Abstract

The relationship between growth rate versus phosphorus concentration and cellular toxin content was determined for Alexandrium minutum AL1V, Alexandrium tamarense MDQ1096, A. tamarense EF04 and Alexandrium andersoni EF12 under different nitrogen and phosphorus supplies. The aim was to determine whether those species with a lower phosphorus uptake affinity, and hence potentially of lower competitive ability at low phosphorus concentrations, were more toxic. The range and mean of toxic content per cell (as fmol per cell) of the species were 13.5-256.5 and 140.2±50.8 for A. tamarense MDQ1096, 0.5-16.5 and 2.9±2.6 for A. minutum, 0-2.0 and 0.2±0.3 for A. tamarense EF04 and, 0-3.3 and 0.06±0.4 for A. andersoni. Ks for culture cell growth (per day),representing the phosphate concentration at which the specific culture cell growth rate is one half the maximum rate, and Kmin (per day), the phosphate concentration at which the specific culture cell growth rate is zero, were used as indicators of species' potential competitive ability at low phosphorus concentrations. Low values for both Ks and K min indicate a high relative ability of the species to use low levels of phosphate and, hence, expected to outcompete higher Ks and Kmin species under phosphorus limitation. Ks and K min were 1.68 and 0.48 for A. tamarense MDQ1096, 1.16 and 0.39 for A. minutum, 1.0 and 0.38 for A. tamarense EF04 and, 0.74 and 0.34 for A. andersoni, respectively. There was a significant positive relationship between toxin content per cell with both Ks and Kmin, indicating that those species with lower ability to compete under phosphorus limitation were more toxic. The findings support the hypothesis that toxin production by dinoflagellates species could be an adaptation evolved to offset the ecological disadvantage of having low nutrient affinity. © 2004 Elsevier B.V. All rights reserved.