Anomalous energy yields in thermodynamic calculations: importance of accounting for pH-dependent organic acid speciation

Methanogenic ecosystems function close to oxidation– reduction equilibrium with change in Gibbs free energy (ΔG) values in the range of −10 to −20 kJ mol−1 (Hoehler et al., 1998; Jackson and McInerney, 2002). With such small margins, attention to detail when making free energy calculations becomes exceedingly important. A pervasive source of confusion, if not error, when making free energy calculations is the fact that organic acids and their anions occur simultaneously (Mazur et al., 2003; Mun et al., 2008). Here, we point out that such confusion is not necessary, as weak acids and their conjugated bases are in thermodynamic equilibrium in anaerobic ecosystems.