Phosphoglycolate Phosphatase-Defident Mutants of <italic>Chlamydomonas reinhardtii</italic> Capable of Growth under Air

Mutants deficient in phosphoglycolate phosphatase (PGPase) require elevated levels of CO2 for growth in the light and cannot grow when photorespiration occurs. Revertants, namely, double mutants capable of growth under air without restoration of the missing PGPase activity, might be expected to have secondary mutations that reduce or eliminate photorespiration. Nineteen revertants were selected from a culture of a PGPase-deficient mutant of Chlamydomonas reinhardtii (pgp-1-18-7F) after a second mutagenesis that involved treatment with 5-fluorodeoxyuridine and ethyl methanesulfonate. There were significant differences in the photosynthetic affinity for CO2 among revertant cells grown under 5% CO2. Eight revertants had five times higher photosynthetic affinity for CO2 than that of wild type 2137 cells grown under 5% CO2, resembling air-adapted wild-type cells, whereas four revertants had less than half the affinity for CO2 of the wild type. In all of the revertant cells with higher affinity grown in 5% CO2, the rates of photosynthesis under levels of CO2 below those in air were apparently higher than that of the wild type, whereas the rates under CO2-saturating conditions were lower than that of wild type, indicating that the efficiency of photosynthesis under air was significantly improved in these revertants. In addition, some revertants had a photosynthetic capacity and a growth rate higher than those of the wild type, without any increased photosynthetic affinity for CO2.