EFFECT OF LIGHT ON DNA SYNTHESIS AND RELATED PROCESSES IN SYNCHRONOUS CULTURES OF CHLORELLA

In autotrophic cultures of Chlorella synchronized by alternating light and dark periods of 16:8 hours the DNA content duplicated normally 4 times successively during the Sm [multi DNA synthetic period] phase, i.e. between the 10th and 18th hour after the beginning of the light period. This finding together with electron microscopical observations revealed that one duplication of the DNA and of the nuclei per cell proceeds every 110 minutes. All nuclei of a cell seem to undergo successive DNA syntheses and nuclear divisions synchronously. The rate of DNA synthesis was independent from illumination. On appropriate reduction of the light period the last duplication cycle fell out and the average final spore number per cell was accordingly lower. If a culture was transferred to darkness or low light intensity 3 hr. before the normal end of the light period the release of spores was promoted by approximately 1 1/2 hr., provided a strong decrease of metabolically accessible carbohydrates was prevented by either an additional short illumination during the dark period or by continuing the weak light A possible explanation for the shortening of the cell development is that, by passing over one DNA duplication and one protoplast division, the cell can enter sooner the respective subsequent developmental stages.