Pyrite formation from FeS and H 2 S is mediated through microbial redox activity

Abstract

The exergonic reaction of FeS with H 2 S to form FeS 2 (pyrite) and H 2 was postulated to have operated as an early form of energy metabolism on primordial Earth. Since the Archean, sedimentary pyrite formation has played a major role in the global iron and sulfur cycles, with direct impact on the redox chemistry of the atmosphere. However, the mechanism of sedimentary pyrite formation is still being debated. We present microbial enrichment cultures which grew with FeS, H 2 S, and CO 2 as their sole substrates to produce FeS 2 and CH 4 . Cultures grew over periods of 3 to 8 mo to cell densities of up to 2 to 9 × 10 6 cells per mL −1 . Transformation of FeS with H 2 S to FeS 2 was followed by 57 Fe Mössbauer spectroscopy and showed a clear biological temperature profile with maximum activity at 28 °C and decreasing activities toward 4 °C and 60 °C. CH 4 was formed concomitantly with FeS 2 and exhibited the same temperature dependence. Addition of either penicillin or 2-bromoethanesulfonate inhibited both FeS 2 and CH 4 production, indicating a coupling of overall pyrite formation to methanogenesis. This hypothesis was supported by a 16S rRNA gene-based phylogenetic analysis, which identified at least one archaeal and five bacterial species. The archaeon was closely related to the hydrogenotrophic methanogen Methanospirillum stamsii, while the bacteria were most closely related to sulfate-reducing Deltaproteobacteria, as well as uncultured Firmicutes and Actinobacteria. Our results show that pyrite formation can be mediated at ambient temperature through a microbially catalyzed redox process, which may serve as a model for a postulated primordial iron−sulfur world.