Elemental Sulfur as a Key Intermediate for Microbial Pyrite Formation

Pyrite (FeS₂) is the end-product of microbial sulfur cycling in reduced environments and is the main burial pathway of sulfur in marine sediments. Pyrite forms by a series of reactions between sulfide and Fe(II)/Fe(III), and sulfur-metabolizing microorganisms play an important role in mediating their formation. Here we tested microbial pyrite formation by the iron(III)- and sulfur-reducing bacterium Geobacter sulfurreducens in the presence of the Fe(III) (oxyhydr)oxide mineral ferrihydrite and elemental sulfur (S⁰). Over 6 months of incubation, two main stages were observed for the geochemical evolution of the system. In the initial ferruginous stage, rapid release of aqueous Fe(II) into the solution is accompanied by mackinawite (FeS) formation through the reaction between sulfide and ferrihydrite. In the second sulfidic stage, sulfide and polysulfides accumulate in solution, catalyzing mackinawite’s transformation to greigite (Fe₃S₄) and eventually to pyrite. Scanning electron microscopy demonstrated that individual spherulitic pyrites formed on the surfaces of elemental sulfur, eventually replacing it completely while still preserving the original shape of the sulfur particles. Hence, elemental sulfur is a significant reactant with key functions in polysulfide formation and templating effect on microbial pyrite formation. Therefore, our results suggest a mechanism for microbial pyrite formation in microenvironments in modern sediments and sulfate-poor ecosystems throughout time (e.g., Archean Earth). Future research will be focused on the bioavailability of microbial pyrite to have a complete picture of the role of pyrite in microbial sulfur cycle.