Cell cycle dependent photosynthesis in the marine dinoflagellate Amphidinium carteri was studied under constant illumination and light/dark (L/D) photocycles to distinguish intrinsic cell cycle control from environmental influences. Cells were grown in constant light and on a 14:10 L:D cycle at light intensities that would yield a population growth rate of I doubling per day. In the former case division was asynchronous, and cells were separated according to cell cycle stage using centrifugal elutriation. Cells grown on the L:D cycle were synchronized, with division restricted to the dark period. Cell cycle stage distributions were quantified by flow cytometry. Various cell age groups from the two populations were compared as to their photosynthetic response (photosynthetic rate versus irradiance) to determine whether or not the response was modulated primarily by cell cycle constraints or the periodic L/D cycle. Cell cycle variation in photosynthetic capacity was found to be determined solely by the L/D cycle; it was not present in cells grown in constant light.In populations synchronized by environmental cues, it is difficult to distinguish cell cycle dependent physiology from responses to the environment. Periodicity in the environment can entrain independently both the cell cycles and the circadian clocks of the individuals in a population (2, 29). Since both of these mechanisms can give rise to coordinated population behavior, it is not clear which is the important factor. Further complicating the problem is the fact that cells have direct reactions to their surroundings. For example, a plant cell transferred into the light may respond to its changed energy status regardless of its cell cycle or clock position. The well known periodic change in photosynthetic capacity in cells grown in photocycle is an example of such an ambiguous situation; while photosynthetic rhythms have been described in detail (6, 13-15), the relative contributions of the clock, the photocycle, and the cell cycle in controlling such rhythms have not been well established. In particular, the contribution of the cell cycle per se (i.e. in the absence of previous entraining stimuli) has never been evaluated.