Environmental and endogenous regulation of cyst germination in two freshwater dinoflagellates

The role of excystment in relation to seasonal succession was investigated in two freshwater dinoflagellates, Ceratium hirundinella (O.F. Muller) Dujardin and Peridinium aciculiferum (Lemmermann). Field studies and laboratory experiments were performed to determine which factors regulate the timing of cyst germination. Environmental factors (temperature, light, nutrients, and anoxia) and endogenous factors (maturation period and biological clock) were investigated. Our main results indicate that temperature and internal maturation period determine when germination can, occur. C. hirundinella had a maturation period of 4.5 months and germinated in the laboratory and in the field at temperatures above 6 degrees C. P. aciculiferum had a maturation period of 2.5 months and germinated in the laboratory and in the field at temperatures below 7 degrees C. In addition, our results indicated that both species were regulated by a biological clock. Furthermore, anoxia prevented the germination of C. hirundinella, contrary to results in earlier studies. To conclude, we could explain the appearance in plankton of the two dinoflagellate species through two main factors regulating excystment, that is, temperature and maturation period.