Journal
LIMNOLOGY AND OCEANOGRAPHY
Volume 49, Issue 5, Pages 1763-1773Publisher
AMER SOC LIMNOLOGY OCEANOGRAPHY
DOI: 10.4319/lo.2004.49.5.1763
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We report the first measurements of coupled nitrogen (N) and oxygen (O) isotopic variations of nitrate (NO3-) during its assimilation by laboratory cultures of marine phytoplankton and derive the N and O kinetic isotope effects for nitrate assimilation by three species of diatoms (Thalassiosira weissflogii, Thalassiosira oceanica, and Thalassiosira pseudonana) and a coccolithophorid (Emiliana huxleyi). Large interspecies and intraspecies variations in the N isotope effects were observed. The O isotope effect associated with nitrate consumption was consistently close to the N isotope effect, such that the O-18/O-16 and N-15/N-14 of nitrate varied in a ratio of similar to1:1, regardless of species or of the magnitude of the isotope effect. In addition, the O-18/O-16 and N-15/N-14 of internal nitrate of T. weissflogii grown under various environmental conditions were elevated relative to the medium nitrate by a proportion of similar to1: 1. These findings are consistent with a nitrate isotopic fractionation mechanism that involves nitrate reduction as the chief fractionating step. The observed N:O isotopic coupling during nitrate assimilation suggests that combined N and O isotopic measurements of water column nitrate can provide new constraints on the ocean N cycle.
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