Interactions between the bloom-forming dinoflagellates Prorocentrum donghaiense and Alexandrium tamarense in laboratory cultures

Abstract

Interactions between Prorocentrum donghaiense and Alexandrium tamarens, two bloom-forming dinoflagellates, were investigated using bi-algal cultures. All P. donghaiense died, but A. tamarense was hardly affected by the end of the experiment when the initial cell density was set at 1.0 × 10 4 cells mL − 1 for P. donghaiense and 0.28 × 10 4 cells mL − 1 for A. tamarense. However, significant growth suppression occurred in either species when the initial cell density of P. donghaiense increased to 1.0 × 10 5 cells mL − 1 in the bi-algal culture, but no out-competement was observed. The simultaneous assay on the culture filtrates showed that P. donghaiense filtrate prepared at a lower initial density (1.0 × 10 4 cells mL − 1 ) stimulated growth of the co-cultured A. tamarense (0.28 × 10 4 cells mL − 1 ), but filtrate at a higher initial density (1.0 × 10 5 cells mL − 1 ) depressed its growth. The filtrate of A. tamarense at a density of 0.28 × 10 4 cells mL − 1 killed all P. donghaiense at a lower density (1.0 × 10 4 cells mL − 1 ), but only exhibited an inhibitory effect on it at a higher density (1.0 × 10 5 cells mL − 1 ). It is likely that these two species of microalgae interfere with each other mainly by releasing allelochemical substance(s) into the culture medium, and a direct cell-to-cell contact was not necessary for their mutual interaction . The allelopathic test further proved that A. tamarense could affect the growth of co-cultured P. donghaiense by producing allelochemical(s); moreover, A. tamarense culture filtrate at the stationary growth phase (SP) had a strongly inhibitory effect on P. donghaiense compared to that at the exponential phase (EP). Results also demonstrated a dose-dependent relationship between the microalgal initial cell density and the degree of the allelopathic effect. The growth of P. donghaiense and A. tamarense in the bi-algal cultures was simulated using a mathematical model to quantify the interaction. The estimated parameters from the model showed that the inhibition exerted by A. tamarense on P. donghaiense was about 17 and 8 times stronger than the inhibition P. donghaiense exerted on A. tamarense, when the initial cell density was set at 1.0 × 10 4 and 1.0 × 10 5 cells mL − 1 for P. donghaiense, respectively, and 0.28 × 10 4 cells mL − 1 for A. tamarense in the bi-algal cultures. A. tamarense seems to have a survival strategy that is superior to that of P. donghaiense in bi-algal cultures under controlled laboratory conditions.