Microbially mediated coupling of nitrate reduction and Fe(II) oxidation under anoxic conditions

The biogeochemical redox processes of iron can influence iron mineralization, contaminant transformation and the fate of nutrients. Fe(II) can be chemically/biologically oxidized by O-2 under oxic conditions or by other oxidants under anoxic conditions. This review focused on microbially mediated NO3--reducing Fe(II) oxidation (NRFO) under anoxic conditions. NRFO processes have been reported to play important roles in Fe/N interactions under neutral-anoxic conditions; however, the mechanism for the metabolic process is largely unknown. Although the Fe-N coupling process was previously identified as a biological process, it has been recently documented that this process is mediated by both chemical and biological mechanisms. Nitrite and NO, the intermediate products of biological nitrate reduction, are able to chemically react with Fe(II) in this system. The importance of enzymatic Fe(II) oxidation has been overestimated. Therefore, the contributions of biological processes and chemical processes to Fe(II) oxidation remain unclear. This review aimed to comprehensively clarify the roles of the biological and chemical processes involved in NRFO. Further studies are necessary to completely answer the question of whether NRFO bacteria can perform enzymatic Fe(II) oxidation coupled with nitrate reduction or if it should be considered a biologically induced, chemical side effect of biological nitrate reduction in Fe(II)-bearing environments.