Potential for Microscale Bacterial Fe Redox Cycling at the Aerobic-Anaerobic Interface

Recent studies of bacterial Fe(II) oxidation at circumneutral pH by a newly-isolated lithotrophic β-Proteobacterium (strain TW2) are reviewed in relation to a conceptual model that accounts for the influence of biogenic Fe(III)-binding ligands on patterns of Fe(II) oxidation and Fe(III) oxide deposition in opposing gradients of Fe(II) and O2. The conceptual model envisions complexation of Fe(III) by biogenic ligands as mechanism which alters the locus of Fe(III) oxide deposition relative to Fe(II) oxidation so as to delay/retard cell encrustation with Fe(III) oxides. Experiments examining the potential for bacterial Fe redox cycling in microcosms containing ferrihydrite-coated sand and a coculture of a lithotrophic Fe(II)-oxidizing bacterium (strain TW2) and a dissimilatory Fe(III)-reducing bacterium (Shewanella algae strain BrY) are described and interpreted in relation to an extended version of the conceptual model in which Fe(III)-binding ligands promote rapid microscale Fe redox cycling. The coculture systems showed minimal Fe(III) oxide accumulation at the sand-water interface, despite intensive O2 input from the atmosphere and measurable dissolved O2 to a depth of 2 mm below the sand-water interface. In contrast, a distinct layer of oxide precipitates formed in systems containing Fe(III)-reducing bacteria alone. Voltammetric microelectrode measurements revealed much lower concentrations of dissolved Fe(II) in the coculture systems. Examination of materials from the cocultures by fluorescence in situ hybridization indicated close physical juxtapositioning of Fe(II)-oxidizing and Fe(III)reducing bacteria in the upper few mm of sand. Together these results indicate that Fe(II)-oxidizing bacteria have the potential to enhance the coupling of Fe(II) oxidation and Fe(III) reduction at redox interfaces, thereby promoting rapid microscale cycling of Fe.