Reduced Inorganic Carbon Transport in a CO2-Requiring Mutant of Chlamydomonas reinhardii

A mendelian mutant of the unicellular green alga Chlamydomonas reinhardii has been isolated that is deficient in inorganic carbon transport. This mutant strain, designated pmp-1-16-5K (gene locus pmp-1), was selected on the basis of a requirement of elevated CO(2) concentration for photoautrophic growth. Inorganic carbon accumulation in the mutant was considerably reduced in comparison to wild type, and the CO(2) response of photosynthesis indicated a reduced affinity for CO(2) in the mutant. At air levels of CO(2) (0.03-0.04%), O(2) inhibited photosynthesis and stimulated the synthesis of photorespiratory intermediates in the mutant but not in wild type. Neither strain was significantly affected by O(2) at saturating CO(2) concentration. Thus, the primary consequence of inorganic carbon transport deficiency in the mutant was a much lower internal CO(2) concentration compared to wild type. From these observations, we conclude that enzyme-mediated transport of inorganic carbon is an essential component of the CO(2) concentrating system in C. reinhardii photosynthesis.