Interactions between and from the East China Sea.

The dinoflagellate is a dominant harmful algal bloom (HAB) species on the East China Sea (ECS) coast. The co-occurrence of with is often observed and can later develop into dense blooms. However, the role of in population dynamics is unknown. In the current study, three (GM1, GM2, and GM3) strains were isolated from the ECS with one (GM1) from a mixed, dense bloom of and other HAB species. All three isolates had identical ITS sequences that were concordant with the species designation. Unique karlotoxin congeners were isolated from one strain (GM2). The sterol compositions of and were consistent with sensitivity to karlotoxin in the former and insensitivity in the latter. Additional experimentation showed that: (1)in monocultures, higher growth rate of than is observed in nutrient-enriched and nutrient-depleted media. In co-cultures, the growth of is inhibited; (2) feeding on by is clearly demonstrated by fluorescent dye tracking; and (3) the isolated karlotoxin is lethal to in a concentration-dependent manner. From these studies we propose that may play a negative role in bloom maintenance and that may in turn be a bloom initiator as a prey item for .