Adaptation of Chlamydomonas reinhardtii High-CO(2)-Requiring Mutants to Limiting CO(2).

Photosynthetic characteristics of four high-CO(2)-requiring mutants of Chlamydomonas reinhardtii were compared to those of wild type before and after a 24-hour exposure to limiting CO(2) concentrations. The four mutants represent two loci involved in the CO(2)-concentrating system of this unicellular alga. All mutants had a lower photosynthetic affinity for inorganic carbon than did the wild type when grown at an elevated CO(2) concentration, indicating that the genetic lesion in each is expressed even at elevated CO(2) concentrations. Wild type and all four mutants exhibited adaptive responses to limiting CO(2) characteristic of the induction of the CO(2)-concentrating system, resulting in an increased affinity for inorganic carbon only in wild type. Although other components of the CO(2)-concentrating system were induced in these mutants, the defective component in each was sufficient to prevent any increase in the affinity for inorganic carbon. It was concluded that the genes corresponding to the ca-1 and pmp-1 loci exhibit at least partially constitutive expression and that all components of the CO(2)-concentrating system may be required to significantly affect the photosynthetic affinity for inorganic carbon.