Apatite-group minerals, the dominant repository of phosphate within Earth’s crust, are insoluble under most conditions 5, and the presence of Fe 2+, solubilised under ancient O 2-poor atmospheres, is thought to have limited phosphate concentrations to ~10 −7 mol/kg in most natural waters and seawater 5, 6. Nevertheless, soluble phosphate is widely thought to have been scarce on the prebiotic Earth. However, beyond its biological utility, recent advances in prebiotic systems chemistry have shown that at high concentration (~0.1–1 mol/kg), phosphate performs an array of chemical functions it facilitates the selective formation of amino acids, lipid precursors, and nucleotides from one multicomponent reaction network 2, 3, 4, assuming a central role in the chemical origins of life. The universal and deep-seated importance of phosphorus in biology, from its role as a structural component to the basic energy currency of all living cells, has long suggested that it was incorporated early in the history of life 1. This suggests that seawater readily met the phosphorus requirements of emergent cellular systems and early microbial life, perhaps fueling primary production during the advent of oxygenic photosynthesis. Experiments and models show that Fe 2+ significantly increases the solubility of all phosphate minerals in anoxic systems, suggesting that Hadean and Archean seawater featured phosphate concentrations ~10 3–10 4 times higher than currently estimated.
Here we determine the solubility of Fe(II)-phosphate in synthetic seawater as a function of pH and ionic strength, integrate these observations into a thermodynamic model that predicts phosphate concentrations across a range of aquatic conditions, and validate these predictions against modern anoxic sediment pore waters. However, current understanding of mineral solubility predicts negligible phosphate concentrations for most natural waters, yet the role of Fe 2+, ubiquitous on early Earth, is poorly quantified. Prebiotic systems chemistry suggests that high phosphate concentrations were necessary to synthesise molecular building blocks and sustain primitive cellular systems.
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