Was hydrogen peroxide present before the arrival of oxygenic photosynthesis? The important role of iron(II) in the Archean ocean

Abstract

We address the chemical/biological history of H₂O₂ back at the times of the Archean eon (2.5-3.9 billion years ago (Gya)). During the Archean eon the pO₂ was million-fold lower than the present pO₂, starting to increase gradually from 2.3 until 0.6 Gya, when it reached ca. 0.2 bar. The observation that some anaerobic organisms can defend themselves against O₂ has led to the view that early organisms could do the same before oxygenic photosynthesis had developed at about 3 Gya. This would require the anaerobic generation of H₂O₂, and here we examine the various mechanisms which were suggested in the literature for this. Given the concentration of Fe²⁺ at 20-200 μM in the Archean ocean, the estimated half-life of H₂O₂ is ca. 0.7 s. The oceanic H₂O₂ concentration was practically zero. We conclude that early organisms were not exposed to H₂O₂ before the arrival of oxygenic photosynthesis.

Keywords: Archean; Fenton reaction; Hydrogen peroxide; Radiation; oxidation‐reduction (redox).