Inverse kinetic isotope fractionation during bacterial nitrite oxidation
Inverse kinetic isotope fractionation during bacterial nitrite oxidation
Date
2008-12-29
Authors
Casciotti, Karen L.
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Abstract
Natural abundance stable isotopes in nitrate (NO3-), nitrite (NO2-), and nitrous oxide (N2O)
have been used to better understand the cycling of nitrogen in marine and terrestrial
environments. However, in order to extract the greatest information from the distributions of
these isotopic species, the kinetic isotope effects for each of the relevant microbial reactions are
needed. To date, kinetic isotope effects for nitrite oxidation and anaerobic ammonium oxidation
(anammox) have not been reported. In this study, the nitrogen isotope effect was measured for
microbial nitrite oxidation to nitrate. Nitrite oxidation is the second step in the nitrification
process, and it plays a key role in the regeneration of nitrate in the ocean. Surprisingly, nitrite
oxidation occurred with an inverse kinetic isotope effect, such that the residual nitrite became
progressively depleted in 15N as the reaction proceeded. Three potential explanations for this
apparent inverse kinetic isotope effect were explored: 1) isotope exchange equilibrium between
nitrite and nitrous acid prior to reaction, 2) reaction reversibility at the enzyme level, and 3) true
inverse kinetic fractionation. Comparison of experimental data to ab initio calculations and
theoretical predictions leads to the conclusion that the fractionation is most likely inverse at the
enzyme level. Inverse kinetic isotope effects are rare, but the experimental observations reported
here agree with kinetic isotope theory for this simple N-O bond-forming reaction. Nitrite
oxidation is therefore fundamentally different from all other microbial processes in which N
isotope fractionation has been studied. The unique kinetic isotope effect for nitrite oxidation
should help to better identify its role in the cycling of nitrite in ocean suboxic zones, and other
environments in which nitrite accumulates.
Description
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 73 (2009): 2061-2076, doi:10.1016/j.gca.2008.12.022.