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

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 +/- 0.4parts per thousand); Chaetoceros simplex (2.7 +/- 0.3parts per thousand); Ditylum brightwellii (3.3 +/- 0.4parts per thousand); Skeletonema costatum (2.7 +/- 0.3parts per thousand); Phaeodactylum tricornutum (4.8 +/- 0.3parts per thousand); Emiliania huxleyi (4.5 +/- 0.2parts per thousand), Isochrysis galbana (3.2 +/- 0.4parts per thousand); Pavlova lutheri, (3.6 +/- 0.5parts per thousand); Amphidinium carterae (2.2 +/- 0.3parts per thousand); Prorocentrum minimum (2.5 +/- 0.3parts per thousand); Dunaliella tertiolecta (2.2 +/- 0.2parts per thousand); and Synechococcus sp. (5.4 +/- 0.6parts per thousand). 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 9parts per thousand). 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 delta(15)N signal generated by phytoplankton in the ocean.