Long-range correlations in redox potential distinguish bacterial ferrous iron oxidation

English Abstract:

Circumneutral ferrous iron-oxidizing bacteria eke out a living precariously balanced between opposing chemical gradients: the reduced iron which acts as their energy source and dissolved oxygen that, while necessary as a terminal electron acceptor for cellular metabolism, will out-compete the bacteria at atmospheric concentrations. In this study, detrended fluctuation analysis was used to quantify changes in redox potential that occurred over time in microcosms with different amounts of bacteriogenic iron oxides (BIOS); concomitant measurements of dissolved iron concentrations were used to determine accompanying rates of iron oxidation. The BIOS and water for the microcosms were obtained from a reduced anoxic groundwater seep where the microbial community consists of mat-forming iron-oxidizing bacteria, predominantly Gallionella ferruginea with a small amount of Leptothrix ocrachea. XRD analysis confirms that the iron precipitates from the live systems consist of x-ray amorphous material. Two abiotic control systems were also tested; one with killed BIOS and one consisting only of filtered creek water. SEM analyses of the live and autoclaved precipitates reveal that iron oxide precipitates were associated with bacterial cells found only in the live systems. Rates of iron oxidation in the living systems were similar to previously reported values for bacterial ferrous iron oxidation. At the same time, the microcosms were distinguished by the presence of persistent long-range correlations in redox potential that are indicative of anomalous diffusion behavior. At the beginning of the experiment, the live systems exhibited strong fractional Brownian motion and superdiffusion; however, as the bacterial consumed ferrous iron, the redox potential scaling exponents shifted to an apparent subdiffusive regime, which resembles a different dynamic state than when the microcosms started.