Circadian Rhythms in Three Unicellular Organisms. The Pecularities of the Organisms, the Evidence Brought on Rhythms and their Specific Practical Problems. Outline of Recent Hypotheses

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Abstract
Three unicellular organisms are considered: Gonyaulax, Euglena and Acetabularia. They differ in morphology, in some specific macromolecules and in cell cycle. Their temporal morphology is reviewed. Circadian rhythms (CR) in single cells were described in Gonyaulax and Acetabularia. Besides common features, the three organisms, owing to some species‐specific peculiarities, contributed to basic knowledge of the CR mechanism and functioning since the sixties. In particular, the CR dependency ‐ or not ‐ on transcription and translation. Light and temperature effects are reviewed as well as the occurrence of a state of arrhythmicity. The question of intercellular communication is discussed. Hormonal effects and their relationships with CR have been approached by two groups, for Gonyaulax and for Acetabularia. They suggested interesting hypotheses related to evolution. Euglena turned out to be a good model system for the study of CR as related to the cell cycle. In all three species, CR dissociation has been described. Photosynthesis and energy metabolism are compared and discussed. Interestingly, the three Protocysts display a photosynthesis CR post‐transcriptionally regulated. The general theories developed by different authors are summarized and our alternative view is developed. Finally, the previously hypothetized role of CR in evolution is briefly presented.