EFFECTS OF ENVIRONMENTAL CONDITIONS ON THE RATE OF PHOTOSYNTHESIS AND SOME PHOTOSYNTHETIC ENZYMES IN DUNALIELLA TERTIOLECTA BUTCHER 1

Exponentially growing Dunaliella tertiolecta raised under continuous light increased both in photosynthetic capacity and in the activities of two enzymes of the photosynthetic reductive pentose cycle during the first 24 hr after transfer to complete darkness at 18C. A rapid decrease in photosynthetic capacity and enzyme activities followed. Three days after transfer to darkness, the photosynthetic capacity was only 25% of the initial value, while the corresponding figures for RuDP carboxylase and aldolase activities were 25 and 20%. Decline of these metabolic parameters at 18C was greatly delayed by continuous illumination of 20 µ w/cm 2 , an intensity considerably below the compensation point. After seven days, the photosynthetic capacity was still 78%, and the RuDP carboxylase and aldolase activities were 65 and 40% of their initial values. The same light intensity at a lower temperature (5C) almost entirely eliminated the decrease in photosynthesis and enzyme activities over a 21‐day period. Changes in RuDP carboxylase activity followed closely those in the rate of photosynthetic dark reactions. Parallel changes in the rate of light reactions suggest that the changes in the dark and light systems are interdependent. Light quality studies indicate that light absorbed by photosynthetic pigments is primarily responsible for decreasing the rate of loss of photosynthetic capacity and RuDP carboxylase activity. It is suggested that light of low intensity, as well as low temperature, may be ecologically important in maintaining the photosynthetic capacity of plankton algae when they are removed from the euphotic zone for a considerable length of time through sinking or vertical mixing.