Abstract. Mass-spectrometric measurements of 1602 and 802 were made to compare the rates of light-dependent 02 evolution and uptake by Chlamydomonas reinhardtiiDang. grown in air (0.035% CO2; low-C~ cells) or CO2-enriched air (5% CO2; high-Ci cells) at pH 5.5 and 8.0.While at pH 5.5, no differences were observed in the isotopic Oz-gas exchange of high-and low-Ci cells, at pH 8.0 the rates of true 02 evolution and uptake were considerably higher in low-C i than in high-C~ cells. The enhanced rates of 02 uptake and evolution by low-Ci cells were completely inducible within 6 h after transferring high-C~ cells to ambient air. At pH 8.0, 02 uptake in the light was inhibited by 2 gM 3-(3,4-dichlorophenyl)-l,1 dimethylurea in both types of alga, but this effect was more pronounced in low-C~ than in high-C~ cells. When the cells were grown at pH 5.5 the activities of the superoxide-radical-degrading enzymes, superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase, were similar regardless of the CO2 concentration provided during growth. At pH 8.0, however, the activities of these enzymes were 4 to 20 times higher in low-C~ than in high-Ci cells. When high-Ci cells were allowed to acclimate to ambient air for 6 h at pH 8.0, the activities of superoxide dismutase, ascorbate peroxidase and monodehydroascorbate dehydrogenase increased to more than 50 % of the level observed with low-C i cells. These results are consistent with an enhanced operation of 02 photoreduction which could provide energy to the * To whom correspondence should be addressed; FAX: 49 (631) 205 29 98Abbreviations." AP = ascorbate peroxidase; Chl = chlorophyll; C i = inorganic carbon (HCO~-+ CO2); DAR = dehydroascorbate reductase; DCMU = 3-(Y,4'-dichlorophenyl)-1,1-dimethylurea; E o = rate of true 02 evolution; GR = glutathione reductase; high-C i cells = cells grown at 5 % CO2; low-C i cells = cells grown at 0.035 % CO 2 ; MDAR = monodehydroascorbate reductase; NET = rate of net 02 evolution; PPFD=photosynthetic photon flux density; SOD = superoxide dismutase; Uo= rate of true O z uptake inorganic-carbon-concentrating mechanism via pseudocyclic photophosphorylation.