Coupled nitrogen and oxygen isotope fractionation of nitrate during assimilation by cultures of marine phytoplankton

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 ¹⁸O/¹⁶O and ¹⁵N/¹⁴N of nitrate varied in a ratio of ∼1:1, regardless of species or of the magnitude of the isotope effect. In addition, the ¹⁸O/¹⁶O and ¹⁵N/¹⁴N of internal nitrate of T. weissflogii grown under various environmental conditions were elevated relative to the medium nitrate by a proportion of ∼1: 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.