The central role of phosphate in biology, from structural components to energy currency, hints at its early involvement in life's history. Recent prebiotic chemistry research demonstrates that high phosphate concentrations (0.1–1 mol/kg) are essential for forming amino acids, lipid precursors, and nucleotides. This raises the question of phosphate availability on the early Earth. Apatite minerals, the primary phosphate source, are largely insoluble. The presence of Fe²⁺, prevalent under the anoxic conditions of early Earth, was believed to limit phosphate concentrations to approximately 10⁻⁷ mol/kg. This scarcity has led to hypotheses proposing alternative phosphorus sources or non-aqueous solvents for prebiotic reactions. However, these alternative scenarios don't fully address the sustained availability of phosphate for primitive cellular systems and early microbial life. The current understanding of phosphate solubility in early Earth environments lacks a robust, data-calibrated geochemical model, particularly concerning the role of Fe²⁺. This study aims to address this gap by investigating Fe(II)-phosphate solubility in synthetic seawater to better constrain phosphate availability during the origin and early evolution of life on Earth.

The importance of phosphate in biology is well-established, with Westheimer (1987) highlighting its unique properties. Powner et al. (2009) and Patel et al. (2015) showed that high phosphate concentrations facilitate the synthesis of essential biomolecules. Pasek (2020) reviewed the thermodynamics of prebiotic phosphorylation, emphasizing the challenge posed by phosphate's low solubility. Several studies have proposed alternative sources of phosphorus (Handschuh & Orgel, 1973; Keefe & Miller, 1995; Karki et al., 2017; Lohmann & Orgel, 1971; Burcar et al., 2019; Pasek & Lauretta, 2005; Ritson et al., 2020), and specific environments like volcanic springs (Damer & Deamer, 2020) or alkaline lakes (Toner & Catling, 2020) have been suggested as potential cradles of life, but these explanations fail to completely account for the ongoing availability of phosphate to support primitive life. The solubility of phosphate in complex solutions like seawater is affected by various cations (Atlas et al., 2018), but quantitative data for the influence of Fe²⁺, especially at higher pH values, is lacking.

This study experimentally determined the solubility of Fe(II)-phosphate (vivianite) in synthetic seawater under anoxic conditions (25°C) as a function of pH and ionic strength. Experiments were performed in an anaerobic chamber to prevent oxidation. Deoxygenated water and reagents were used, and the atmosphere was constantly monitored to maintain <1 ppm O₂. Solubility was determined by measuring dissolved Fe²⁺ and phosphate concentrations after achieving apparent equilibrium. Major cation concentrations were measured using ICP-OES, while total dissolved phosphate was determined spectrophotometrically. Solid materials were analyzed using powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) to confirm the mineral phase and prevent oxidation. A thermodynamic model was developed using the Pitzer framework to incorporate the experimental data and predict phosphate concentrations across various aquatic conditions. The model uses ion activity coefficient functions, accounting for interactions between phosphate anions and different cations (Ca²⁺, Mg²⁺, and Fe²⁺) through pairing functions. The model incorporates Pitzer ion interaction coefficients for major salts, and utilizes infinite dilution constants for phosphoric acid. The model was optimized against experimental data, including the newly acquired solubility data and previously published data. The optimized model was then validated against natural anoxic sediment pore waters. Reaction path models of evaporation were also conducted to evaluate how phosphate concentrations change with evaporation under different initial alkalinity and cation compositions, simulating possible scenarios for prebiotic seawater.

The experimental solubility data showed that vivianite solubility increases dramatically between pH 4 and 8.5, indicating strong complexation between Fe²⁺ and phosphate anions. The thermodynamic model successfully reproduced the measured stoichiometric dissociation constants of phosphoric acid in seawater media as a function of ionic strength. The model demonstrated that the presence of Fe²⁺, along with Ca²⁺ and Mg²⁺, significantly increases the solubility of all phosphate minerals in anoxic systems. The model accurately predicted dissolved phosphate concentrations in multi-component systems, with pore water saturation states matching equilibrium vivianite solubility. The analysis of phosphate solubility in anoxic seawater as a function of pH, salinity, and cation concentrations revealed that at pH < 6.7–7.2, total P ranges from ~200–400 µmol/kg, three orders of magnitude higher than current estimates. Evaporation of prebiotic seawater is shown to further concentrate phosphate, but the pH evolution of the evaporating fluid depends strongly on the relative proportion of total alkalinity (ALK) to cations partitioned into carbonate minerals. Low alkalinity seawater evaporation can increase phosphate concentrations by three orders of magnitude while pH decreases. Conversely, high alkalinity scenarios lead to Mg-Cl-rich fluids with >1 mol/kg total phosphate at pH 6–7. These findings indicate that the Hadean oceans likely delivered significant phosphate to various marine systems.

The significantly higher phosphate concentrations predicted by this study challenge the prevailing view of phosphate scarcity on early Earth. The results suggest that prebiotic synthesis could have readily occurred in a wide range of marine environments, and that the ocean provided a suitable reservoir of phosphate to support primitive cellular systems and early microbial life. The findings reconcile the apparent contradiction between the low solubility of apatite and the requirement for high phosphate concentrations in prebiotic chemistry. The significant role of Fe²⁺ in enhancing phosphate solubility highlights the importance of considering redox conditions in early Earth geochemical models. The results also have implications for understanding early primary production and the evolution of oxygenic photosynthesis, potentially offering a new perspective on the factors that drove the Great Oxidation Event.

This research provides a new geochemical framework for understanding phosphate availability in early Earth oceans. The substantially higher phosphate concentrations predicted by the model strongly support the hypothesis that phosphate was readily available to support prebiotic chemistry and the emergence of life. Future research could focus on further refining the model by incorporating additional factors, such as the impact of other trace elements or organic ligands on phosphate solubility. Also, exploring the implications of these findings for understanding early microbial ecosystems and the evolution of phosphorus cycling on Earth is critical.

The study's conclusions are based on a model and experimental data obtained under controlled laboratory conditions, which may not perfectly reflect the complex conditions of early Earth oceans. The model relies on certain assumptions about the chemical composition and redox state of early Earth seawater, introducing some uncertainties in the predictions. Although the model was validated against modern anoxic sediment pore waters, extrapolating to the vastly different conditions of the Hadean and Archean eons has inherent limitations.