Pyrite formation from FeS and H2S is mediated through microbial redox activity

Abstract

The exergonic reaction of FeS with H₂S to form FeS₂ (pyrite) and H₂ 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, H₂S, and CO₂ as their sole substrates to produce FeS₂ and CH₄ Cultures grew over periods of 3 to 8 mo to cell densities of up to 2 to 9 × 10⁶ cells per mL^-1 Transformation of FeS with H₂S to FeS₂ was followed by ⁵⁷Fe Mössbauer spectroscopy and showed a clear biological temperature profile with maximum activity at 28 °C and decreasing activities toward 4 °C and 60 °C. CH₄ was formed concomitantly with FeS₂ and exhibited the same temperature dependence. Addition of either penicillin or 2-bromoethanesulfonate inhibited both FeS₂ and CH₄ 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.

Keywords: biogenic mineral transformation; deep biosphere; origin of life; sulfur cycle; syntrophy.

Fig. 1.

Time-resolved CH₄ formation in comparison with iron−sulfur mineral composition after nearly 7 mo of incubation (207 d) in culture J5 compared with abiotic controls and incubations of culture J5 with penicillin-G (1,000 U⋅ml⁻¹) or 2-bromoethanesulfonate (BES, 10 mM). (A and C) The mean ± SD of CH₄ measurements of three independent incubations. SDs are often smaller than the actual symbol size. (B and D) Mössbauer spectra corresponding to the last time point in the presented time series, with FeS₂ in red, FeS in blue, Fe₃S₄ in green, and intermediate FeS_x phases in orange. Black dots represent the measured raw data. Corresponding black lines represent the sum of all fitted mineral phases.

Fig. 2.

(A) Representative X-ray diffractograms of mineral precipitates formed in culture J5 and an abiotic control setup after 9 mo of incubation (281 d). Diffraction data of the two FeS₂ dimorphs pyrite and marcasite as well as of Fe₃S₄ (greigite) and FeS (mackinawite) are given as reference. (B) (Left) Scanning electron microscopy images of a nearly 7-mo-old (211 d) culture J5 in comparison with freshly prepared medium without inoculum. (Scale bar, 2 µm.) (Right) The corresponding results from EDX (images correspond to a twofold down-scaling of the SEM images to the left). Besides atoms from medium salts, iron and sulfur were the only elements discovered in the mineral phases.

Fig. 3.

Temperature-dependent pyrite and methane formation in culture J5 after nearly 7 mo of incubation (207 d). (A) Mössbauer spectra showing the temperature-dependent iron−sulfur mineral composition (FeS₂ in red, an intermediate FeS₂−FeS phase in gray, FeS in blue, Fe₃S₄ in green, and intermediate FeS_x phases in orange). Black dots represent the measured raw data. Corresponding black lines represent the sum of all fitted mineral phases. (B) Relative abundance of pyrite in comparison with all other measured iron−sulfur minerals plotted against temperature as the explanatory variable. Details are provided in SI Appendix, Table S2. (C) Average amount of methane (n = 2) plotted against temperature as the explanatory variable.

Fig. 4.

Bacterial and archaeal community composition of enrichment culture J5. RAxML trees based on (A) bacterial and (B) archaeal 16S rRNA gene sequences obtained from culture J5. Representative sequences of OTUs at the approximate species level (99% sequence identity) are shown. Sequence identity to the next cultured relative is given in percent in square brackets. Numbers of clones from the same OTU are presented in parenthesis followed by the GenBank accession number of a representative sequence. Bootstrap support is indicated by closed (≥90%) and open (≥70%) circles at the respective branching points. [Scale bar, 10% (Bacteria) and 2% (Archaea) estimated sequence divergence.] (C) Combined phase contrast image and fluorescent image of DAPI-stained cells and (D) scanning electron microscopy image with cells indicated by white arrows of culture J5 after 7.4 and 10 mo of incubation, respectively.