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

Increasing reports of anoxic pathways 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 low oxygen, ostensibly via oxidoreductase (NXR) acting reverse. Estimates for 15N effects NXR-based reduction ranged 27 55‰, far larger than previously reported values catalyzed by reductase, while isotopes exhibited very little change. We suggest these observations are best explained enzyme-catalyzed equilibrium between and nitrate, similar anammox microbial carbon sulfur cycling. Enzyme-catalyzed may play an underappreciated role application N cycling studies, including rate measurements models Our results underscore several considerations redox transition zones emphasize need better understand potential studies cycle.