A thermodynamic feasibility study was applied as a means of predicting suitable energy-yielding substrates for growth of sulphate-reducing microorganisms. The average free energy release per electron pair for a substrate-sulphate oxidoreduction may be more or less than the energy requirement for ATP synthesis from ADP and Pi. Substrates were divided into two groups on this thermodynamic basis and the division was shown to accord with previous experimental reports; those substrates which released an average of at least 8� 4 kcal per electron pair (35�2 kJ per electron pair) were able to support growth whilst those releasing less than 8� 4 kcal were unable to do so. It is proposed that the thermodynamic assessment could be applied to a wide range of possible substrates to predict the likelihood of their serving as sole substrates for growth of these organisms.

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