In this study, we investigated seasonal changes in chlorophyll a (Chl.a), microcystin, and the 16S rRNA gene-based cyanobacterial community structures using 16S rRNA gene amplicon analysis of the surface water of Lake Hachiro, Akita Prefecture, Japan. Moreover, we examined the growth characteristics of four Microcystis spp. strains (N1, N6, N7, and 0824) isolated from Lake Hachiro. In 2014, Chl.a and total microcystin concentrations (173.4 and 1.7 μg/L, respectively) were the highest in October, when the water temperature was less than 20°C. Although algal blooms were observed for a longer term in 2015, the concentrations of Chl.a and total microcystin also increased transiently at the end of September. The cyanobacterial compositions of algal blooms included the genera Aphanizomenon, Anabaena, Prochlorothrix, and Microcystis in 2014 and Aphanizomenon, Anabaena, and Microcystis in 2015. In October of both years, algal blooms were dominated by Microcystis spp., suggesting the presence of major producers of microcystins during this term. In laboratory cultures, Microcystis strain 0824 showed a high specific growth rate at 15°C (0.17 1/d). Low temperature -adapted Microcystis spp., such as strain 0824, may be responsible for the occurrence of harmful algal blooms during the low water temperature season in Lake Hachiro.
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