Life at the Energetic Edge: Kinetics of Circumneutral Iron Oxidation by Lithotrophic Iron-Oxidizing Bacteria Isolated from the Wetland-Plant Rhizosphere

Batch cultures of a lithotrophic Fe(II)-oxidizing bacterium, strain BrT, isolated from the rhizosphere of a wetland plant, were grown in bioreactors and used to determine the significance of microbial Fe(II) oxidation at circumneutral pH and to identify abiotic variables that affect the partitioning between microbial oxidation and chemical oxidation. Strain BrT grew only in the presence of an Fe(II) source, with an average doubling time of 25 h. In one set of experiments, Fe(II) oxidation rates were measured before and after the cells were poisoned with sodium azide. These experiments indicated that strain BrT accounted for 18 to 53% of the total iron oxidation, and the average cellular growth yield was 0.70 g of CH2O per mol of Fe(II) oxidized. In a second set of experiments, Fe(II) was constantly added to bioreactors inoculated with live cells, killed cells, or no cells. A statistical model fitted to the experimental data demonstrated that metabolic Fe(II) oxidation accounted for up to 62% of the total oxidation. The total Fe(II) oxidation rates in these experiments were strongly limited by the rate of Fe(II) delivery to the system and were also influenced by O2 and total iron concentrations. Additionally, the model suggested that the microbes inhibited rates of abiotic Fe(II) oxidation, perhaps by binding Fe(II) to bacterial exopolymers. The net effect of strain BrT was to accelerate total oxidation rates by up to 18% compared to rates obtained with cell-free treatments. The results suggest that neutrophilic Fe(II)-oxidizing bacteria may compete for limited O2 in the rhizosphere and therefore influence other wetland biogeochemical cycles.