Excystment in the freshwater dinoflagellate Peridinium cunningtonii.

The environmental conditions regulating excystment of Peridinium cunningtonii which periodically formed blooms in the summer in Nagase Reservoir, Kochi Prefecture, were examined by use of clonal population and synthetic medium (MW1).Cysts, which were sexually produced by inoculating into nitrogen and phosphorus deficient medium, were dormant and remained viable for at least 6months at 5°C in darkness. The darkcold treated cysts which had been stored at 5°C for 30days excysted at the frequency of 90%, when the incubation temperature was shifted up to 22°C. The temperature range for the excystment was between 18° and 26°C. The rates of excystment at 18° and 26°C were 30% and 44%, respectively. The optimum temperature for excystment was 22°C. No excystment occurred below 15°C. The excystment was independent of the nutritional conditions (nitrogen and/or phosphorus free medium (a)) and of the presence or absence of light. The cysts excysted and produced one vegetative cell retaining a large red body. This cell divided within 24h into daughter cells each with a red body. These results strongly suggest that the excystment of overwintering cysts of P. cunningtonii provides seed population for the bloom of this dinoflagellate in the reservoir.