THE IMPORTANCE OF CO2 ON THE AVAILABILITY OF PHOSPHATE ON THE PREBIOTIC EARTH

Phosphate is a biogenic element critical to extant membranes (phospholipids), bioinformation systems (RNA, DNA), and metabolism (adenosine phosphate). While the origin of life itself remains one of the most significant scientific questions to be solved, any plausible scenario requires sufficient concentrations of phosphate ions to allow for the formation of phosphate-containing molecular building blocks that would eventually lead to life. Assuming that life originated in an aqueous environment, any scenario has to overcome the low total inorganic phosphate concentration (PO₄) imposed by the low solubility of hydroxyapatite, which is referred to as the “phosphate problem”.

We used detailed, quantitative water-rock interaction modeling to explore the constraints on phosphate concentration on the early Earth. Building on earlier work, we calculated the composition of water interacting with komatiite—a proxy for a primitive crust—as a function of the partial pressure of CO₂ (P_CO2) at a temperature of 75°C. The resulting fluid (essentially primitive seawater) was then subjected to a maximum of ten evaporation cycles. Using present-day P_CO2 as a constraint, the primitive seawater and the brine after ten cycles of evaporation has a molar PO₄/Ca ratio that is smaller than unity (PO₄/Ca = 0.3 in final brine). Elevating P_CO2 to twice the present-day levels pushes the primitive seawater and its brine across a chemical divide that leads to a molar PO₄/Ca ratio that is larger than unity (PO₄/Ca = 8.2 in final brine). While the modest increase in P_CO2 leads to PO₄/Ca >1, the total PO₄ concentration in the final brine was less than 4 10^-4 M. Further increasing P_CO2 to tenfold the present-day level leads to PO₄ concentration in the final brine in the mM range, which is within biologically relevant values. With P_CO2 at 1 atm, the PO₄ concentration in the final brine reaches 59 mM. Further increasing P_CO2 has no effect.

In summary, this study shows for the first time that it is possible to overcome the “phosphate problem” by constraining P_CO2-levels on the early Earth up to ~ 1 atm or higher. Independent published estimates of the atmospheric composition of the early earth suggest P_CO2 levels well above 1 atm, supporting the notion that primitive seawater and its brines could have had phosphate concentrations that are biologically relevant.