Evidence for rapid microscale bacterial redox cycling of iron in circumneutral environments

The potential for microscale bacterial Fe redox cycling was investigated 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 alga strain BrY). The Fe(II)-oxidizing organism was isolated from freshwater wetland surface sediments which are characterized by steep gradients of dissolved O-2 and high concentrations of dissolved and solid-phase Fe( II) within mm of the sediment-water interface, and which support comparable numbers (10(5)-10(6) mL(-1)) of culturable Fe(II)-oxidizing and Fe(III)-reducing reducing. The coculture systems showed minimal Fe( III) oxide accumulation at the sand-water interface, despite intensive O-2 input from the atmosphere and measurable dissolved O-2 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. 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. Our 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.