Dual nitrogen-oxygen isotopic analysis and kinetic model for enzymatic nitrate reduction coupled with Fe(II) oxidation by Pseudogulbenkiania sp. strain 2002

Abstract

While microbially mediated coupling process of nitrate reduction and Fe(II) oxidation (NRFO) plays important role in widespread nitrogen and iron biogeochemical cycles in the natural environments, the underlying mechanism of NRFO is still not clearly understood. Here, N and O isotope fractionation during microbial nitrate reduction by Pseudogulbenkiania sp. strain 2002 with or without Fe(II) were investigated. The nitrate reduction rate decreased substantially in the presence of Fe(II), and cells became encrusted with Fe(III) minerals. However, the N isotope enrichment factors (ε) with or without Fe(II) were identical within uncertainty (24.1 ± 2.4‰ and 25.1 ± 2.4‰), implying that Fe(II) oxidation had little influence on the extent of N isotope fractionation and that cell encrustation was unlikely to cause additional mass transfer limitation. Isotopic composition ratios of 18O to 15N (18ε: 15ε) were 0.73 ± 0.13 with and 0.50 ± 0.08 without Fe(II), close to reported values for nitrate reduction by periplasmic nitrate reductases, implying that enzymatic nitrate reduction mainly occurs in strain 2002's periplasmic space. A kinetic model based on elementary reactions was established, and rate constants of enzymatic nitrate reducing reactions were obtained in a living cell system. Combining dual N-O isotope fractionation analysis with kinetic modeling is a promising approach for interpreting diverse enzymatic pathways during NRFO processes.