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The variation in Skeletonema cells grown at 3 klux continuous illumination and 20°C is reported. Four different types of lamps gave no difference in the photosynthetic characteristics. The average diameter of the cells decreased from 8-3.5 μ during their six months vegetative period. The ratio between the pigment content in the largest and the smallest cells was about 2:1. A good correlation between cell volume and chlorophyll a content was found for this species. The content of chlorophyll c generally varied between 4 and 17 per cent of the chlorophyll a content -- A distinct correlation between the chlorophyll a content and the rate of photosynthesis per unit of cells at low light intensity was found. The rate of photosynthesis, in mg C per mg chlorophyll a and hour at 1 klux, varied between 0.40 and 0.70 for all 60 experiments with an average value of 0.56. The corresponding value for cells deficient in phosphorus was 0.19 and for cells deficient in nitrogen 0.09. -- The material also showed a good correlation between the rate of photosynthesis per cell at 1 klux and the light-saturated rate of photosynthesis. Ik varied between 7 and 13 klux. [ABSTRACT FROM AUTHOR]

Curves showing the rate of photosynthesis as a function of light intensity are presented for Chlorella vulgaris growing at a constant light intensity of either 3 or 30 Klux. The rate is given a) per number of cells, b) per unit of weight of chlorophyll (a + b), and c) per unit of wet volume or unit of weight of dry matter. At an illumination of about 3 Klux the average cell size is at its maximum. At the same time the content of chlorophyll per cell is maximal and the content of enzymes active in photosynthesis is very nearly maximal. If we increase the illumination the cell size decreases. At the same time the content of active enzymes per cell increases slightly, but the content of chlorophyll per cell decreases. If we decrease the illumination (from 3 Klux) cells are produced with the maximum content of chlorophyll but decreasing content of enzymes active In photosynthesis. At the same time the cell size decreases. Growth rates at the two light intensities are computed by means of the measured rates of photosynthesis and respiration. They are identical with the growth rates measured by means of the increase in number of cells. After a shift from 3 Klux to 30 Klux it takes only 24 hours before the ratio chlorophyll a+b: dry weight has changed to that normally found at the high light intensity. The same is true concerning the cell size. When shifting from 30 Klux to 3 Klux it takes a little longer. However, a chlorophyll concentration nearly as high as that ordinarily found at 3 Klux was measured. About 36 hours after the shift from 30 Klux to 3 Klux, or vice versa, the shape of the curve giving the rate of photosynthesis as a function of light intensity approaches that characteristic of algae grown constantly at the new light intensity. If Chlorella grown constantly at 30 Klux are transferred to the dark for 22 hours the shape of the tight photosynthesis curve is only moderately changed. IK decreases from 13 to 9 Klux. If instead the algae are transferred to 3 Klux, IK is decreased to 5 Klux after 22 hours. The importance is stressed of using such dilute concentrations of algae in experiments of both growth and photosynthesis that no self shading takes place. [ABSTRACT FROM AUTHOR]