Microbially mediated nitrate-reducing Fe(II) oxidation: Quantification of chemodenitrification and biological reactions

Abstract Redox reactions between iron and nitrogen drive the global biogeochemical cycles of these two elements and, concomitantly, change the fate of nutrients in and the mineralogy of the cycles. The microbially mediated NO3−-reducing Fe(II) oxidation process (NRFO) plays a key role in Fe/N interactions under neutral-anoxic conditions. Microbially mediated NRFO was considered a biological process, yet recently it has been documented that chemical mechanisms are also at play. However, the relative contributions of biological processes and chemical processes to Fe(II) oxidation remain largely unquantified owing to the co-occurrence of the reactions. Herein, the kinetics and secondary minerals of microbially mediated NRFO by Pseudogulbenkiania sp. strain 2002 and Acidovorax sp. strain BoFeN1 were investigated with acetate as electron donor unless otherwise stated. The results of Cells + NO3− suggested the two strains could biologically reduce NO3− to NO3−/NOx/N2O/N2 and concomitantly oxidize acetate and result in cell growth. Fe(II) oxidation and NO3− reduction occurred simultaneously in the presence of Fe(II) (Cells + Fe(II) + NO3−). For strain BoFeN1, the presence of Fe(II) slightly enhanced the NO3− reduction, acetate consumption, and cell growth, all of which were substantially retarded by Fe(II) for strain 2002. When compared with the microbial nitrite reduction, the relatively higher rate of chemical reaction between NO2− and dissolved Fe(II) confirmed the occurrence of chemodenitrification in the microbially mediated NRFO processes. After 5 days’ incubation, no green rust was observed, and lepidocrocite, goethite, and magnetite were observed with the Cells + Fe(II) + NO3− treatment, but only goethite was found with the Fe(II) + NO2−. The spectra for the EPSs + Fe(II) treatment suggested that the oxidized c-Cyts in the EPSs could oxidize Fe(II), which show the theoretical capability of taking electrons from Fe(II) into the cells via c-Cyts. A brief model was established by combining the verified reactions of (1) biological reduction of NO3− to NO2−/NOx/N2O/N2, (2) Fe(II) oxidation by NO2−, and (3) Fe(II) oxidation by c-Cyts in EPSs. Based on the model, the rate constant of Fe(II) oxidation by c-Cyts in EPSs was derived. For nitrite reduction, the relative contribution of biological processes to the nitrite reduction was higher than that of chemodenitrification. For Fe(II) oxidation, the relative contribution of the chemical process via nitrite to Fe(II) oxidation was higher than that of biological processes. These findings provide a quantitative interpretation of the chemodenitrification and biological reactions in the microbially mediated NRFO processes, which could assist the mechanistic understanding of the global biogeochemical cycles of iron and nitrogen in subsurface environments.