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Effect of iron limitation on the isotopic composition of cellular and released fixed nitrogen in Azotobacter vinelandii

McRose, D. L. and Lee, A. and Kopf, S. H. and Baars, O. and Kraepiel, A. M. L. and Sigman, D. M. and Morel, F. M. M. and Zhang, X. (2019) Effect of iron limitation on the isotopic composition of cellular and released fixed nitrogen in Azotobacter vinelandii. Geochimica et Cosmochimica Acta, 244 . pp. 12-23. ISSN 0016-7037. doi:10.1016/j.gca.2018.09.023.

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Most biological nitrogen transformations have characteristic kinetic isotope effects used to track these processes in modern and past environments. The isotopic fractionation associated with nitrogen fixation, the only biological source of fixed nitrogen (N), provides a particularly important constraint for studies of nitrogen cycling. Nitrogen fixation using the ‘canonical’ Mo-nitrogenase produces biomass with a δ^(15)N value of ca. −1‰ (vs. atmospheric N_2). If the ‘alternative’ V- and Fe-only nitrogenases are used, biomass δ^(15)N can be between −6‰ and −7‰. These biomass values are assumed to be relatively invariant and to reflect the cellular level expressed isotope effect of nitrogen fixation. However, field and laboratory studies report wide ranges of diazotrophic biomass δ^(15)N (from −3.6‰ to +0.5‰ for Mo-based nitrogen fixation). This variation could be partly explained by the release of dissolved organic N (DON) that is isotopically distinct from biomass. The model nitrogen fixer Azotobacter vinelandii secretes siderophores, small molecules that aid in Fe uptake and can comprise >30% of fixed nitrogen. To test whether siderophores (and other released N) can decouple biomass δ^(15)N from the isotope effect of nitrogen fixation we measured the isotopic composition of biomass and released N in Fe-limited A. vinelandii cultures fixing nitrogen with Mo- and V-nitrogenases. We report that biomass δ^(15)N was elevated under Fe limitation with a maximum value of +1.2‰ for Mo-based nitrogen fixation. Regardless of the nitrogenase isozyme used, released nitrogen δ^(15)N was also 2–3‰ lower than biomass δ^(15)N. Siderophore nitrogen was found to have a slightly higher δ^(15)N than the rest of the DON pool but was still produced in large enough concentrations to account for increases in biomass δ15N. The low δ^(15)N of siderophores (relative to biomass) is consistent with what is known from compound specific isotope studies of the amino acids used in siderophore biosynthesis, and indicates that other amino-acid derived siderophores should also have a low δ^(15)N. The implications for studies of nitrogen fixation are discussed.

Item Type:Article
Related URLs:
URLURL TypeDescription
McRose, D. L.0000-0001-9637-7176
Kopf, S. H.0000-0002-2044-0201
Sigman, D. M.0000-0002-7923-1973
Morel, F. M. M.0000-0002-6311-7826
Additional Information:© 2018 Elsevier Ltd. Received 11 April 2018, Accepted 22 September 2018, Available online 1 October 2018. We thank M.A. Weigand for her help and guidance with the denitrifier method. We are also grateful to X.T. Wang, E.R. Kast, V. Luu, J.A. Lueders-Dumont, S.E. Fawcett and A.R. Babbin for helpful discussions and assistance with methods. This work was supported by the Princeton Environmental Institute, the National Science Foundation (grant numbers EAR-1631814 and OCE 1657639) and NASA 80NSSC17K0667.
Funding AgencyGrant Number
Princeton Environmental InstituteUNSPECIFIED
Subject Keywords:Nitrogen isotopes; Nitrogen fixation; Alternative nitrogenase; Iron; Kinetic isotope effect; Siderophore; Azotobacter vinelandii
Record Number:CaltechAUTHORS:20181205-163500871
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Official Citation:D.L. McRose, A. Lee, S.H. Kopf, O. Baars, A.M.L. Kraepiel, D.M. Sigman, F.M.M. Morel, X. Zhang, Effect of iron limitation on the isotopic composition of cellular and released fixed nitrogen in Azotobacter vinelandii, Geochimica et Cosmochimica Acta, Volume 244, 2019, Pages 12-23, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:91521
Deposited By: George Porter
Deposited On:06 Dec 2018 15:35
Last Modified:16 Nov 2021 03:41

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