Effects of nutritional factors and different parental crosses on the encystment and excystment of Alexandrium catenella (Dinophyceae) in culture

13 pages, 33 figures, 3 tablesAlexandrium catenella is a cyst-forming dinoflagellate causative of paralytic shellfish poisoning. Strain and nutritional factors affecting cyst formation and germination in this species were studied in cultures of several clonal crosses. Sexual reproduction was monitored and the effect of nutrients on sexual stages was proved because planozygotes isolated in medium with no phosphates added encysted in lower percentages than those placed in media with no nitrates added or in replete conditions, where the highest encystment was achieved. However, other unknown factors prevented encystment, because, even in replete conditions, some planozygotes were unable to encyst. Dormancy period of cysts ranged from 5 to 65 days at 24°C. A progressive germination inside this time interval was recorded for cysts from all six clonal crosses employed, and therefore, identified as a species characteristic. This gradual germination was modulated by nutrient levels in both encystment and excystment media: (1) cysts formed in conditions where nitrates and phosphates were added needed more time to excyst than those from media lacking one of these nutrients and (2) cysts from any encystment treatment germinated faster, and in higher percentages in poor external conditions compared to those placed in replete L1 medium. More than 90% of the cysts isolated to seawater germinated in less than 20 days, whereas those isolated to L1 medium had not begun to excyst, and, after 60 days, the percentage of germinated cysts in replete conditions remained below 40%. However, ultimate germination frequencies and postmeiotic viability of cysts might depend on genetic characteristics because cysts from the clonal crosses of A. catenella employed showed significant differences. Ecdysal cysts were mainly observed under the treatments in which the smallest number of resting cysts were produced. In 65% of the cases germination of these cysts under replete conditions occurred within 15 daysResearch supported by Instituto Español de Oceanografía grant for training of researching staff and the Strategy project, No. EVK3-CT-2001-0046Peer Reviewe