Impact of nitrogen chemical form on the isotope signature and toxicity of a marine dinoflagellate

Stable isotopes are used to identify and track nitrogen (N) sources to water bodies and thus can be used to ascertain the N source(s) used by the phytoplankton in those systems. To focus this tool for a particular harmful algal species, however, the fundamental patterns of N isotope fractionation by that organism must first be understood. While literature is available describing N isotope fractionation by diatoms and coccolithophores, data are lacking regarding dinoflagellates. Here we investigated the effects of N chemical form on isotope fractionation (Δ) and toxin content using isolates of the dinoflagellate Alexandrium catenella in single-N and mixed-N experiments. Growth of A. catenella exclusively on nitrate (NO₃ ^–), ammonium (NH₄ ⁺), or urea resulted in Δ of 2.7 ± 1.4, 29 ± 9.3, or 0.3 ± 0.1, respectively, with the lowest cellular toxicity reported during urea utilization. Cells initially utilized NH₄ ⁺ and urea when exposed to mixed-N medium and only utilized NO₃ ^– after NH₄ ⁺ decreased below 2 to 4 μM. This pattern of N preference was similar across all N treatments, suggesting that there is no effect of preconditioning on N chemical preference by A. catenella. In NO₃ ^–- and urea-rich environments, the δ¹⁵N of A. catenella would resemble the source(s) of N utilized, supporting this tool’s utility as a tracer of N source(s) facilitating bloom formation, but caution is advisable in NH₄ ⁺-rich environments, where the large Δ value could lead to misinterpretation of the signal.