Energy available for microbial respiration

By analogy to higher animals, we might expect that respiring microbes would use reduced carbon compounds as electron donors and O2 as the electron acceptor indeed, aerobic bacteria that do just this are common in oxic environments. Microbes, however, are notably versatile, employing H2, H2S, NHj , CH4, Fe++, and many other species as electron donors. They can similarly use SO4-, NO3, NO2, HCOJ, and so on as electron acceptors. The microbes can even use ferric 

Respiration, as we have described, drives two half-reactions, one to donate electrons and one to accept them. Iron-reducing bacteria, for example, can live on acetate, which is produced during the breakdown of organic matter. Oxidizing acetate provides electrons, 

The difference AEh between the redox potentials for the electron accepting and 

The SPECE8 input script below describes the analysis of a hypothetical ground-water, assuming equilibrium with ferric hydroxide and a soil gas in which fco2 = 10-2. In the script, we decouple a number of redox pairs so that we can constrain the amounts of several elements in two or more redox states. 

The program reports in its output the resulting redox potential for each redox couple, as calculated from the Nernst equation. The Nernst potentials, arranged in decreasing order, are 

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