Exploring the impact of silica and divalent cations on microbial iron respiration and secondary mineral products in Archean ocean simulations

Abstract

Widespread chemical sedimentary deposits known as Banded Iron Formations (BIFs) archive Archean ocean chemistry and, potentially, traces of ancient microbial life. Microbial Fe(III) respiration likely influenced the mineralization of early BIF sediments. One hypothesis suggests that the early diagenetic bioreduction of primary iron oxides in BIFs resulted in the formation of other iron phases, including iron-rich silicates, carbonates, and magnetite common in BIF assemblages. Here, we simulated this proposed pathway in experimental/laboratory incubations of Shewanella putrefaciens CN32, a model iron-reducing bacterium, grown under conditions mimicking the predicted Archean seawater geochemistry. We examined the variability in the secondary mineral precipitates that formed in the presence or absence of calcium, magnesium, and dissolved silica to assess the impact of silica and divalent cations on the resultant mineral product. We analyzed the reduced mineral phases harvested from these experiments using Raman spectroscopy, electron microscopy, powder x-ray diffraction, and spectrophotometric techniques to identify mineral precipitates and track the distributions of Fe2+/Fe3+. We detected a diverse range of calcium carbonate morphologies and polymorphism in incubations with calcium. We also identified aggregates of wavy, iron- and silica-rich, amorphous precipitates in all DIR incubations amended with silica. Our observations indicate that microbial iron reduction of ferrihydrite is a viable pathway for precursor iron silicate phases. This finding allows us to draw parallels between our experimental proto-silicates and the iron silicate nano-inclusions in BIF chert deposits, suggesting that early iron silicates could be signatures of iron-reducing metabolisms on early Earth.