Killing potential protist predators as a survival strategy of the newly described dinoflagellate Alexandrium pohangense

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Blooms caused by some species belonging to the dinoflagellate genus Alexandrium are known to cause large-scale mortality of fish. Thus, the dynamics of these species is important and of concern to scientists, officials, and people in the aquaculture industry. To understand the dynamics of such species, their growth and mortality due to predation need to be assessed. The newly described dinoflagellate Alexandrium pohangense is known to grow slowly, with a maximum autotrophic growth rate of 0.1d−1. Thus, it may not form bloom patches if its mortality due to predation is high. Therefore, to explore the mortality of A. pohangense due to predation, feeding on this species by the common heterotrophic dinoflagellates Gyrodinium dominans, Gyrodinium moestrupii, Luciella masanensis, Noctiluca scintillans, Oxyrrhis marina, Oblea rotunda, Polykrikos kofoidii, and Pfiesteria piscicida, as well as by the ciliate Tiarina fusus, was examined. None of these potential predators was able to feed on A. pohangense. In contrast, these potential predators were killed and their bodies were dissolved when incubated with A. pohangense cells or cell-free culture filtrates. The survival of G. moestrupii, O. marina, P. kofoidii, and T. fusus on incubation with 10cellsml−1 of A. pohangense was 20–60%, while that at the equivalent culture filtrates was 20–70%. With increasing A. pohangense cell-concentration (up to 1000cellsml−1 or equivalent culture filtrates), the survival rate of G. moestrupii, O. marina, P. kofoidii, and T. fusus rapidly decreased. The lethal concentration (LC50) for G. moestrupii, O. marina, P. kofoidii, and T. fusus at the elapsed time of 24h with A. pohangense cells (cultures of 11.4, 13.3, 1.6, and 3.3cellsml−1, respectively) was lower than that with A. pohangense filtrates (culture filtrates of 35.5, 30.6, 5.5, and 5.0cellsml−1, respectively). Furthermore, most of the ciliates and heterotrophic dinoflagellates in the water collected from the coast of Tongyoung, Korea, were killed when incubated with cultures of 1000 A. pohangense cells ml−1 and equivalent culture filtrates. The relatively slow growing A. pohangense may form blooms by reducing mortality due to predation through killing potential protist predators.