Microbial iron cycling during deposition and diagenesis of Banded Iron Formations

Banded Iron Formations (BIFs) are marine sediments consisting of alternating iron (Fe)-rich and silica (Si)-rich layers which were deposited between 3.8 to 1.85 Ga during the Precambrian era. BIFs represent important proxies for the geochemical composition of Precambrian seawater and provide evidence for early microbial life. Iron present in BIFs was likely precipitated in the form of Fe(III) minerals, such as ferrihydrite (Fe(OH)3), either by anoxygenic photoautotrophic Fe(II)-oxidizing bacteria (photoferrotrophs), by microaerophilic Fe(II)-oxidizing bacteria, or by the abiotic oxidation of dissolved Fe(II) (i.e. Fe2+) by O2 produced by cyanobacteria. However, the mineralogy found in BIFs today shows not only oxidized minerals like hematite but also (partially) reduced minerals like magnetite and siderite. This might be due to the presence of ancient dissimilatory Fe(III)-reducing microbes, which were able to reduce parts of the Fe in the primary precipitated Fe(III) minerals back to Fe(II). In our current work, we combine both oxidative and reductive Fe transformation processes in cycling experiment by co-culturing marine Fe(II)-oxidizing (oxygen-producing cyanobacteria Synechoccocus PCC 7002) and Fe(III)-reducing bacteria (Shewanella colwelliana) in the presence of different iron and silica concentrations to investigate the microbial impact on Fe mineral formation. These experiments and the resulting data will allow to discern processes occurring during the deposition and diagenesis of BIFs and better interpret early Earth conditions. Here, we show first results from the cycling experiments, which reveal the mineral assemblages that can be formed under such conditions that mimic processes in marine sediments of the early Earth.