Marine phosphate availability and the chemical origins of life on Earth

Phosphate is fundamental to biology, serving structural, metabolic, and energetic roles. Prebiotic systems chemistry further indicates that high phosphate concentrations (∼0.1–1 mol/kg) can drive selective formation of amino acids, lipid precursors, and nucleotides from a common reaction network. Despite this importance, soluble phosphate on the prebiotic Earth has been considered scarce because apatite-group minerals are generally insoluble and Fe²⁺, abundant under anoxic conditions, was thought to suppress dissolved phosphate to ~10⁻⁷ mol/kg in natural waters. This perceived scarcity motivated hypotheses invoking alternative phosphorus sources or non-aqueous solvents. However, these scenarios do not readily explain sustained phosphate supply for primitive cells and early microbial ecosystems. Critically, phosphate solubility in multicomponent solutions like seawater depends on ionic strength and cation composition (Ca²⁺, Mg²⁺, Fe²⁺) through strong complexation, yet quantitative constraints for Fe²⁺ complexation at circumneutral pH and as a function of ionic strength have been lacking. This knowledge gap leads to highly variable predictions of phosphate availability in anoxic waters. The study aims to quantify Fe(II)-phosphate solubility in seawater-like solutions, develop a thermodynamic model to predict phosphate concentrations across plausible early Earth conditions, and test these predictions against modern anoxic pore waters to reassess marine phosphate availability on the prebiotic Earth.

Previous work suggests low phosphate availability due to low solubility of apatite-group minerals and potential scavenging by Fe²⁺ under anoxic atmospheres. Alternatives proposed include reduced phosphorus from meteoritic schreibersite weathering producing soluble P species (though not organophosphates without subsequent oxidation), photochemical oxidation to phosphate, and concentration in acidic springs or alkaline lakes. Nonetheless, these settings do not resolve how phosphate could sustain emerging cellular systems. Data for phosphate complexation with major seawater cations (Ca²⁺, Mg²⁺) exist, but quantitative Fe(II)-phosphate complexation above acidic pH and solubility of Fe(II)-phosphate minerals at circumneutral pH and across ionic strengths were poorly constrained, leading to order-of-magnitude discrepancies in predicted phosphate solubility for anoxic modern environments.

Experimental solubility of Fe(II)-phosphate (vivianite) was measured in synthetic seawater at 25 °C under strictly anoxic conditions across pH and ionic strength. Experiments were conducted in an anaerobic glovebox (94% H₂/6% N₂) with Pd catalyst and desiccant, <1 ppm O₂, using deoxygenated DI water. Fe²⁺ and Mg²⁺ stock solutions (from FeCl₂·4H₂O and MgCl₂·6H₂O) were prepared from deoxygenated water; Fe²⁺ stock was treated with Fe powder and HCl to reduce any ferric iron, then filtered under N₂. Experimental pH was set (oxygenated NaOH additions prior to FeCl₂ addition) and solutions were allowed to reach apparent solubility equilibrium (stable Fe²⁺ and phosphate concentrations over 24–48 h). Solutions were 0.1 μm filtered anoxically; solids were preserved anoxically. Aqueous Fe, Mg, Ca and other cations were measured by ICP-OES; total dissolved phosphate by the molybdate blue HACH PhosVer method (880 nm). Solids were characterized by powder XRD (Rietveld refinement) and FT-IR (KBr pellets) to confirm vivianite and assess minor element incorporation. A thermodynamic model was developed using the Pitzer ion-interaction framework to compute activity coefficients in saline, multicomponent media. The model includes ion-pairing between phosphate species and major cations (Ca²⁺, Mg²⁺, Fe²⁺) with formation constants from literature and this study, and interaction parameters for Na⁺, K⁺, Cl⁻, SO₄²⁻, CO₃²⁻, OH⁻, and relevant triplet interactions. Infinite-dilution dissociation constants for phosphoric acid and seawater stoichiometric dissociation constants as a function of salinity were reproduced. New experimental vivianite solubility data were combined with prior datasets to optimize a vivianite solubility product (Ksp) and Fe(II)-phosphate aqueous complex formation constants (ionic strength independent). The model was validated by recalculating saturation states for vivianite in modern anoxic sediment pore waters, comparing predicted to observed equilibrium conditions. Reaction path evaporation models were implemented in Geochemist’s Workbench using the calibrated thermodynamic dataset, simulating evaporation of anoxic seawater and basaltic waters under CO₂-rich atmospheres. Mineral equilibria for carbonates, silicates (e.g., greenalite), Fe carbonates, Mg phosphates, and Ca phosphates (including octacalcium phosphate, OCP) were included. Scenarios varied total alkalinity relative to Ca+Mg removal into carbonates to evaluate pH evolution and phosphate concentration during evaporation. All calculations assumed amorphous silica saturation and specified initial Fe and phosphate concentrations consistent with prebiotic scenarios.

