Photosynthetic Characteristics of Several High-CO2-Requiring Mutants of Chlamydomonas

Abstract

Photosynthesis in Chlamydomonas reinhardtii (2137 mt+) cells grown under air condition (low CO2 cells) have a very high affinity for COp, and have little or no CO2-sensitive O2 inhibition of photosynthesis (1,2,3). These characteristics, which have also been reported in low CO2 cells of several other algae (4, 5,6), is thought to be caused mainly by the CO2-concentrating mechanism. This mechanism apparently raised the CO2/O2 concentration ratio at the site of RuBP carboxylase/oxygenase, although the details of the CO2-concentrating mechanism are still unclear. Several mutants of C. reinhardtii have been isolated which apparently require elevated CO2 concentrations for photoautotrophic growth. Two such mutants have previously been described, a carbonic anhydrase deficient mutant (ca-1–12–1C) (2), and an inorganic carbon transport deficient mutant (pmp-1-l6–5K) (3). These two mutants indicate that these two components, at least, are essential to supply sufficient CO2 to the site of RuBP carboxylase during photosynthesis under low CO2 conditions.