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, D. H. Turpin

Limnol. Oceanogr., 43(2), 1998, 215-224 | DOI: 10.4319/lo.1998.43.2.0215

ABSTRACT: Knowledge of the fractionation of nitrogen isotopes by phytoplankton is a key requirement for the calibration of the new δ15N 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 δ15N of batch cultures of Thalassiosira pseudonana grown on nitrate, nitrite, ammonium, and urea was determined. This paper reports the first δ15N study of phytoplankton growth on urea (e.g. organic N substrate). The δ15N of the particulate nitrogen (PN) collected during the logarithmic growth phase, thus for N-sufficient cells, was lower than the δ15N of the source due to kinetic isotope fractionation. With increasing drawdown of the N substrate, the δ15N of the accumulating PN increased in accordance with the Rayleigh distillation model. Enrichment factors (ε) derived from a least-squares analysis of the accumulated δ15NPN 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, ε 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.

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