Short-Term Interactions of Noctiluca scintillans with the Toxic Dinoflagellates Dinophysis acuminata and Alexandrium minutum : Growth, Toxins and Allelopathic Effects.

The Galician Rías (NW Iberian Peninsula) are an important shellfish aquaculture area periodically affected by toxic episodes often caused by dinoflagellates such as Dinophysis acuminata and Alexandrium minutum , among others. In turn, water discolorations are mostly associated with non-toxic organisms such as the heterotrophic dinoflagellate Noctiluca scintillans , a voracious non-selective predator. The objective of this work was to study the biological interactions among these dinoflagellates and their outcome in terms of survival, growth and toxins content. To that aim, short experiments (4 days) were carried out on mixed cultures with N. scintillans (20 cells mL -1 ) and (i) one strain of D. acuminata (50, 100 and 500 cells mL -1 ) and (ii) two strains of A. minutum (100, 500 and 1000 cells mL -1 ). Cultures of N. scintillans with two A. minutum collapsed by the end of the assays. Both D. acuminata and A. minutum exposed to N. scintillans arrested its growth, though feeding vacuoles in the latter rarely contained any prey. Toxin analyses at the end of the experiment showed an increase in intracellular OA levels in D. acuminata and a significant reduction in PSTs in both A. minutum strains. Neither OA nor PSTs were detected in N. scintillans . Overall, the present study indicated that the interactions among them were ruled by negative allelopathic effects.