Phylogeny, growth and toxicity of the noxious red-tide dinoflagellate Alexandrium leei in Japan

A red tide of the dinoflagellate Alexandrium leei occurred at the beginning of April 2017 in Nomi Bay, Japan, killing aquacultured fishes, such as red sea breams and great amberjacks. This was the first known reported case of an ichthyotoxic red tide of A. leei in Japan, so we collected basic information about this dinoflagellate in the field and laboratory. Wild and cultured A. leei cells from Japan had similar morphological characteristics to cells from foreign waters, and phylogenetic analysis revealed that the Japanese isolate belonged to a northeast Asian group. Meteorological data from 2014 to 2018 showed that precipitation was lowest and the duration of bright sunshine was longest during January–May 2017 than in any other year. Field studies further showed that the water temperature during the red tide was ∼18°C and the salinity and dissolved inorganic nitrogen concentrations during January–May were higher in 2017 than in any other year from 2014 to 2018. A strain of A. leei that was cultured at 10–25°C grew at photosynthetic photon flux densities >15–30μmol photons m−2 s−1, exhibiting a maximum growth rate at 150 μmol photons m−2 s−1 at 20°C. Toxicity bioassays showed that most red sea breams showed signs of death within 2 h of exposure to cell suspensions containing >3 × 103 cells ml−1 of A. leei in both the logarithmic (log) and stationary phases of growth, rotifers died in a cell suspension containing <3 × 103 cells ml−1, with log-phase cells exhibiting higher toxicity than stationary-phase cells, and haemolytic activity increased in a cell concentration-dependent manner during the log phase of growth to reach <90% at 5 × 103 cells ml−1. The present study indicates that A. leei may affect fishes and other organisms in Japanese coastal waters.