Controls on Upper Ocean Enzymatic Nitrate-Nitrite Isotopic Exchange in and beyond Southern Ocean

Increasing studies have recognized that the enzymatic nitrate-nitrite isotopic exchange reaction may be a potential breakthrough to update our understanding of the nitrogen cycle. However, fundamental aspects of this reaction remain poorly understood, limiting our comprehensive understanding of the nitrogen cycle. In this study, we present the first coupled measurements of nitrate and nitrite dual isotopes at natural abundance across the Antarctic summer to uncover the environmental factors influencing enzymatic isotopic exchange. East Antarctic surface waters exhibit the most anomalous nitrate and nitrite isotope signatures and a more pronounced equilibrium isotope effect compared to West Antarctica. This feature may be attributed to the regulation of enzymatic isotopic exchange reaction by temperature, and we infer that there might be a tipping point in the expression of its intensity. Given the warming of Antarctic waters due to global climate change, particularly in West Antarctic, we hypothesize that such reaction could have an amplified impact on nitrogen isotope dynamics. Further analyses incorporating data from beyond the Southern Ocean also suggest that functional differences in the nitrite oxidoreductase enzyme itself are a critical contributing factor. Overall, our study provides new insights into the mechanisms underlying the enzymatic isotopic exchange reaction, with broad implications for models of the modern upper ocean nitrogen cycle and paleoceanographic reconstructions of ancient nitrogen cycle dynamics.