• Experimental data show vivianite apparent solubility increases by several orders of magnitude from pH 4 to 8.5, indicating strong Fe²⁺–phosphate complexation controls solubility as pH rises.
• The thermodynamic model reproduces measured stoichiometric dissociation constants of phosphoric acid in seawater across salinity and demonstrates that Ca²⁺, Mg²⁺, and especially Fe²⁺ substantially increase the solubility of phosphate minerals in anoxic systems.
• Vivianite solubility decreases with decreasing salinity, explaining its preferential formation in brackish/lacustrine settings and rarity in normal marine/hypersaline environments.
• Model-predicted pore water saturation states in vivianite-bearing natural systems align with equilibrium at vivianite solubility, reconciling previously variable reports of phosphate mineral saturation in anoxic waters.
• In anoxic seawater at 25 °C, phosphate availability is limited by different minerals depending on pH: at pH > 7.2–7.7, greenalite (Fe(II) silicate) and OCP limit total phosphate; at pH < 7.2–7.7, OCP and vivianite limit phosphate.
• For Fe(II)-bearing waters at pH < 6.7–7.2, total dissolved phosphate reaches ~200–400 µmol/kg, approximately three orders of magnitude higher than prior estimates for anoxic seawater.
• Evaporation of prebiotic seawater can further concentrate phosphate by up to several orders of magnitude. Depending on total alkalinity relative to carbonate precipitation, pH may decrease (low-ALK cases) or remain elevated/increase (higher-ALK cases). In the latter, reaction paths yield Mg–Cl-rich fluids with >1 mol/kg total phosphate buffered near pH 6–7.
• Overall, early anoxic oceans could have supplied phosphate at concentrations compatible with prebiotic synthesis thresholds, challenging the notion of pervasive phosphate scarcity on the early Earth.

By quantifying Fe(II)-phosphate solubility and incorporating strong Fe²⁺–phosphate complexation into a seawater-based thermodynamic framework, the study demonstrates that anoxic seawater could sustain much higher phosphate concentrations than previously inferred from apatite solubility alone. The findings resolve discrepancies in reported saturation states of phosphate minerals in anoxic environments and identify the pH-dependent mineral controls (OCP, vivianite, greenalite) that cap phosphate concentrations across realistic prebiotic conditions. These higher phosphate levels—further enhanced by evaporation under certain alkalinity regimes—meet or exceed concentrations used in prebiotic synthesis experiments, implying that marine settings could have been viable venues for the emergence and maintenance of early metabolic and genetic chemistry. The results also suggest that as Fe(II) availability and cation ratios (Ca/Mg) evolved, the locus of phosphate limitation shifted among mineral phases, with Ca-phosphate phases often imposing the ultimate ceiling on phosphate in Fe-bearing waters. This framework links seawater composition, redox state, and mineral equilibria to phosphate supply, with implications for early primary productivity and the timing of oxygenic photosynthesis.

The study provides experimentally calibrated constraints on Fe(II)-phosphate solubility in seawater-like solutions and integrates them into a thermodynamic model that accurately predicts phosphate concentrations across pH, salinity, and cation compositions relevant to early Earth. Validation against modern anoxic pore waters supports the model’s realism. Key outcomes indicate that Fe²⁺ markedly elevates phosphate solubility, that mineral controls on phosphate are pH-dependent (OCP, vivianite, greenalite), and that evaporation under appropriate alkalinity regimes can generate phosphate-rich fluids at favorable pH. Collectively, these results overturn the assumption of pervasive phosphate limitation in prebiotic oceans and support marine environments as plausible nurseries for the origin and early evolution of life. Future work should refine constraints on ancient seawater cation ratios and Fe(II) concentrations, assess kinetic effects and mineral transformation pathways (e.g., OCP to CFA), evaluate trace substitutions (e.g., Mn) on solubility, and extend models to variable temperatures and redox gradients to explore spatial and temporal heterogeneity in phosphate availability.

• Experiments were conducted at a single temperature (25 °C) and under laboratory-controlled anoxic conditions, which may not capture natural temperature variability and dynamic redox fluctuations.
• Synthetic seawater compositions and assumed ancient seawater cation ratios (Ca/Mg) are uncertain; end-member scenarios were used to bracket possibilities.
• Some calculations omitted sulfate and assumed equilibrium with specified CO₂ partial pressures; real systems may deviate.
• Thermodynamic modeling emphasizes equilibrium; kinetic barriers, mineral nucleation pathways, and back-reactions (explicitly not considered in evaporation paths) could alter trajectories.
• Effects of trace element substitutions (e.g., Mn in vivianite) on solubility were not quantified.
• Validation against natural systems relies on reconstructed pore water chemistries and assumptions where data (e.g., DIC) were incomplete.