Nitrogen isotope fractionation during the uptake and assimilation of nitrate, nitrite, ammonium, and urea by a marine diatom

Knowledge of the fractionation of nitrogen isotopes by phytoplankton is a key requirement for the calibration of the new δ 15 N 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 δ 15 N of batch cultures of Thalassiosira pseudonana grown on nitrate, nitrite, ammonium, and urea was determined. This paper reports the first δ 15 N study of phytoplankton growth on urea (e.g. organic N substrate). The δ 15 N of the particulate nitrogen (PN) collected during the logarithmic growth phase, thus for N‐sufficient cells, was lower than the δ 15 N of the source due to kinetic isotope fractionation. With increasing drawdown of the N substrate, the δ 15 N of the accumulating PN increased in accordance with the Rayleigh distillation model. Enrichment factors (ε) derived from a least‐squares analysis of the accumulated δ 15 N PN data were 5.2 ± 0.2‰, 0.9 ± 0.6‰, 20 ± 1‰, and 0.8 ± 0.6‰ for NO 3 − , NO 2 − , NH 4 + , and urea incorporation, respectively. Overall, ε values for nitrate incorporation were consistent with field estimates of ~6‰ and could be used to estimate past relative nitrate utilization as estimated from the 615N of bulk sedimentary organic matter.