Natural synthesis of bioactive greigite by solid–gas reactions

Greigite, a ferrimagnetic iron sulfide Fe(II)Fe(III)2S4, is thought to have played an essential role in chemical evolution leading to the origin of life. Greigite contains a [4Fe–4S] cluster-like structure and has been synthesized in the laboratory by liquid-state reactions. However, it is unclear how greigite can be synthesized in nature. Herein, we show that greigite is synthesized by the solid–gas reaction of Fe(III)-oxide-hydroxides and H2S. We discovered that the hyperthermophilic hydrogenotrophic methanogen Methanocaldococcus jannaschii reduced elemental sulfur, and the resulting sulfide generated greigite from hematite. The time course and pH dependence of the reaction respectively indicated the involvement of amorphous FeS and H2S as reaction intermediates. An abiotic solid–gas reaction of hematite and H2S (g) under strictly anaerobic conditions was developed. The solid–gas reaction fully converted hematite to greigite/pyrite at 40–120 °C within 12 h and was unaffected by the bulk gas phase. Similar abiotic reactions occurred, but relatively slowly, with aqueous H2S in acidulous liquids using hematite, magnetite, or amorphous FeO(OH) as starting materials, suggesting that greigite was extensively produced in the Hadean Eon as these Fe(III)-oxide-hydroxides were shown to be present or routinely produced during that era. Surprisingly, the obtained greigite induced methanogenesis and growth of hydrogenotrophic methanogens, suggesting that the external greigite crystals enhanced reactions that would otherwise require enzymes, such as [4Fe–4S] cluster-harboring membrane-bound hydrogenases. These data suggested that the greigite produced by the solid–gas and solid–dissolved gas reactions was bioactive.