H2 — thermodynamic, kinetic and physical controls on the observed levels and the relation to specific redox processes

Hydrogen is an important intermediate produced through fermentation of organic matter and used in terminal electron acceptor processes. In systems dominated by sulfate reduction and methanogenesis the level of H₂ is related to the Gibbs energy of the reactions so that the energy yield is constant for the given reaction. The calculated energy yield is close to the minimum energy required for energy storage by ATP synthesis. Also for non-dissolved electron acceptors, such as Fe-oxides, data indicate that the observed H₂ level is related to the Gibbs energy, though the nature of Fe-oxides makes this difficult to confirm. Concentrations of H₂ are also influenced by physical sediment properties, and advective flow.