Energetics of Reactions Mediated by Microbes

Most redox reactions in vitro reach equilibrium only extremely slowly with half times of the order of months or years, even though they may be highly favoured thermodynamically. This is illustrated by the persistence of N2 in oxic systems even though its oxidation to NOs is strongly favoured (Table 4.1). However, microbes in soil and water are capable of catalysing particular reactions from which they obtain energy for metabolism. The half times of such microbially catalysed reactions are of the order of hours or days. 

Oxi + Red2 = Redi - - 0x2 for which the reduction half reactions are  

As discussed earlier, the dependence of pe on the concentrations of reductants and oxidants is often small in comparison with its dependence on pH. The term in the square brackets in Equation (4.25) can therefore be replaced by pe terms giving for the approximate standard free energy change  

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