The Effect of External pH on the Apparent CO2Affinity of Chlorella saccharophila

Abstract

The carbon dioxide compensation point of the unicellular green alga, Chloretla saccharophila, was determined in aqueous medium by a gas chromatographic method. Compensation points decreased markedly from 63 cm³ m⁻³ at an external pH of 4.0 to 3.2 cm³ m⁻³ at pH 8.0 and were not affected by the O₂ concentration of the medium. The calculated CO² concentration required to support the half-maximum photosynthetic rate of the algal cells ranged from 6.0 mmol m_–3 at an external pH of 60 to 1.5 mmol m⁻³ at pH 8.0 and these values were not affected by O₂ concentration. The K_m(CO₂) of nbulose-l,5-bisphosphate carboxylase isolated from cells grown either at pH 4.0 or pH 8.0 was determined to be 64 mmol m⁻³. These results indicate that loss of CO₂ by photorespiration does not occur in C. saccharophila cells at acid pH and the disparity between the apparent affinity for CO₂ of the intact cells and that of the carboxylase indicates the operation of a ‘CO₂ concentrating mechanism’ in this alga at acid pH.