Impact of silica on the identity of minerals formed in Archean oceans during Fe cycling by cyanobacteria and iron(III)-reducing bacteria

Banded Iron Formations (BIF) are iron- and silica-rich marine sediments deposited between 3.8 and 1.85 Ga. With the evolution of cyanobacteria, iron cycling via abiotic Fe(II) oxidation with O₂ produced by cyanobacteria and Fe(III) reduction by Fe(III)-reducing microorganisms is hypothesized to be responsible for BIF mineral deposition and transformation both in the water column and in the resulting sedimentary deposits. However, the impact of silica on iron mineralogy during microbially influenced iron cycling has not been determined experimentally under relevant ancient conditions. Hence, we set up batch incubation experiments with different concentrations of silica (0–2.2 mM) in the presence of varying concentrations of Fe(II) (0.5–5 mM), a marine cyanobacterium (Synechococcus sp. PCC 7002) and a marine dissimilatory Fe(III)-reducer (Shewanella colwelliana). We found that the fluctuation between microbial production of O₂ (cyanobacteria) and the activity of Fe(III)-reducing bacteria led to alternating Fe(II) oxidation and Fe(III) reduction and the precipitation of various Fe(II)- and Fe(III)-bearing minerals. Using a combination of X-ray diffraction and ⁵⁷Fe Moessbauer spectroscopy we identified poorly crystalline Fe(III)-bearing minerals (e.g., ferrihydrite), and the well crystalline ones (e.g., goethite, and lepidocrocite) in the absence of silica. Fe minerals precipitated in the presence of silica showed more pronounced reflections in µXRD, indicating higher crystallinity, while ⁵⁷Fe Moessbauer spectroscopy suggested the formation of Fe(II) silicate minerals and Fe(III) (oxyhydr)oxide minerals associated with silica. Furthermore, the presence of silica led to higher oxidation and reduction rates but more incomplete Fe(II) oxidation, while the reduction extent was higher in the presence of silica. In summary, our experiments showed that the presence of silica clearly affects Fe cycling and Fe mineral (trans)formation by cyanobacteria and Fe(III)-reducing bacteria, with relevance for the deposition of Precambrian Banded Iron Formations.