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

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 (S-0), 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 S-0 is indicated to have a direct bandgap of 4.4 eV. The UV-excited S-0 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 S-0 and assisting their photoreduction. Assuming that terrestrial hot springs covered 1% of primitive Earth's surface, S-0 at 10 mu M could have produced maximal 10(9) kg/year HCOOH within 10-cm-thick photic zones, underlying its remarkable contributions to the accumulation of prebiotic biomolecules.