Glycolate Excretion and the Oxygen to Carbon Dioxide Net Exchange Ratio during Photosynthesis in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii cells were grown in high (5% v/v) or low (0.03% v/v) CO(2) concentration in air. O(2) evolution, HCO(3) (-) assimilation, and glycolate excretion were measured in response to O(2) and CO(2) concentration. Both low- and high-CO(2)-grown cells excrete glycolate. In low-CO(2)-grown cells, however, glycolate excretion is observed only at much lower CO(2) concentrations in the medium, as compared with high-CO(2)-adapted cells. It is postulated that the activity of the CO(2)-concentrating mechanism in low-CO(2)-grown cells is responsible for the different dependence of glycolate excretion on external CO(2) concentration in low- versus high-CO(2)-adapted cells.The O(2)/CO(2) net exchange ratio is dependent on the CO(2) concentration in the medium and is linearly dependent on the fraction of glycolate excreted per CO(2) taken up. Glycolate excretion, however, is too low to account for the deviation of the O(2)/CO(2) net exchange ratio from unity.