Microbially mediated nitrate reduction coupled with Fe(II) oxidation (NRFO) plays an important role in the Fe/N interactions in pH-neutral anoxic environments. However, the relative contributions of the chemical and microbial processes to NRFO are still unclear. In this study, N-O isotope fractionation during NRFO was investigated. The ratios of O and N isotope enrichment factors (18ε:15ε)-NO3- indicated that the main nitrate reductase functioning in Acidovorax sp. strain BoFeN1 was membrane-bound dissimilatory nitrate reductase (Nar). N-O isotope fractionation during chemodenitrification [Fe(II) + NO2-], microbial nitrite reduction (cells + NO2-), and the coupled process [cells + NO2- + Fe(II)] was explored. The ratios of (18ε:15ε)-NO2- were 0.58 ± 0.05 during chemodenitrification and -0.41 ± 0.11 during microbial nitrite reduction, indicating that N-O isotopes can be used to distinguish chemical from biological reactions. The (18ε:15ε)-NO2- of 0.70 ± 0.05 during the coupled process was close to that obtained for chemodenitrification, indicating that chemodenitrification played a more important role than biological reactions during the coupled process. The results of kinetic modeling showed that the relative contribution of chemodenitrification was 99.3% during the coupled process, which was consistent with that of isotope fractionation. This study provides a better understanding of chemical and biological mechanisms of NRFO using N-O isotopes and kinetic modeling.
Keywords: N and O isotope fractionation; cell encrustation; chemical and microbial processes; nitrate-reducing Fe(II) oxidation; nitrite reduction.