A new screening method for algal photosynthetic mutants - CO2-insensitive mutants of the green alga Chlorella ellipsoidea

A new method has been developed for screening algal photosynthetic mutants. This method uses autoradiography to assess gross photosynthetic C-14 fixation by green algal colonies on agar plates and allows the identification of clones that differ in photosynthetic characteristics from wild-type cells. Three wild-type cells, high-CO2-grown Chlorella ellipsoidea, air-grown C. ellipsoidea, and air-grown Chlorella saccharophila, had K-0.5 values for dissolved inorganic carbon (DIG) of 1083, 250, and 50 mu M, respectively, and as plaques on agar plates at Chi densities greater than 25 mu g cm(-2) exhibited relative amounts of C-14 fixation of 15, 55, and 100%, respectively. Cells of C. ellipsoidea were mutagenized with x-rays and screened by this method. Growth of C. ellipsoidea in high CO2 represses DIC transport and thus lowers its affinity for DIG. Five of the mutants detected by this method showed high-affinity photosynthesis similar to air-grown wild-type cells even when grown in high CO2. Seven other mutants when grown in high CO2 showed affinities for DIC intermediate between air-grown and high-CO2-grown wild-type cells. The affinities of high-CO2-grown mutants were reflected precisely in their capacities to accumulate DIC intracellularly. These results indicate that the mutants are fully or partially insensitive to the repressive effect of ambient CO2 concentration on DIC transport.