Mixotrophic ability of the phototrophic dinoflagellates Alexandrium andersonii, A. affine, and A. fraterculus

The dinoflagellate Alexandrium spp. have received much attention due to their harmful effects on diverse marine organisms, including commercially important species. For minimizing loss due to red tides or blooms of Alexandrium spp., it is very important to understand the eco-physiology of each Alexandrium species and to predict its population dynamics. Its trophic mode (i.e., exclusively autotrophic or mixotrophic) is one of the most critical parameters in establishing prediction models. However, among the 35 Alexandrium species so far described, only six Alexandrium species have been revealed to be mixotrophic. Thus, mixotrophic ability of the other Alexandrium species should be explored. In the present study, whether each of three Alexandrium species (A. andersonii, A. affine, and A. fraterculus) isolated from Korean waters has or lacks mixotrophic ability, was investigated. When diets of diverse algal prey, cyanobacteria, and bacteria sized micro-beads were provided, A. andersonii was able to feed on the prasinophyte Pyramimonas sp., the cryptophyte Teleaulax sp., and the dinoflagellate Heterocapsa rotundata, whereas neither A. affine nor A. fraterculus fed on any prey item. Moreover, mixotrophy elevated the growth rate of A. andersonii. The maximum mixotrophic growth rates of A. andersonii on Pyramimonas sp. under a 14:10 h light/dark cycle of 20 μE m−2 s−1 was 0.432 d−1, while the autotrophic growth rate was 0.243 d−1. With increasing mean prey concentration, the ingestion rate of A. andersonii increased rapidly at prey concentrations <650 ng C ml−1 (ca. 16,240 cells ml−1), but became saturated at the higher prey concentrations. The maximum ingestion rate by A. andersonii of Pyramimonas sp. was 1.03 ng C predator−1 d−1 (25.6 cells predator−1 d−1). This evidence suggests that the mixotrophic ability of A. andersonii should be taken into consideration in predicting the outbreak, persistence, and decline of its harmful algal blooms.