Cell Cycle-related and Endogenously Controlled Circadian Photosynthetic Rhythms in Euglena

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Abstract
The data presented for three strains of Euglena gracilis corroborate previous reports of a diel rhythm in photosynthetic capacity in division-synchronized cultures of this alga and extend these studies to free running, dividing and nondividing (stationary) cultures maintained in either 24-hour or 40-minute cycles of light and darkness. During synchronous growth entrained by LD: 12,12 or free running under LD: 1/3,1/3, photosynthetic CO2 fixation was rhythmic with a period (24.0 or about 30 hours) corresponding to the period of the cell division rhythm in the population. Furthermore, the rhythm in CO2 fixation (per cell) found in nondividing cultures maintained in LD: 12,12 persisted in LD: 1/3,1/3 for weeks with a free running, circadian period of approximately 30 hours. An endogenous, circadian rhythm in cellular chlorophyll was found to exist, independently of cell division, under both light regimens and in each individual experiment; this observation could reflect changes in the functional role of the pigment. In cultures maintained in LD: 1/3,1/3, the phase relationship between the rhythm of photosynthetic capacity and that of chlorophyll content varied, suggesting the possibility of desynchronization among circadian rhythms in a multioscillator, unicellular organism.