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

Kemeny, P. C. and Weigand, M. A. and Zhang, R. and Carter, B. R. and Karsh, K. L. and Fawcett, S. E. and Sigman, D. M. (2016) Enzyme-level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean. Global Biogeochemical Cycles, 30 (7). pp. 1069-1085. ISSN 0886-6236. doi:10.1002/2015GB005350.

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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.

Item Type:Article
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Sigman, D. M.0000-0002-7923-1973
Additional Information:© 2016 American Geophysical Union. Issue online: 10 August 2016; Version of record online: 13 July 2016; Accepted manuscript online: 20 June 2016; Manuscript Accepted: 13 June 2016; Manuscript Revised: 12 June 2016; Manuscript Received: 8 December 2015. The stable isotope data presented in this study will be merged into the P16S CCHDO product ( This research was funded by the U.S. NSF through grant OPP-1401489 (D.M.S.), by the Princeton Environmental Institute's Undergraduate Research Fund for senior thesis research at Princeton University (P.C.K.), and by the Princeton University Department of Geosciences Fund for senior thesis research (P.C.K.). This is PMEL contribution 4518. R.Z. appreciates the support of the CSC Fellowship, and S.E.F. is grateful to the University of Cape Town URC fund. We thank S. Oleynik, W. Abouchami, and V. Luu for help with isotopic analyses and the captain and crew of the RVIB Nathaniel B. Palmer for a successful voyage.
Funding AgencyGrant Number
Princeton UniversityUNSPECIFIED
University of Cape TownUNSPECIFIED
Subject Keywords:nitrate; nitrite; interconversion; seasonality; fall; antarctic
Issue or Number:7
Record Number:CaltechAUTHORS:20161020-083251171
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Official Citation:Kemeny, P. C., M. A. Weigand, R. Zhang, B. R. Carter, K. L. Karsh, S. E. Fawcett, and D. M. Sigman (2016), Enzyme-level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean, Global Biogeochem. Cycles, 30, 1069–1085, doi:10.1002/2015GB005350
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71310
Deposited By: Tony Diaz
Deposited On:20 Oct 2016 22:00
Last Modified:11 Nov 2021 04:43

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