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Microbial iron oxidation in the wetland plant rhizosphere.
Neubauer, Scott*,1, Megonigal, Patrick1, Emerson, David2, 1 Smithsonian Environmental Research Center, Edgewater, MD, USA2 American Type Culture Collection, Manassas, VA, USA
ABSTRACT- The formation of iron plaque on the roots of wetland plants can occur when oxygen leaking from plant roots oxidizes soluble ferrous iron in wetland porewaters. Although the chemical oxidation of Fe(II) proceeds very rapidly at circumneutral pH, lithotrophic Fe(II) oxidizing bacteria (FeOB) are common on the roots of many wetland plants and can be a significant fraction of the total root-associated microbial community, indicating that they may play a role in rhizosphere Fe(II) oxidation. To assess the significance of microbial iron oxidation, a series of laboratory experiments was performed using a purified strain of FeOB (strain BrT) that was isolated from the roots of Typha latifolia. The first set of lab experiments used batch cultures of strain BrT grown in bioreactors under tightly controlled pH, O2, and Fe(II) supply conditions. A second set of experiments examined iron oxidation in hydroponic microcosms that contained surface-sterilized roots of live wetland plants that were subsequently inoculated with live BrT cells or autoclaved iron oxides. In the microcosm experiments, the only oxygen source to the hydroponic solution was radial oxygen loss from the plant roots. In both sets of experiments, rates of iron oxidation were measured by monitoring the disappearance of dissolved Fe(II) over periods of hours to days. Results from these experiments indicate that iron oxidizing bacteria can accelerate rates of Fe(II) oxidation and may account for over 50% of total Fe(II) oxidation. Additionally, the FeOB were able to successfully compete with chemical oxidation for Fe(II) (i.e. grow), despite evidence that total Fe(II) oxidation was limited by the Fe(II) supply (bioreactor experiments) or the rate of Fe(II) diffusion to the root surface (microcosm experiments). Taken together, these experiments indicate that iron oxidizing bacteria are likely to contribute to iron plaque formation in the rhizosphere.
Key words: microbial iron oxidation, rhizosphere, lithotrophic bacteria, radial oxygen loss