Nitrogen (N) balancing metabolism in the toxic dinoflagellate Alexandrium pacificum against N shift revealed by physiology and N-related genes regulation

The dinoflagellate Alexandrium pacificum is responsible for harmful algal blooms and paralytic shellfish poisoning in marine environments. Its physiology is greatly affected by nitrogen (N) sources; however, the molecular mechanisms involved in N acquisition and balancing are not clearly understood. Here, we determined the full-length gene sequences of nitrate (NO₃⁻) transporter (ApNRT), NO₃⁻ reductase (ApNR), and ammonium (NH₄⁺) transporter (ApAMT) from the dinoflagellate A. pacificum. In addition, we examined physiological and transcriptional responses of these three genes under diverse concentrations of NO₃⁻ (0.00–8.82 mM) and NH₄⁺ (0.00–1.76 mM). The open reading frames of ApNRT, ApNR, and ApAMT were determined as 1767 bp, 3312 bp, and 1363 bp, without introns in their genomic coding regions. Their encoded proteins were phylogenetically close to those of other photosynthetic eukaryotes. NO₃⁻ supplementation promoted cell growth, while NH₄⁺ inhibited it. Expression of ApNRT and ApNR were correlated in both low and high N conditions. Sufficient uptake of one of the N forms (NO₃⁻ and NH₄⁺, respectively) suppressed the regulation of the other dissolved inorganic nitrogen (DIN) transporter (ApAMT and ApNRT, respectively). These results showed that A. pacificum may have a selective mechanism for N uptake depending on the available N sources, suggesting a proliferation strategy of dinoflagellate in eutrophic environments.