Evidence of an endogenous circannual rhythm in growth-rates in dinoflagellates

Prorocentrum micans and Gonyaulax excavata clonal cultures grown under constant laboratory environmental conditions (continuous light and 20 +/- 1 degree C), over a two-year period, exhibit significative changes in growth rate. When rhythmometrically analyzed by cosinor, a pattern resembling a circannual rhythm became apparent. Significant rhythmometric differences appeared between species. Endogenous rhythms of microorganisms may have ecological implications.