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 −3 . Cysts germinated at temperatures between 14 and 23°C with a final germination rate over 93%. Strains TIO523 and GCLY02 displayed growth at temperatures between 17 and 26°C and 14 and 26°C, respectively, with both strains displaying the highest growth rate of ca. 0.5 divisions d -1 at 23°C. The PSTs of the three strains and cysts from the sediments were analyzed by liquid chromatography with tandem mass spectrometry (LC-MS/MS). All strains were able to produce PSTs, which were dominated by N-sulfocarbamoyl C toxins (C1/2, 53.0-143.5 pg cell −1 ) and decarbamoyl gonyautoxins (dcGTX2/3, 26.7-52.1 pg cell −1 ), although they were not detected in cysts. However, hydroxybenzoyl (GC) toxins were detected in both cells and cysts. Our results suggested that the population in the Taiwan Strait belonged to a warm water ecotype and has a unique toxin profile. Our results also suggested that the persistence of cells in the water column may have initiated the bloom.Gymnodinium catenatum fits the emended description of Gymnodinium (Daugbjerg et al., 2000), but it is distant from the type species of Gymnodinium in terms of its molecular phylogeny (Luo et al., 2018).A spherical, microreticulate cyst is produced by G. catenatum (Anderson et al., 1988), but later two other species, Gymnodinium nolleri and Gymnodinium microreticulatum, were reported to also produce similar microreticulate cysts (Bolch et al., 1999; Ellegaard and Moestrup, 1999). However, the motile cells of G. nolleri and G. microreticulatum do not form chains and their cysts are much smaller than tho...

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The dinoflagellate Alexandrium pacificum can produce paralytic shellfish toxins and is mainly distributed in the Pacific. Blooms of A. pacificum have been frequently reported in offshore areas of the East China Sea, but not along the coast. To investigate the bloom dynamics of A. pacificum and their potential origins in the Taiwan Strait, we performed intensive sampling of both water and sediments from 2017 to 2020. Ellipsoidal cysts were identified as A. pacificum and enumerated based on microscopic observation. Their abundances were quite low but there was a maximum of 9.6 cysts cm−3 in the sediment near the Minjiang River estuary in May 2020, consistent with the high cell abundance in the water column in this area. Cells of A. pacificum were examined using a quantitative polymerase chain reaction, and they appeared to be persistent in the water column across the seasons. High densities of A. pacificum (103 cells L−1) were observed near the Jiulongjiang and Minjiang River estuary in early May 2020, where high nutrients (dissolved inorganic nitrogen and phosphate), and relatively low temperatures (20–21 °C) were also recorded. Strains isolated from the East and South China Sea exhibited the highest division rate (0.63 and 0.93 divisions d−1) at 20 and 23 °C, respectively, but the strain from the Yellow Sea showed the highest division (0.40 divisions d−1) at 17–23 °C. Strains from the East and South China Sea shared similar toxin profiles dominated by the N-sulfocarbamoyl toxins C1/2, but the strain from the Yellow Sea predominantly produced the carbamoyl toxins GTX1/4 and no C1/2. Our results suggest that both cyst germination and persistent cells in the water column might contribute to the bloom formation in the Taiwan Strait. Our results also indicate that the East and South China Sea populations are connected genetically through similar toxin formation but separated from the Yellow Sea population geographically.