Chlorophyll Synthesis in Chlorella II. Effect of Glucose and Light Intensity on the Lag Phase.

Dense cultures of Chlorella grown under our conditions contain large amounts of chlorophyll on a packed cell basis. Upon dilution the cells are, in effect, put into an environment in which light is not limiting and chlorophyll seemingly in excess (at least temporarily). Thus it is not economical for the cell to produce more chlorophyll until it is the limiting factor. It is our contention that the cell may sense this situation by the piling up of products such as glucose or starch. The observed light intensity effect which causes a stimulation of chlorophyll production at lowered intensities is analogous to that previously reported by others and is felt to be related to the rate of photosynthate production.Chlorophyll synthesis commences after a period of about 12 hours. Once it has become established, added substrate or light have a smaller effect. At 36 hours addition of glucose stimulates growth markedly but chlorophyll production continues at the control rate. This suggests that the effect of substrate is not upon the action of an enzyme involved in the actual synthesis of the chlorophyll molecule. Glucose has been known to influence production of certain enzymes involved usually in substrate catabolism. It is quite possible that the role of glucose here is quite similar. The data are consistent with such an explanation. The substrate would have to influence the production of an enzyme in the magnesium branch of the biosynthetic pathway of porphyrin compounds and not in the portion common to both iron and magnesium porphyrins. Accordingly the lag period is viewed as both an inhibitory period as well as a preparatory period for subsequent chlorophyll synthesis.