Comparative study of the life cycles of Alexandrium tamutum and Alexandrium minutum (Gonyaulacales, Dinophyceae) in culture

The microalgal genus Alexandrium includes species known to produce paralytic shellfish poisoning (PSP). Due to the importance of discriminating between HAB-forming species, we compared the undescribed life-cycle pattern of Alexandrium tamutum Montresor, Beran et U. John and of its toxic relative Alexandrium minutum Halim. Sexual stages, asexual and sexual division, mating type, and nuclear morphology were studied in both species. Sexual cysts are known to be morphologically identical. However, the relative size of the U-shaped nucleus may be used to differentiate between the cysts of these species since DNA packaging in the resting cysts was lower in A. tamutum than in A. minutum, species in which the planozygote nucleus was reduced to half its volume prior to encystment. The dormancy period of the cysts was < 20 d for A. tamutum, but longer than 1 month for A. minutum. In both species, cyst appearance needed to be explained by the existence of more than two sexual types (+/-), which indicates a complex heterothallic mating type. However, planozygotes of both species may divide instead of encysting. This characteristic was used for nutritional and heritage studies. Isolated planozygotes of both species encysted in larger percentages in medium deficient in both nitrates and phosphates (L/15) than in medium without phosphates added (L-P), a medium in which most planozygotes neither divide nor encyst. Parental strains of A. minutum with and without the ventral pore formed planozygotes and, later, offspring with the ventral pore, although apparently smaller than usual. A synchronization-flow cytometry method for discriminating diploids formed by sexual fusion (planozygotes) from cells with 2C DNA content resulting from self-duplication of DNA (dividing cells) was described. The results indicated that the maximum percentage of A. minutum planozygotes (20%) was achieved only 3 to 5 d after crossing the parental strains, and that light might not be needed for the sexual fusion and formation of planozygotes.