Toxic dinoflagellate blooms of Gymnodinium catenatum and their cysts in Taiwan Strait and their relationship to global populations.

Gymnodinium catenatum is able to produce paralytic shellfish toxins (PSTs) and was responsible for a massive bloom in the Taiwan Strait, East China Sea, in June 2017, which resulted in serious human poisoning and economic losses. To understand the origin of the bloom and determine the potential for blooms in subsequent years, water and sediment samples collected in the Taiwan Strait from 2016 to 2019 were analyzed for cells and cysts using light microscopy (LM) and/or quantitative polymerase chain reaction (qPCR). The morphology of both cells and cysts from the field and cultures was examined with LM and scanning electron microscopy (SEM). Large subunit (LSU) and/or internal transcribed spacer (ITS)-5.8S rRNA gene sequences were obtained in 13 isolates from bloom samples and five strains from cysts. In addition, cells of strains TIO523 and GCLY02 (from the Taiwan Strait and Yellow Sea of China, respectively) were subjected to growth experiments, and cysts from the field were used for germination experiments under various temperatures. Our strains shared identical LSU and ITS-5.8S rRNA gene sequences with those from other parts of the world, and therefore belonged to a global population. A low abundance of G. catenatum cells were detected during most of the sampling period, but a small bloom was encountered in Quanzhou on June 8, 2018. Few cysts were observed in 2016 but a marked increase was observed after the bloom in 2017, with a highest density of 689 cysts cm

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.