Put it in Reverse: Isotopic Insights From a Nitrite-oxidizer Performing Nitrate Reduction

Increasing evidence for the occurrence of anoxic pathways of nitrite oxidation in marine oxygen deficient zones (ODZs) challenges longstanding ideas about the nature of biogeochemical nitrogen turnover in these systems and others. Here, we present stable isotope data from a series of experiments examining the nitrite oxidizing bacterium, Nitrococcus mobilis, grown under anaerobic conditions in the presence of nitrate. Our results confirm previously reported observations that under these conditions N. mobilis is able to reduce nitrate to nitrite, presumably via nitrite oxidoreductase (NXR) acting in reverse. Using a Rayleigh model, our data exhibited an ¹⁵N isotope effect for NXR-based nitrate reduction falling between 27 to 49‰ – larger than any previously reported nitrogen kinetic isotope effect, and varying as a function of initial nitrate concentration. Rather than a normal kinetic isotope effect, however, we suggest that these observations reflect an enzyme-catalyzed equilibrium between nitrite and nitrate – similar to that reported for nitrite and nitrate during anammox and to that reported to occur in microbial carbon and sulfur cycling. Given the prevalence of nitrite oxidoreductase in the global ocean, and especially ODZs, it is conceivable that enzyme-catalyzed isotope equilibrium may play an underappreciated role in stable isotope dynamics. For example, large, routinely observed differences between δ¹⁵N of nitrite and nitrate (Dδ¹⁵N) in ODZs are often explained as rapid and rampant recycling between nitrate and nitrite. As a nuanced, yet alternative explanation, this could stem in part from the activity of nitrite-oxidizing bacteria driving the system towards N isotope equilibrium catalyzed by reversible action of nitrite oxidoreductase – rather than the combined activity of nitrate-reducing and nitrite-oxidizing bacteria. Following from this then, the same phenomenon could also impact ¹⁵N-based measurements of nitrite oxidation under low O₂. In sum, our results raise a number of new considerations about N cycling in redox transition zones and emphasize the need to better understand the potential for enzyme catalyzed isotope equilibrium in the marine nitrogen cycle.