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

Alkaline hydrothermal vents are plausible environments for the 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 crystallographic association of Fe⁰ surrounded by magnetite suggests the coupling of Fe³⁺ / H₂ co-production at ambient temperature by amakinite oxidation with the thermodynamically unfavorable reduction of Fe²⁺ to Fe⁰. This abiotic disproportionation process coupling exergonic and endergonic reactions may serve as a proto-bioenergetic mechanism increasing the non-equilibrium reduction state of the system and offers an interesting analog of the biological electronic bifurcation reaction, the free energy coupling being a fundamental thermodynamic trait of life-as-we-know-it.