The green marine algaBryopsis maximashowed a circadian rhythm in the rate of oxygen evolution in photosynthesis. The rhythm lasted several days in constant light and seemed to be endogenous. It disappeared during darkness and reappeared under natural or artificial light-dark cycle, which shows that it is light-dependent and entrainable by an exogenous light-dark cycle. In the rhythm, the oxygen evolution rate at midnight was 50 to 70of that at noon, and the amplitude of the rhythm was larger at higher intensities of actinic light. The light-intensity dependency of the rhythm showed that the rhythmic change in the activities was due to an alteration of the dark-reaction rate in photosynthesis and not due to a change of the light-reaction rate.
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