Anaerobic community structure from a nonequilibrium thermodynamic perspective

Thermodynamic analysis of the degradation of selected compounds showed that aerobic and denitrifying processes are heat driven (large enthalpy change). Owing to the smaller enthalpy changes, other anaerobic processes depend more on the entropy flow of chemical substances exchanged with the environment. This results in partial degradation of the substrate and the excretion of end products which have a high free-energy content. This creates niches for organisms to use these end products as energy sources. Mechanistically, the efflux of end products can be used to conserve energy by coupling the efflux with the translocation of protons or other ions. Analysis of published data strongly suggests that the movement of succinate, acetate, and possibly longer chain fatty acids is carrier mediated. Hypothetical calculations showed that these compounds can accumulate to high internal concentrations relative to extracellular concentrations, which suggests that the efflux of these compounds can generate a proton-motive force. This results in the development of mutualistic relationships, where the first species gains additional energy by the continued electrogenic excretion of the end product which enhances the flow of the compound to the second species. Nonequilibrium thermodynamic analysis shows that increasing the flow of materials will favor reactions such as fatty acid degradation which have small changes in free energy.