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Photosynthetic Unit Size during the Synchronous Life Cycle of Scenedesmus¹

^a Departments of Botany and Zoology, The University of Texas, Austin, Texas 78712

Apparent size of the photosynthetic unit (chlorophyll/O₂ per flash) was estimated by O₂ yield of repetitive short flashes on cell samples taken at various times from a synchronized culture (14-hour light, 10-hour dark) of Scenedesmus obliquus. Unit size was essentially invariant (< 10% variation) with a mean value of 1750 chlorophyll/O₂ per flash. In contrast, the light-saturated photosynthetic rate per unit chlorophyll, or turnover rate of the photosynthetic unit, varied with the life cycle, rising 40% in the first three hours of the light period and decaying slowly thereafter. The results are taken as evidence that the metabolic machinery is subject to far greater control and adjustment than is the photochemical machinery.

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