Toxic Characteristics and Action Mode of the Mixotrophic Dinoflagellate on Co-Occurring Phytoplankton and Zooplankton.

The mixotrophic dinoflagellate frequently forms harmful algal blooms around the world and has caused massive deaths of shellfish, finfish and birds, yet its toxic mechanism is still unclear. In this study, toxic effects of on co-culturing phytoplankton and zooplankton were investigated. The results showed that sonicated cultures of JX13 and JX14, isolated from the Pearl River Estuary, had a significant lethal effect on the rotifer , with the highest mortality rate of 80%. The highest inhibition rates of cultures JX13 (90%) and JX14 (80%) on were much higher than that of AS2 (20%). Toxicity varied with the growth stage, during which cells in the exponential stage showed the highest toxicity (40%), while filtrate had the highest toxicity (10%) in the decline stage. The action mode of toxicity on plankton was explored through an osmotic membrane culture device. It was found that JX13 displayed an inhibitory effect on coexisting phytoplankton, whether they had contact or not, but the inhibition rate increased by 25% with contact. A lethal effect of JX13 on rotifer was observed only in contact treatment. This study suggests that direct contact is the key action mode to trigger the release of toxins and induce toxic effects of on co-occurring plankton.