Microbial Iron Redox Metabolism in Circumneutral pH Environments

Redox cycling of iron (Fe) is a key process governing energy flow as well as the speciation and mobility of a wide variety of aqueous and solid-phase constituents in soil and sedimentary environments. Both reduction and oxidation of Fe are microbially catalyzed, and available evidence suggests that microbial Fe redox metabolism takes place across a wide range of spatial and temporal scales natural systems. The coupling of microbial Fe reduction and oxidation has been proposed in various situations where a redox transition zone is observed. This talk will review and synthesize several case studies of the potential for Fe redox cycling in circumneutral pH subsurface environments. Of specific interest are novel pathways and organisms involved in the oxidation of insoluble reduced Fe phases with oxygen or nitrate, and the coupling of Fe oxidation and reduction in field and experimental systems that model potential redox gradients and fluctuations in the subsurface. Recent cultivation studies and physiological experiments indicate that a variety of Proteobacteria are able to oxidize Fe(II)-silicates (e.g. biotite, smectite) and other insoluble Fe(II)-bearing minerals (e.g. pyrite). These findings, together with recent genomic insights from pure and mixed cultures, set the stage for rapid expansion in our knowledge of the range of extracellular electron transfer mechanisms utilized by subsurface microorganisms. These mechanisms permit both closely coupled oxidation and reduction of Fe, as well as previously unrecognized pathways for microbial acceleration of neutral-pH oxidative weathering processes.