Vertical migration, nutrition and toxicity in the dinoflagellate Alexandrium tamarense

The effect of nitrate-N availability on paralytic shellfish toxin production by the dinoflagellate Alexandrium tamarense was studied in a vertically stratified laboratory water column (tank) where swimming behavior could influence photosynthesis and nutrition. Results were compared with those from batch and semi-continuous cultures in which migratory behavior was not a factor. The batch and semi-continuous cultures demonstrated a direct positive relationship between N availability and toxin content. Steady-state cultures, maintained at 2 contrasting rates of semi-continuous N supply, also demonstrated significantly different cellular toxin profiles (relative proportion of toxins). The tank experiment was carried out in a 2.1 m PVC cylinder (0.29 m internal diameter) and lasted for 24 d. Initially, nitrate was replete throughout the water column (50 mu M) and the highly toxic cells formed a thin surface 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 nitracline. During this phase the toxin content of the cells decreased gradually as the C:N of the cells increased. In the third phase, the deep nitrate pool was exhausted and the cells penetrated deeper during the 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 was again restricted to the nitracline; toxicity of the cells increased and C:N declined. Finally, N was added to the surface layer. During this fifth and final phase, cellular toxicity continued to rise, C:N declined further, and the cells continued to migrate to the thermocline during the dark period. The toxicity of the cells during the N-stratified phases of the water column experiment was intermediate between the N-replete and N-depleted phases, indicating that A. tamarense is capable of producing PSP toxins from N acquired during a nocturnal descent. It is concluded that toxic dinoflagellates inhabiting N-depleted coastal waters are likely capable of sustaining growth and a moderate level of toxicity through nocturnal migrations to deep N pools.