Rethinking early Earth phosphorus geochemistry

Abstract

Phosphorus is a key biologic element, and a prebiotic pathway leading to its incorporation into biomolecules has been difficult to ascertain. Most potentially prebiotic phosphorylation reactions have relied on orthophosphate as the source of phosphorus. It is suggested here that the geochemistry of phosphorus on the early Earth was instead controlled by reduced oxidation state phosphorus compounds such as phosphite (HPO(3)(2-)), which are more soluble and reactive than orthophosphates. This reduced oxidation state phosphorus originated from extraterrestrial material that fell during the heavy bombardment period or was produced during impacts, and persisted in the mildly reducing atmosphere. This alternate view of early Earth phosphorus geochemistry provides an unexplored route to the formation of pertinent prebiotic phosphorus compounds, suggests a facile reaction pathway to condensed phosphates, and is consistent with the biochemical usage of reduced oxidation state phosphorus compounds in life today. Possible studies are suggested that may detect reduced oxidation state phosphorus compounds in ancient Archean rocks.

Fig. 1.

Structures of biological P molecules at pH 8. (Left) Inorganic P molecules are shown. (Right) Representative organic P molecules are shown.

Fig. 2.

Thermodynamic stability diagrams for P species. (A) Eh-pH diagram for P species at 298 K, 10⁻⁶ M, with dashed lines representing the present-day atmosphere (Upper) and lower limit on water stability (Lower). (B) Condensation sequence for P minerals assuming 10⁻⁴ bar and solar elemental abundances.

Fig. 3.

NMR spectrum of Fe²⁺, H₂O₂, and HPO₃²⁻ in solution after 1 day. From left to right, the peaks are orthophosphate (6.5 ppm), phosphite (4 ppm), pyrophosphate (−4 ppm), and triphosphate (small peak at −17.5 ppm).

Fig. 4.

Oxidative half-life for 1 mM solution of phosphite (HPO₃²⁻) vs. atmospheric H₂ content. The top profile is based on the estimate of UV flux under a CO₂-rich atmosphere, whereas the bottom profile assumes no CO₂ (70).

Fig. 5.

Eh-pH diagram for radical aqueous P species, 10⁻⁶ M, with dashed lines representing the present-day atmosphere (Upper) and lower limit on water stability (Lower).