Many iron (Fe) redox processes that were previously assumed to be purely abiotic, such as photochemical Fe reactions, are now known to also be microbially mediated. Owing to this overlap, discerning whether biotic or abiotic processes control Fe redox chemistry is a major challenge for geomicrobiologists and biogeochemists alike. Therefore, to understand the network of reactions within the biogeochemical Fe cycle, it is necessary to determine which abiotic or microbially mediated reactions are dominant under various environmental conditions. In this Review, we discuss the major microbially mediated and abiotic reactions in the biogeochemical Fe cycle and provide an integrated overview of biotic and chemically mediated redox transformations.

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Understanding the mechanisms of anaerobic microbial iron cycling is necessary for a full appreciation of present‐day biogeochemical cycling of iron and carbon and for drawing conclusions about these cycles on the ancient Earth. Towards that end, we isolated and characterized an anaerobic nitrate‐dependent Fe(II)‐oxidizing bacterium from a freshwater sediment. The 16SrRNA gene sequence of the isolated bacterium (strain BoFeN1) places it within the β‐Proteobacteria, with Acidovorax sp. strain G8B1 as the closest known relative. During mixotrophic growth with acetate plus Fe(II) and nitrate as electron acceptor, strain BoFeN1 forms Fe(III) mineral crusts around the cells. The amount of the organic cosubstrate acetate present seems to control the rate and extent of Fe(II) oxidation and the viability of the cells. The crystallinity of the mineral products is influenced by nucleation by Fe minerals that are already present in the inoculum.

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