FORM OF INORGANIC CARBON UTILIZED FOR PHOTOSYNTHESIS IN CHLORELLA-VULGARIS 11H-CELLS

The rate of photosynthetic 14CO2 fixation in C. vulgaris 11h cells in the presence of 0.55 mM NaH14CO3 at pH 8.0 (20.degree. C) was greatly enhanced by the addition of carbonic anhydrase (CA). When air containing 400 ppm 14CO2 was bubbled through the algal suspension, the rate of 14CO2 fixation immediately after the start of the bubbling was suppressed by CA. These effects of CA were observed in cells which were grown in air containing 2% CO2 (high-CO2 cells) and in those grown in ordinary air (containing 0.04% CO2, low-CO2 cells). Irrespective of the CO2 concentration given to the algal cells during growth, apparently the active species of inorganic C absorbed by Chlorella cells is free CO2 and they cannot utilize bicarbonate. The effects observed in the high-CO2 cells were much more pronounced than those in the low-CO2 cells. This difference was accounted for by the difference in the affinity for CO2 in photosynthesis between the high- and low-CO2 cells.