Inverse assimilation: Was the hydrogen escape from the earth's primary atmosphere enhanced by H₂-evolution coupled with biophoto-oxidation of methane?

Summary

Photosynthesis (assimilation) is currently understood as a photoreduction of CO₂ by water into carbohydrates; reoxidation by O₂ (dissimilation) closes the biological carbon cycle. If, originally, there was a surplus of H₂, thermodynamical equilibrium would require that C, Fe, Mn, S and N exist as CH₄, Fe⁺⁺, Mn⁺⁺, S⁻⁻ and NH₃. The redox equilibrium is believed to have been shifted gradually as a result of the escape of H₂ formed photochemically in the atmosphere. However, in a system containing H₂, CO₂ is an energy-rich compound affording 31.2 kcal on reduction to CH₄; it should be so rare that even CaCO₃ would dissolve. How, then, could photoreduction evolve, lacking CO₂ and a thermodynamical advantage? Also H₂ escape, governed by diffusion, does not account for so much H₂ during geological time (Miller andUrey [19]). The hypothesis of areductive primeval carbon cycle between biomass (C(H₂O)) and methane involvinginverse assimilation and dissimilation (Decker [6]), as presented in the following scheme, would evercome these difficulties. The present oxidative carbon cycle requires 114 kcal supplied by two consecutive photoreactions: regular:

$$CO_2 + H_2 OC(H_2 O) + O_2 + 114 kcal.$$

A reductive carbon cycle involving inverse assimilation and dissimilation would require only 32 kcal, i.e. only one photoreaction: inverse:

$$CH_4 + H_2 OC(H_2 O) + 2H_2 + 32 kcal.$$

It could actively support an elevated H₂ level necessary to explain the observed escape rate, until all CH₄ was consumed and burried as elemental C or carbonate, the latter resulting from fermentations in ‘anaerobic’ environments. All enzymatic steps of such a cycle are present in contemporary organisms except a photochemical CH₄ assimilation in a CO₂-free atmosphere, which, however, has apparently never seriously been looked for. Further facts pro and contra should be available in the geological record, e.g. in Precambrian ‘anaerobic’ Fe³⁺ and CaCO₃ deposits.