Enzyme-level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean

In the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO₃⁻) have been used to investigate NO₃⁻ assimilation and N cycling in the summertime period of phytoplankton growth, both today and in the past. However, recent studies indicate the significance of processes in other seasons for producing the annual cycle of N isotope changes. This study explores the impact of fall conditions on the ¹⁵N/¹⁴N (δ¹⁵N) and ¹⁸O/¹⁶O (δ¹⁸O) of NO₃⁻ and nitrite (NO₂⁻) in the Pacific Antarctic Zone using depth profiles from late summer/fall of 2014. In the mixed layer, the δ¹⁵N and δ¹⁸O of NO₃⁻ + NO₂⁻ increase roughly equally, as expected for NO₃⁻ assimilation; however, the δ¹⁵N of NO₃⁻-only (measured after NO₂⁻ removal) increases more than does NO₃⁻-only δ¹⁸O. Differencing indicates that NO₂⁻ has an extremely low δ¹⁵N, often < −70‰ versus air. These observations are consistent with the expression of an equilibrium N isotope effect between NO₃⁻ and NO₂⁻, likely due to enzymatic NO₃⁻-NO₂⁻ interconversion. Specifically, we propose reversibility of the nitrite oxidoreductase (NXR) enzyme of nitrite oxidizers that, having been entrained from the subsurface during late summer mixed layer deepening, are inhibited by light. Our interpretation suggests a role for NO₃⁻-NO₂⁻ interconversion where nitrifiers are transported into environments that discourage NO₂⁻ oxidation. This may apply to surface regions with upwelling, such as the summertime Antarctic. It may also apply to oxygen-deficient zones, where NXR-catalyzed interconversion may explain previously reported evidence of NO₂⁻ oxidation.