Reversible pH Changes in Cells of Chlamydomonas reinhardi Resulting from CO2 Fixation in the Light and Its Evolution in the Dark

Illumination of a suspension of Chlamydomonas reinhardi causes an increase in the pH of the medium which is reversed in the dark. This pH change is a manifestation of CO(2) uptake in the light and its evolution in the dark. Simultaneous measurements of pH changes and oxygen evolution reveal that the photosynthetic coefficient approaches one.Intact cells of F-60, a mutant strain of C. reinhardi that lacks an active phosphoribulokinase, do not exhibit the light-dependent pH increase or oxygen evolution. However, chloroplast fragments prepared from the cells of the mutant strain exhibit a normal "proton pump" activity.The light-dependent pH increase shown by intact cells can be inhibited by KCN, by uncouplers of photosynthetic phosphorylation, and by Dio-9. It is markedly increased upon the addition of potassium bicarbonate, and all inhibitors tested inhibit the pH increase in both the presence and absence of potassium bicarbonate.The results of the present work negate the conclusion of other workers that the light-dependent pH changes in intact cells of C. reinhardi (and probably in other algae as well) are due to the operation of the "proton pump."