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

• Published in: Global Biogeochemical Cycles
• Main Authors: Kemeny, PC, Weigand, MA, Zhang, R, Carter, BR, Karsh, KL, Fawcett, SE, Sigman, DM
• Format: Article in Journal/Newspaper
• Language: English
• Published: Amer Geophysical Union 2016
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Antarctic; Southern Ocean; The Antarctic; Pacific; Antarctic Ocean; Antarc*
• Online Access: https://doi.org/10.1002/2015GB005350; http://ecite.utas.edu.au/114803

In the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO 3 − ) have been used to investigate NO 3 − 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 15 N/ 14 N (δ 15 N) and 18 O/ 16 O (δ 18 O) of NO 3 − and nitrite (NO 2 − ) in the Pacific Antarctic Zone using depth profiles from late summer/fall of 2014. In the mixed layer, the δ 15 N and δ 18 O of NO 3 − + NO 2 − increase roughly equally, as expected for NO 3 − assimilation; however, the δ 15 N of NO 3 − -only (measured after NO 2 − removal) increases more than does NO 3 − -only δ 18 O. Differencing indicates that NO 2 − has an extremely low δ 15 N, often < −70 versus air. These observations are consistent with the expression of an equilibrium N isotope effect between NO 3 − and NO 2 − , likely due to enzymatic NO 3 − -NO 2 − 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 NO 3 − -NO 2 − interconversion where nitrifiers are transported into environments that discourage NO 2 − 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 NO 2 − oxidation.