The cycling of iron is driven by abiotic and biotic reactions. Bacteria can actboth directly and indirectly to oxidize or reduce iron (Fortin and Langley,2005). Direct interactions involve the enzymatic conversion of iron to bothoxidized and reduced forms in order to gain energy (Ghiorse, 1984; Widdel etal., 1993). Indirect interactions involve localized alteration of pH and Ehconditions through their metabolic activities, which then promotes eithermineral precipitation or solubilisation with subsequent metal mobilization(Fortin et al., 1998). Also, bacteria can accumulate iron by passivephysicochemical sorption of Fe(III) and Fe(II) ions onto the negatively chargedmicrobial and extracellular polymeric substance (EPS) surfaces (McLean et al.,1996; Jiao et al., 2010), which may accelerate the rate of iron precipitation.Iron oxides formed in close association with bacteria (either as internal orexternal precipitates) are referred to as biogenic minerals. The formation,occurrence, and potential use of biogenic iron-rich minerals have beenthoroughly reviewed (Fortin and Langley, 2005).
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