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.

越来越多的关于缺氧系统中亚硝酸盐氧化的缺氧途径的报道挑战了关于生物地球化学氮周转的长期观点。在这里，我们展示了在硝酸盐还原条件下生长的亚硝酸盐氧化细菌Nitrococcus mobilis的稳定同位素数据。结果证实，运动链霉菌在低氧条件下将硝酸盐还原为亚硝酸盐，表面上是通过亚硝酸盐氧化还原酶 (NXR) 反向作用。^{基于 NXR 的硝酸盐还原的15} N 同位素效应估计值在 27 至 55‰ 之间，远大于先前报道的硝酸盐还原酶催化的硝酸盐还原值，而氧同位素的变化很小。我们认为这些观察结果最好用亚硝酸盐和硝酸盐之间的酶催化同位素平衡来解释，类似于厌氧氨氧化和微生物碳和硫循环的报道。酶催化同位素平衡可能在将稳定同位素应用于 N 循环研究（包括循环速率测量和 N 周转的生物地球化学模型）中发挥的作用未被充分认识。我们的研究结果强调了关于氧化还原过渡区 N 循环的几个考虑因素，并强调需要更好地了解酶催化同位素平衡在氮循环研究中的潜力。