Evidence for the contribution of pseudocyclic photophosphorylation to the energy requirement of the mechanism for concentrating inorganic carbon in Chlamydomonas

Abstract

Mass-spectrometric measurements of 16O2 and 18O2 were made to compare the rates of light-dependent O2 evolution and uptake by Chlamydomonas reinhardtii Dang. grown in air (0.035% CO2; low-Ci cells) or CO2-enriched air (5% CO2; high-Ci cells) at pH 5.5 and 8.0. While at pH 5.5, no differences were observed in the isotopic O2-gas exchange of high- and low-Ci cells, at pH 8.0 the rates of true O2 evolution and uptake were considerably higher in low-Ci than in high-Ci cells. The enhanced rates of O2 uptake and evolution by low-Ci cells were completely inducible within 6 h after transferring high-Ci cells to ambient air. At pH 8.0, O2 uptake in the light was inhibited by 2 μM 3-(3,4-dichlorophenyl)-1,1 dimethylurea in both types of alga, but this effect was more pronounced in low-Ci than in high-Ci cells.