Sulfur Biogeochemistry—Past and Present

Microbially mediated sulfur-redox 25propanoic acid, NH 4+, and Fe 2+ . In reactions 4.8–4.14, sulfate isreduced to S 0 , a six-electron transfer. Values of ∆G rfor the reactionsin Table 4A are given in Table 4B for each of the seven sitesconsidered. In Figure 2, values of ∆G rare depicted as a functionof reaction number.It can be seen in Table 4B and Figure 2 that all reactions inwhich H 2, CH 4, or carboxylic acids serve as electron donors (4.1–4.5, 4.8–4.12) are exergonic at all seven sites, with values of ∆G rranging from −9 to −208 kJ/mol SO 42−. Reactions in which NH 4+or Fe 2+ act as electron donors (4.6, 4.7, 4.13, 4.14) are stronglyendergonic, with values of ∆G rbetween +112 and +540 kJ/molSO 42−. In terms of per mole of sulfate reduced, reactions to H 2Sare more exergonic than reactions to S 0 . The largest energy yield(170–208 kJ/mol SO 42−) is observed from reaction (4.3) at Punto1, Punto 7, Stinky Surf Rock, and Pozzo Vasca. In this reaction,formic acid serves as the electron donor. The range of ∆G rvaluesat the different sites for most of the sulfate-reduction reactionsis relatively large; with only two exceptions (reactions 4.6 and4.13), the range is >50 kJ/mol, commonly it is ~100 kJ/mol, andin one case (reaction 4.14), even >200 kJ/mol. At times, it isadvantageous to normalize values of ∆G rper electron transferred(∆G r/e − ), because this enables a more direct comparison of redoxreaction energetics (Amend et al., 2003b). When normalized inthis way, analogous reactions in which SO 42−is reduced to H 2S orto S 0 (reactions 4.3 and 4.10) have very similar energy yields. Forexample, ∆G 4.3/e − ranges from −8 to −26 kJ/mol e − , and ∆G 4.10/e −is between −6 and −25 kJ/mol e − .It should be pointed out that at sites where the pH is onlyslightly acid (5–6), the carboxylate anions formate, acetate, andpropanoate dominate over formic, acetic, and propanoic acid.Therefore, at Grip, Acque Calde 2, and Pozzo Istmo, values of∆G rfor reactions 4.3–4.5 and 4.10–4.12 in Table 4B are not forthe reactions as written, but for their counterparts, which considerthe carboxylate anion instead of the acid. This is perhaps best illustratedwith an example. At Punto 1, Punto 7, Stinky Surf Rock,and Pozzo Vasca, where the pH is between 2.0 and 3.7, sulfatereductioncoupled to formic acid-oxidation can be represented bySO 42−+ 4HCOOH + 2H + → H 2S + 4CO 2+ 4H 2O. (3)At Grip, Acque Calde 2, and Pozzo Istmo, the correspondingreaction is instead