A sulfur-cycling co-culture alters iron sulfide diagenesis and pyritization

Abstract

Iron sulfide minerals play a key role in modulating the oxygenation of Earth s atmosphere and represent an archive for reconstructing past environmental conditions. In anoxic sedimentary environments, microbially produced sulfide reacts with iron, thereby forming metastable iron sulfides (mackinawite, greigite) which are expected to transform into pyrite, the most thermodynamically stable phase. However, our understanding of this process mostly stems from pure cultures grown on sulfate, which fails to capture the complexity of natural microbial communities and subsequent sedimentary changes. To this end, we set up pure and co-culture incubations of sulfate- and sulfur-reducing bacteria with selected ferric minerals (goethite, ferric phosphate and ferrihydrite) or ferrous iron and later subjected the samples to experimental diagenesis at controlled temperature (80 degrees C). We hypothesized that the co-culture would enhance pyrite formation by maintaining an internal cryptic S cycle that replenishes polysulfides and ferrous iron via the sulfidation of ferric compounds. Our findings showed that iron sulfide morphology was strongly dependent on the reactivity of ferric compounds, rather than the presence of bacteria. Consistent with this, pyrite formed through diagenesis in both pure cultures and abiotic incubations amended with sodium sulfide, indicating that pyritization is a primarily chemically controlled process. Contrary to our expectations, the presence of the co-culture led to longer diagenetic preservation of metastable minerals like greigite and marcasite, ultimately hindering pyritization. Surprisingly, we could also detect spherulitic pyrite after experimental diagenesis in sterile samples without exogenous sulfide addition. This was explained by the abiotic disproportionation of elemental S which took place in temperature regimes usually associated with microbial sulfide production. Overall, these results highlight the influence of mixed microbial communities, but also overlooked abiotic reactions, on iron sulfide (trans)formations in sedimentary environments, with implications for paleo(magnetic) reconstructions and the usability of these minerals as biosignatures.