Mechanisms of Pyrite Formation Promoted by Sulfate-Reducing Bacteria in Pure Culture

Arnaud Duverger,Jasmine S Berg,Vincent Busigny,François Guyot,Jennyfer Miot,Sylvain Bernard

doi:10.3389/feart.2020.588310

Abstract

Pyrite, or iron disulfide, is the most common sulfide mineral at Earth’s surface and is widespread through the geological record. Because sulfides are mainly produced by sulfate-reducing bacteria (SRB) in modern sedimentary environments, microorganisms are assumed to drive the formation of iron sulfides, in particular pyrite. However, the exact role played by microorganisms in pyrite formation remains unclear and, to date, the precipitation of pyrite in microbial cultures has only rarely been achieved. The present work relies on chemical monitoring, electron microscopies, X-ray diffraction and synchrotron-based spectroscopy to evaluate the formation of iron sulfides by sulfate-reducing bacteria Desulfovibrio desulfuricans as a function of the source of iron, either provided as dissolved Fe2+ or as FeIII-phosphate nanoparticles. Dissolved ferrous iron led to the formation of increasingly crystalline mackinawite (FeS) with time, encrusting bacteria cell walls, hence preventing further sulfate reduction upon day 5 and any evolution of iron sulfides into more stable phases, e.g. pyrite. In contrast, ferric phosphate was transformed into a mixture of large flattened crystals of well crystallized vivianite (Fe3(PO4)2 . 8 H2O) and a biofilm-like thin film of poorly crystallized mackinawite. Although being hosted in the iron sulfide biofilm, most cells were not encrusted. Excess-sulfide delivered by the bacteria and oxidants (such as polysulfides) promoted the evolution of mackinawite into greigite (Fe3S4) and the nucleation of pyrite spherules. These spherules were several hundreds nanometer wide and occurred within the extracellular polymeric substance (EPS) of the biofilm after only 1 month. Altogether, the present study demonstrates that the mineral assemblage induced by the metabolic activity of SRB strongly depends on the source of iron, which has strong implications for the interpretation of the presence of pyrite and vivianite in natural environments.

Highlights

Iron is the fourth most abundant element on the Earth’s surface, but due to modern oxic conditions, dissolved Fe2+ concentration in modern ocean is low
Dissimilatory sulfate reduction was attested in the two conditions by the formation of black precipitates of iron sulfides, whereas no precipitation was observed in abiotic controls
After 30 days of biomineralization, only 4 mM of sulfate was consumed in the Fe-diss condition (i.e., 20 % of initial sulfate concentration) (Figure 1A), while up to 10 mM was used in the FP-nano condition (Figure 1B)

Summary

Introduction

Iron is the fourth most abundant element on the Earth’s surface, but due to modern oxic conditions, dissolved Fe2+ concentration in modern ocean is low (around 20 nM). In some modern ferruginous environments, analogous to ancient ocean, does dissolved ferrous iron concentration reach millimolar levels (Lake Pavin and Kabuno Bay (Busigny et al, 2014; Crowe et al 2014; Llirós et al, 2015)). In phosphate-rich anoxic environments, iron minerals are dominated by ferrous phosphate vivianite (Fe3(PO4)2 · 8H2O), e.g., in lacustrine environments (Rothe et al, 2016). Anoxic euxinic settings are dominated by iron sulfide minerals such as pyrite (FeS2), greigite (Fe3S4), or mackinawite (FeS)

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Results

Conclusion