Fe2+ disproportionation within iron-rich alkaline vent analogues reveals proto-bioenergetic systems

Alkaline hydrothermal vents are plausible environments for emergence of life on Earth. By means of a simplified analogical reconstruction of the vent-ocean interface of these systems reproducing early Earth conditions, we show that iron (oxy-hydr)oxide minerals may have carried out proto-bioenergetic processes driven by pH and redox gradients. The initial pH gradient precipitates the iron (oxy-hydr)oxide mineral barriers (magnetite, green rust and amakinite) and yields reducing conditions, enabling the production of metallic iron at room temperature via the disproportionation of Fe²⁺ to Fe³⁺ and Fe⁰. The association of Fe⁰ with magnetite suggests the coupling of Fe³⁺ / H₂ co-production by amakinite oxidation with the thermodynamically unfavorable reduction of Fe²⁺ to Fe⁰. This abiotic disproportionation process coupling exergonic and endergonic reactions resembles a proto-bioenergetic mechanism increasing the non-equilibrium reduction state of the system and offers an interesting analogue of the electronic bifurcation reaction, fundamental to the thermodynamic requirements of life.