Circadian rhythm of chloroplast ultrastructure in Gonyaulax polyedra, concentric organization around a central cluster of ribosomes

A circadian rhythm of chloroplast ultrastructure in the marine dinoflagellate Gonyaulax polyedra is described. Thylakoid stacks in the parts of the chloroplasts that lie closest to the interior of the cell are more widely separated during the day than at night. No ribosomes are present in the stroma of this region when it is expanded. Changes in thylakoid spacing are found in the absence of environmental light-dark cycles and thus represent an additional manifestation of circadian rhythmicity in Gonyaulax. Changes in thylakoid spacing occur only in parts of the chloroplasts which lie within a given distance from the center of the cell. In Gonyaulax, the nucleus, the Golgi bodies, and the developing trichocysts are also arranged concentrically. At the center of the cell, a large aggregation of densely packed cytoplasmic ribosomes has been discovered. We speculate that these ribosomes may control the arrangement of organelles in Gonyaulax and the rhythmic changes in thylakoid spacing.