Enzyme-catalyzed isotope equilibrium: A hypothesis to explain apparent N cycling phenomena in low oxygen environments

Increasing reports of anoxic pathways of nitrite oxidation in oxygen deficient systems challenge longstanding ideas about biogeochemical nitrogen turnover. Here, we present stable isotope data from experiments examining the nitrite-oxidizing bacterium, Nitrococcus mobilis, grown under nitrate-reducing conditions. Results confirm that N. mobilis reduces nitrate to nitrite under low oxygen, ostensibly via nitrite oxidoreductase (NXR) acting in reverse. Estimates for 15N isotope effects for NXR-based nitrate reduction ranged from 27 to 55‰, far larger than previously reported values for nitrate reduction catalyzed by nitrate reductase, while oxygen isotopes exhibited very little change. We suggest these observations are best explained by enzyme-catalyzed isotope equilibrium between nitrite and nitrate, similar to that reported for anammox and microbial carbon and sulfur cycling. Enzyme-catalyzed isotope equilibrium may play an underappreciated role in the application of stable isotopes to N cycling studies, including cycling rate measurements and biogeochemical models of N turnover. Our results underscore several considerations about N cycling in redox transition zones and emphasize the need to better understand the potential for enzyme-catalyzed isotope equilibrium in studies of the nitrogen cycle. •Nitrite-oxidizing bacterial activity during nitrate reduction yields unusual isotope dynamics.•During nitrate reduction, exceptionally large 15N isotope effects are observed.•N isotope effects are distinctly decoupled from O isotope dynamics.•N and O isotope dynamics are consistent with enzyme-catalyzed equilibration between NO2− and NO3−.•Enzyme reversibility under low oxygen may impact rate estimates and other isotope-based studies.