Ecological advantages of toxin production by the dinoflagellate Alexandrium minutum under phosphorus limitation

Unialgal and mixed cultures of the toxic dinoflagellate Alexandrium minutum and the non-toxic dinoflagellate Prorocentrum micans were cultured under phosphate (P) limitation (<1 muM), in the presence or absence of the copepod Acartia clausi. The aim was to determine the possible effects of interspecific competition, predation and nutrient limitation on the production of Paralytic Shellfish Poisoning (PSP) toxin by A. minutum. The growth rate of A. minutum was higher in the cultures of A. minutum and copepods than in the mixed cultures of A. minutum and P. micans, which indicates that A. minutum was more negatively affected by interspecific competition than by predation, Toxin content per cell in A. minutum increased in all cultures, but toxin production rate was higher in cultures with P. micans and/or A. clausi. Toxin concentration in A. minutum increased as PO43- concentration diminished. In the mixed cultures with copepods, analyses of toxin content in the copepods showed that at cell toxin contents lower than approximately 20 fmol cell(-1), mainly cells of A. minutum were ingested by copepods, whereas at higher toxins content per cell, copepods fed mainly on P. micans. We conclude that one of the possible advantages of toxin production by A. minutum under P limitation is to enhance interspecific competition, by redirecting grazing pressure to non-toxic phytoplankton species. It would allow a possibly low competitive ability of A. minutum to be offset under low nutrient concentrations.