Nitrogen isotope fractionation during the uptake and assimilation of nitrate, nitrite, ammonium, and urea by a marine diatom
Waser, N. A. D., P. J. Harrison, B. Nielsen, S. E. Calvert, and D. H. Turpin
Limnol. Oceangr. 43:215-224

Knowledge of the fractionation of nitrogen isotopes by phytoplankton is a key requirement for the calibration of the new ¹⁵N paleotracer. An essential part of information required in this calibration concerns the magnitude of isotopic fractionation during the incorporation of N substrates by phytoplankton. To this end, the ¹⁵N of batch cultures of Thalassiosira pseudonana grown on nitrate, nitrite, ammonium, and urea was determined. This paper reports the first ¹⁵N study of phytoplankton growth on urea (e.g. organic N substrate). The ¹⁵N of the particulate nitrogen (PN) collected during the logarithmic growth phase, thus for N-sufficient cells, was lower than the ¹⁵N of the source due to kinetic isotope fractionation. With increasing drawdown of the N substrate, the ¹⁵N of the accumulating PN increased in accordance with the Rayleigh distillation model. Enrichment factors () derived from a least-squares analysis of the accumulated ¹⁵N_PN data were 5.2 ± 0.2ppt, 0.9 ± 0.6ppt, 20 ± 1ppt, and 0.8 ± 0.6ppt for NO3-, NO2-, NH5+, and urea incorporation, respectively. Overall, values for nitrate incorporation were consistent with field estimates of ~6ppt and could be used to estimate past relative nitrate utilization as estimated from the 615N of bulk sedimentary organic matter.