Photoreduction of inorganic carbon(+IV) by elemental sulfur: Implications for prebiotic synthesis in terrestrial hot springs

Abstract

Terrestrial hydrothermal systems have been proposed as alternative birthplaces for early life but lacked reasonable scenarios for the supply of biomolecules. Here, we show that elemental sulfur (S⁰), as the dominant mineral in terrestrial hot springs, can reduce carbon dioxide (CO₂) into formic acid (HCOOH) under ultraviolet (UV) light below 280 nm. The semiconducting S⁰ is indicated to have a direct bandgap of 4.4 eV. The UV-excited S⁰ produces photoelectrons with a highly negative potential of -2.34 V (versus NHE, pH 7), which could reduce CO₂ after accepting electrons from electron donors such as reducing sulfur species. Simultaneously, UV light breaks sulfur bonds, benefiting the adsorption of charged carbonates onto S⁰ and assisting their photoreduction. Assuming that terrestrial hot springs covered 1% of primitive Earth's surface, S⁰ at 10 μM could have produced maximal 10⁹ kg/year HCOOH within 10-cm-thick photic zones, underlying its remarkable contributions to the accumulation of prebiotic biomolecules.