Resting cysts, and effects of temperature and salinity on the growth of vegetative cells of the potentially harmful species Alexandrium insuetum Balech (Dinophyceae)

•There are no unique features that can accurately identify the resting cysts of Alexandrium species, morphologically.•The resting cysts of minutum group differ from those of ostenfeldii and andersonii groups.•A. insuetum may become a candidate organism for outbreaks of future blooms and PSP in Jinhae-Masan Bay, Korea. The potentially harmful species Alexandrium insuetum established by the incubation of resting cysts isolated from sediment trap samples collected at Jinhae-Masan Bay, Korea was characterized by morphological and phylogenetic analysis. The effects of temperature and salinity on the growth of A. insuetum were also investigated. The resting cysts are characterized by a spherical shape, a small size (20–25μm) and the presence of either three or four red accumulation bodies. The similarity of morphological features of the resting cysts to those of other species of the minutum group (consisting of Alexandrium minutum and A. tamutum) indicates that the morphological features of resting cysts might improve the accuracy of the grouping of Alexandrium species. A. insuetum germinated from the resting cysts is morphologically consistent with vegetative cells reported from Korean and Japanese coastal areas, and has an partial large subunit (LSU) rDNA sequence identical to that from Japanese strains. The growth of A. insuetum was observed between salinity 20 and 35, with increasing temperature; however at 25°C, A. insuetum could grow even at the salinity of 15. The highest growth rate (0.60d−1) was observed at 25°C and the salinity of 25, which is higher than the previously reported growth rate of A. tamarense, which is responsible for outbreaks of paralytic shellfish poisoining and blooms in Jinhae-Masan Bay. These results suggest that the proliferation of A. insuetum in Jinhae-Masan Bay is likely to be highest during the summer.