Variability in toxicity of the dinoflagellate Alexandrium tamarense in response to different nitrogen sources and concentrations.

Nitrogen (N) supply in pulses was simulated by exposing Alexandrium tamarense which was acclimatized at low N concentration (6 microM-N) to sudden increase in concentrations of nitrate, ammonium and urea, and the variability in toxicity due to nutrient status of A. tamarense was examined. The toxin composition did not vary dramatically among the three N sources, however, ammonium induced the highest concentration of intracellular toxin, followed by urea and then nitrate. Therefore, populations utilizing high ammonium concentration could be more toxic than those growing on nitrate or urea. The toxin content was dependent on the cellular N status of nitrate grown cells only, suggesting that the competition for N in toxin production with other metabolic pathways such as growth may be different among N sources. The relationship between toxin and nutrient status is a complex interaction and it involves the redistribution of cellular N within the cells. Understanding the toxin dynamics of natural populations in relation to nutrient is essential for the mitigation of harmful dinoflagellates in a given coastal ecosystem.