A "point of no return" in the cell cycle of Chlorella

In a Chlorella culture growing synchronously at pH 6.3 under a 12 hr light/12 hr dark regime, DNA replication occurs between the 8th and the 12th hour of the cycle, the main period of protein and chlorophyll synthesis occurring between the 4th and 12th hour of the cycle. When the culture is transferred to alkaline pH at any time up to the 8 hr of the cycle, autospore release is prevented, and the pattern of synthesis of DNA, protein and chlorophyll is altered. However, when the culture is transferred to alkaline conditions after the 8th hour of the cycle, the pattern follows that of a culture growing at pH 6.3 with respect to cell number and volume, as well as protein, chlorophyll and DNA contents. Thus, a transition point seems to occur after the 8 hr of the cycle. The existence of such a point was also demonstrated by reciprocal experiments in which Chlorella was cultured at an alkaline pH and transferred to pH 6.3 at various times in the cell cycle.