DIFFERENTIATING CHEMICAL AND BACTERIAL IRON OXIDATION WITH LONG-RANGE CORRELATIONS IN OXIDATION-REDUCTION POTENTIAL

Bacterial oxidation of Fe²⁺ is significant because of its role as one of the earliest forms of metabolism on Earth, and because it contributes to global biogeochemical cycling and physicochemical speciation of iron. At circumnetural pH, rapid chemical oxidation of Fe²⁺ in the presence of atmospheric oxygen restricts Fe²⁺- oxidizing bacteria to low pO₂ environments where chemical oxidation is slower. In environments where neutrophilic Fe²⁺-oxidizing bacteria compete with chemical oxidation, rapid hydrolysis and precipitation of Fe³⁺ produces flocculent mats of bacteriogenic iron oxides (BIOS) that consist of poorly-ordered hydrous ferric-oxides intermixed with bacterial cells. Despite underpinning all biochemical reactions, the oxidation-reduction (redox) dynamics of microbial communities are poorly understood. This is particularly true of BIOS mats, which are complex heterogeneous systems with a number of chemical and biological processes that exist under far from equilibrium conditions. Using detrended fluctuation analysis (DFA), we have calculated α scaling exponents for fluctuations of oxidation-reduction potential in the presence and absence of Fe²⁺-oxidizing bacteria. The α values for both systems are indicative of fractional Brownian motion; however, a mean α value of 1.74 + 0.03 was determined in the absence of bacteria, whereas the mean α value was 1.89 + 0.02 in the presence of Fe²⁺-oxidizing bacteria . These α values are significantly different at p < 0.01 and demonstrate that bacterial Fe²⁺-oxidation can be distinguished from chemical oxidation using DFA. This signal processing technique holds promise as a method for remote monitoring of in situ bacterial activity using time series measurements of oxidation-reduction potential.