VERTICAL MIGRATION, NUTRITION AND TOXICITY IN THE DINOFLAGELLATE ALEXANDRIUM-TAMARENSE

The effect of nitrate-N availability on paralytic shellfish toxin prod uction by the dinoflagellate Alexandrium tamarense was studied in a ve rtically stratified laboratory water column (tank) where swimming beha vior could influence photosynthesis and nutrition. Results were compar ed with those from batch and semi-continuous cultures in which migrato ry behavior was not a factor. The batch and semi-continuous cultures d emonstrated a direct positive relationship between N availability and toxin content. Steady-state cultures, maintained at 2 contrasting rate s of semi-continuous N supply, also demonstrated significantly differe nt cellular toxin profiles (relative proportion of toxins). The tank e xperiment was carried out in a 2.1 m PVC cylinder (0.29 m internal dia meter) and lasted for 24 d. Initially, nitrate was replete throughout the water column (50 mu M) and the highly toxic cells formed a thin su rface layer which persisted throughout the 14 h light:10 h dark cycle. When nitrate was depleted in the surface layer as a result of uptake by the phytoplankton, the cells began a nocturnal migration to the nit racline. During this phase the toxin content of the cells decreased gr adually as the C:N of the cells increased. In the third phase, the dee p nitrate pool was exhausted and the cells penetrated deeper during th e dark period. The toxin content of the cells reached the lowest level during this phase. When nitrate was added to the deep layer, a fourth phase began, during which nocturnal descent of the migrating cells wa s again restricted to the nitracline; toxicity of the cells increased and C:N declined. Finally, N was added to the surface layer. During th is fifth and final phase, cellular toxicity continued to rise, C:N dec lined further, and the cells continued to migrate to the thermocline d uring the dark period. The toxicity of the cells during the N-stratifi ed phases of the water column experiment was intermediate between the N-replete and N-depleted phases, indicating that A. tamarense is capab le of producing PSP toxins from N acquired during a nocturnal descent. It is concluded that toxic dinoflagellates inhabiting N-depleted coas tal waters are likely capable of sustaining growth and a moderate leve l of toxicity through nocturnal migrations to deep N pools.