 0 -disproportionationreactions listed in Table 5A (data given in Table 5B) plotted againstreaction number. Symbols are as in Figure 2. Equilibrium (∆G r= 0) isindicated by a solid horizontal line.tionation reaction, given in Table 5A. In each of the S 0 -reductionreactions (5.1–5.7), two electrons are transferred from the electrondonor to S 0 , which, in turn, is reduced to H 2S. In the disproportionationreaction (5.8), S 0 is both oxidized to SO 42−and reducedto H 2S without an additional TEA or electron donor. Values of∆G rfor reactions in Table 5A are listed in Table 5B for the sevenVulcano sites. Tabulated values of ∆G rfor reactions 5.1–5.8 arealso plotted as a function of reaction number in Figure 3.Note that in Table 5B and Figure 3, reactions 5.1–5.5are exergonic at all sites considered, yielding between 14and 61 kJ/mol S 0 . The electron donors in these reactions areH 2, CH 4, and carboxylic acids. Again, at Grip, Acque Calde2, and Pozzo Istmo, values of ∆G rin Table 5B are calculatedfor reactions written with the carboxylate anion and not theprotonated carboxylic acid. In general, values of ∆G rare morenegative for those reactions in which the carboxylic acids (orcarboxylate anions) serve as the electron donors (5.3–5.5) thanfor those with inorganic reductants. Although all five reactionsmentioned (5.1–5.5) are exergonic, they generally yieldless energy per mole as written than most sulfate-reductionexamples discussed above. It should be pointed out, however,that per mole of electrons transferred, these two sets of reactionsyield comparable energy, 7–30 kJ/mol e − for S 0 -reductionand 1–26 kJ/mol e − for sulfate-reduction. Analogous toreactions in Table 4, S 0 -reduction reactions in which NH 4+andFe 2+ serve as the electron donor are endergonic, consuming92–105 kJ/mol S 0 for reaction 5.6 and 20–115 kJ/mol S 0 for