Effect of O2 and CO2 on net CO2 exchange in a high-CO2-requiring mutant of Chlamydomonas reinhardtii during dark-light-dark transitions

Net CO₂ exchange was monitored through a dark-light-dark transition, under 2% and 21% O₂ in the presence and absence of CO₂, in Chlamydomonas reinhardtii wild type and the high-CO₂-requiring mutant ca-1-12-1C. Upon illumination at 350 l/l CO₂, ca-1-12-1C cell exhibited a large decrease in net CO₂ uptake following an initial surge of CO₂ uptake. Net CO₂ uptake subsequently attained a steady-state rate substantially lower than the maximum. A large, O₂-enchanced post-illumination burst of CO₂ efflux was observed after a 10-min illumination period, corresponding to a minimum in the net CO₂ uptake rate. A smaller, but O₂-insensitive post-illumination burst was observed following a 30-min illumination period, when net CO₂ uptake was at a steady-state rate. These post-illumination bursts appeared to reflect the release of an intracellular pool of inorganic carbon, which was much larger following the initial surge of net CO₂ uptake than during the subsequent steady-state CO₂ uptake period.With the mutant in CO₂-free gas, O₂-stimulated, net CO₂ efflux was observed in the light, and a small, O₂-dependent post-illumination burst was observed. With wild-type cells no CO₂ efflux was observed in the light in CO₂-free gas under either 2% or 21% O₂, but a small, O₂-dependent post-illumination burst was observed. These results were interpreted as indicating that photorespiratory rates were similar in the mutant and wild-type cells in the absence of CO₂, but that the wild-type cells were better able to scavenge the photorespiratory CO₂.