The Photosynthetic Response to a Shift in the Chlorophyll a to Chlorophyll b Ratio of Chlorella
The chlorophyll a:b ratio was shifted in Chlorella vannielii by varying the illuminance under which the cells were cultured-the ratio increased from 2.9, 3.0, 4.0, and 4.8 to 6.2, respectively, at 100, 300, 900, 2,700 and 6,000 foot candles. The 6,000-foot candle cells retained an optimal growth rate at the chlorophyll a:b ratio of 6.2 which was the upper limit of normal growth. Comparisons were made between the 300-and 6,000-foot candle cultures to determine the significance to the
... to the photosynthetic mechanism of a shift in the chlorophyll a:b ratio.The high light cells (6,000 foot candles) contained only one-tenth the total amount of chlorophyll of the low light cells (300 foot candles) based on dry weight. The total chlorophyll per cell of the high light cells was one-fifth of that in the low light cells. Electron micrographs indicated differences in chloroplast structure. An average of five or six thylakoids composed a granum-like region of the low light chloroplasts, whereas only a pair of thylakoids at most was found in the high light chloroplasts. The high light chloroplasts had more starch. On a dry weight basis, the high light cells had a respiration rate 3 times that of the low light cells. Based on chlorophyll, the respiration rate of the high light cells was 26 times greater. Based on dry weight, the oxygen evolution for both cultures was essentially the same at 6,000 foot candles; however, at 300 foot candles the rate for the low light cells was about 5 times faster than that of the high light cells. With chlorophyll as the index, the rates of the high light cells were higher than those of the low light cells-7 times faster at 6,000 and 2 times faster at 300 foot candles. At 10,000 foot candles, the low light-grown cells underwent photooxidation, whereas the high light grown cells photosynthesized at a rate slightly higher than at 6,000 foot candles.Action spectra of system II (oxygen evolution) from a modulated polarograph indicated photochemical participation of chlorophyll b in the high light deficient cells, although the participation was much less than in the low light cells. Enhancement was 1.11 for the low light cells, and 1.05 for the high light cells.In order to account for the energy balance in the two cultures it was concluded that perhaps oxidative phosphorylation supplemented a reduced photophosphorylation in the high light cells. Experiments with peroxyacetyl nitrate support the view that cyclic photophosphorylation is less in the chlorophyll b-deficient cells. Chlorophyll b served also to broaden absorption for the photosynthetic unit-a detrimental role when cells are illuminated above saturation.