CHANGES IN TOXIN CONTENT, BIOMASS AND PIGMENTS OF THE DINOFLAGELLATE ALEXANDRIUM-MINUTUM DURING NITROGEN REFEEDING AND GROWTH INTO NITROGEN OR PHOSPHORUS STRESS

Two strains of the paralytic shellfish toxin (PST) producing dinoflagellate Alexandrium minutum Halim (highly toxic AL1V and weakly toxic AL2V) were grown in batch culture with either nitrate or phosphate as the Limiting nutrient. In comparison with cells of the strain AL1V, cells of AL2V grew at a similar C-specific rate, had a higher CIN ratio, and lower ratios of chl a/chl c(2) and chl a/peridinin. Neither chlorophylls nor carotenoids could be used to estimate C-biomass, N-biomass or toxin content for this organism. The toxin profile for both strains was dominated (up to 95%) by the gonyautoxin GTX4, with smaller proportions of GTX1, GTX2 and GTX3. The rate of toxin synthesis for both strains was greatest 1 to 2 d after the N-refeeding of N-deprived cells, with the net rate of toxin synthesis exceeding that of C-biomass and cell division by a factor of up to 4. Toxin synthesis was not enhanced by short-term P-stress. N-stress alone led to a decrease in toxin cell(-1), but P-stress followed by N-stress did not result in such a decline, implicating phosphorus in the regulation of toxin metabolism. Although arginine is a major precursor for PST synthesis, taurine, glycine, glutamine, and cell N showed similar relations to that observed for arginine with respect to toxin content. Furthermore, the mole ratio of arginine/toxin could vary by a factor of up to 5 between AL1V and AL2V at peak values of toxin cell(-1), and by more than 5 within a strain when growing under different conditions. These observations suggest that the relationship between free arginine content and toxin content is complex. No explanation for the higher toxin content of AL1V is apparent, except that AL1V has a higher N-content per cell. and this may be conducive to a higher rate of synthesis of the N-rich toxins.