Form of inorganic carbon utilized for photosynthesis in Chlorella vulgaris 11 h cells

Abstract

The rate of photosynthetic ¹⁴CO₂ fixation in Chlorella vulgaris 11h cells in the presence of 0.55 mM NaH¹⁴CO₃ at pH 8.0 (20°C) was greatly enhanced by the addition of carbonic anhydrase (CA). However, 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 had been grown in air containing 2% CO₂ (high-CO₂ cells) as well as those grown in ordinary air (containing 0.04% CO₂, low-CO₂ cells). We therefore concluded that, irrespective of the CO₂ concentration given to the algal cells during growth, the active species of inorganic carbon 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 high-CO₂ cells. This difference was accounted for by the difference in the affinity for CO₂ in photosynthesis between the high- and low-CO₂ cells.