Abiotic and Microbial Interactions during Anaerobic Transformations of Fe(II) and [Formula: see text]

Microbial Fe(II) oxidation using [Formula: see text] as the terminal electron acceptor [nitrate-dependent Fe(II) oxidation, NDFO] has been studied for over 15 years. Although there are reports of autotrophic isolates and stable enrichments, many of the bacteria capable of NDFO are known organotrophic [Formula: see text]-reducers that require the presence of an organic, primary substrate, e.g., acetate, for significant amounts of Fe(II) oxidation. Although the thermodynamics of Fe(II) oxidation are favorable when coupled to either [Formula: see text] or [Formula: see text] reduction, the kinetics of abiotic Fe(II) oxidation by [Formula: see text] are relatively slow except under special conditions. NDFO is typically studied in batch cultures containing millimolar concentrations of Fe(II), [Formula: see text], and the primary substrate. In such systems, [Formula: see text] is often observed to accumulate in culture media during Fe(II) oxidation. Compared to [Formula: see text] abiotic reactions of biogenic [Formula: see text] and Fe(II) are relatively rapid. The kinetics and reaction pathways of Fe(II) oxidation by [Formula: see text] are strongly affected by medium composition and pH, reactant concentration, and the presence of Fe(II)-sorptive surfaces, e.g., Fe(III) oxyhydroxides and cellular surfaces. In batch cultures, the combination of abiotic and microbial Fe(II) oxidation can alter product distribution and, more importantly, results in the formation of intracellular precipitates and extracellular Fe(III) oxyhydroxide encrustations that apparently limit further cell growth and Fe(II) oxidation. Unless steps are taken to minimize or account for potential abiotic reactions, results of microbial NDFO studies can be obfuscated by artifacts of the chosen experimental conditions, the use of inappropriate analytical methods, and the resulting uncertainties about the relative importance of abiotic and microbial reactions. In this manuscript, abiotic reactions of [Formula: see text] and [Formula: see text] with aqueous Fe(2+), chelated Fe(II), and solid-phase Fe(II) are reviewed along with factors that can influence overall NDFO reaction rates in microbial systems. In addition, the use of low substrate concentrations, continuous-flow systems, and experimental protocols that minimize experimental artifacts and reduce the potential for under- or overestimation of microbial NDFO rates are discussed.