Native iron reduces CO₂ to intermediates and end-products of the acetyl-CoA pathway

Autotrophic theories for the origin of life propose that CO₂ was the carbon source for primordial biosynthesis. Among the six known CO₂ fixation pathways in nature, the acetyl-CoA (AcCoA; or Wood-Ljungdahl) pathway is the most ancient, and relies on transition metals for catalysis. Modern microbes that use the AcCoA pathway typically fix CO₂ with electrons from H₂, which requires complex flavin-based electron bifurcation. This presents a paradox: how could primitive metabolic systems have fixed CO₂ before the origin of proteins? Here, we show that native transition metals (Fe⁰, Ni⁰ and Co⁰) selectively reduce CO₂ to acetate and pyruvate-the intermediates and end-products of the AcCoA pathway-in near millimolar concentrations in water over hours to days using 1-40 bar CO₂ and at temperatures from 30 to 100 °C. Geochemical CO₂ fixation from native metals could have supplied critical C₂ and C₃ metabolites before the emergence of enzymes.