Heat flows solubilize apatite to boost phosphate availability for prebiotic chemistry

Abstract

Phosphorus is an essential building block of life, likely since its beginning. Despite this importance for prebiotic chemistry, phosphorus was scarce in Earth's rock record and mainly bound in poorly soluble minerals, with the calcium-phosphate mineral apatite as key example. While specific chemical boundary conditions have been considered to address this so-called phosphate problem, a fundamental process that solubilizes and enriches phosphate from geological sources remains elusive. Here, we show that ubiquitous heat flows through rock cracks can liberate phosphate from apatite by the selective removal of calcium. Phosphate's strong thermophoresis not only achieves its 100-fold up-concentration in aqueous solution, but boosts its solubility by two orders of magnitude. We show that the heat-flow-solubilized phosphate can feed the synthesis of trimetaphosphate, increasing the conversion 260-fold compared to thermal equilibrium. Heat flows thus enhance solubility to unlock apatites as phosphate source for prebiotic chemistry, providing a key to early life's phosphate problem.

Fig. 1. A geothermal solution to the phosphate problem on the early Earth.

a Phosphorus only constitutes around 0.1 wt% of Earth’s crust and is mostly bound as phosphate in apatite minerals, which renders it inaccessible for nascent life. b Heat flows in geothermal systems are able to enrich phosphate against calcium, boosting phosphate solubility at neutral pH and its absolute concentrations for downstream synthesis of energy-rich trimetaphosphate (TMP).

Fig. 2. Heat-flow-driven solubilization of phosphate from apatite.

a Experiment. Acidic-dissolved phosphate (pH 1.6 to pH 4), is flushed through a heat flow chamber (∆T), leading to the selective enrichment of phosphate at the bottom outlet by the interplay of convection (black) and thermophoresis (white). Downstream neutralization (pH_neut) mimics the transition to prebiotic chemistry conditions that allow, for instance, the formation of trimetaphosphate (TMP). b Dissolved phosphate was accumulated from initially low concentrations (bulk, gray) and extracted in the bottom outlet of the heat flow chamber (∆T, red). c Here, calcium concentrations were depleted relative to phosphate by the thermal non-equilibrium, which shifts the Ca:PO₄ ratio from 5:3 found in apatite (bulk, gray) to 1:1 (∆T, red) (see Supplementary Fig. 3). d Under the neutral conditions required for prebiotic chemistry, previously acidic-dissolved phosphate and calcium precipitated (see Supplementary Figs. 4, 5). In contrast to the bulk case (gray), the heat-flow-driven removal of calcium (∆T, red), boosted the solubility of phosphate up to 100-fold. The results were verified by geochemical modeling (black boxes, indicating mean ± SD, see “Methods”). e Moderate heating to 180 °C triggered the formation of TMP from the heat-flow-altered solutions (∆T, red), increasing yields more than 100-fold compared to the absence of thermal gradients (bulk, gray). All error bars show the SD.

Fig. 3. Phosphate-rich habitats formed by heat-flow-driven accumulation.

a Leaching from geomaterials with different weight percentages of phosphorus (see Supplementary Table 2) in solutions of different pH_init yielded low phosphate concentrations: VCG volcanic glass, BF2 basalt F2, KAO kaolinite, ILL illite, SCS siliciclastic sand, BSS basalt sand, CAS carbonate sand, MON montmorillonite (for SEM images and compositions see Supplementary Figs. 7, 8 and Supplementary Table 2). b Heat flows across water-filled fractures boosted the phosphate concentrations from the top-feeding inflows (“bulk”, gray) by a factor of 130-fold in the lowest 25 % of the pore (“niche”, dark red), or 40-fold averaged over the whole crack (“pore”, light red). All error bars show the SD.