MEPS 255:81-91 (2003)  -  doi:10.3354/meps255081

Nitrogen isotope fractionation in 12 species of marine phytoplankton during growth on nitrate

Joseph A. Needoba1,*, N. A. Waser2, P. J. Harrison1,3, S. E. Calvert2

1Department of Botany, and
2Department of Earth and Ocean Sciences, University of British Columbia, 6270 University Boulevard, Vancouver, British Columbia V6T 1Z4, Canada
3Atmospheric and Marine Coastal Environmental Program, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR

ABSTRACT: The nitrogen isotopic composition of 12 species of marine phytoplankton were determined by isotope ratio mass spectrometry in order to investigate isotope fractionation associated with growth on nitrate. The species, representing diatoms, coccolithophores, dinoflagellates, green algae, and cyanobacteria, were grown in batch cultures in artificial seawater under the same laboratory conditions of constant light and temperature. The species (with isotope fractionation values in parenthesis) were: Thalassiosira weissflogii (6.2 ± 0.4‰); Chaetoceros simplex (2.7 ± 0.3‰); Ditylum brightwellii (3.3 ± 0.4‰); Skeletonema costatum (2.7 ± 0.3‰); Phaeodactylum tricornutum (4.8 ± 0.3‰); Emiliania huxleyi (4.5 ± 0.2‰), Isochrysis galbana (3.2 ± 0.4‰); Pavlova lutheri, (3.6 ± 0.5‰); Amphidinium carterae (2.2 ± 0.3‰); Prorocentrum minimum (2.5 ± 0.3‰); Dunaliella tertiolecta (2.2 ± 0.2‰); and Synechococcus sp. (5.4 ± 0.6‰). There was no relationship between isotope fractionation and organism group, nor was there a direct effect of cell size or growth rate on the degree of isotope fractionation among all the groups. Overall, the results show that isotope fractionation during growth on nitrate is lower than values obtained from field samples (i.e. 4 to 9‰). These results indicate that there is no simple mechanism for describing differences in isotope fractionation between groups of phytoplankton, and that a physiological understanding of isotope fractionation during uptake and assimilation of nitrate is needed to properly understand the δ15N signal generated by phytoplankton in the ocean.

KEY WORDS: Marine phytoplankton · Nitrogen isotope fractionation · Nitrate · 15N:14N · Stable isotopes · Batch culture

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