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Photorespiration rates under air-equilibrated conditions (0.04% CO2 and 21% O2) were measured in Chlamydomonas reinhardtii wild-type 2137, a phosphoglycolate- phosphatase-deficient (pgpl) mutant and a suppressor double mutant (7FR2N) derived from the pgpl mutant. In both cells grown under 5% CO2 and adapted air for 24 h in the suppressor double mutant, the maximal rate of photorespiration (phosphoglycolate synthesis) was only about half of that in either the wild type or the pgpl mutant (18-7F) cells. In the progeny, the reduced rate of photorespiration was accompanied by increased photosynthetic affinity for inorganic carbon and the capacity for growth under air whether accompanied by the pgpl background or not. Tetrad analyses suggested that these three characteristics all resulted from a nuclear single-gene mutation at a site unlinked to the pgpl mutation. The decrease in photorespiration was, however, not due to an increase in the CO2/O2 relative specificity of ribulose-l,5-bisphosphate carboxylase/ oxygenase of 7FR2N or of any other suppressor double mutants tested. The relationship between the decrease in the rate of photorespiration and the CO2-concentrating mechanism is discussed.