Effect of O2 and CO 2 on Net CO 2 Exchange in a High-CO 2-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 μl/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.