The Path of Carbon in Photosynthesis by Chlamydomonas segnis Adapting to Low and High Carbon Dioxide Tension

Abstract

Summary The kinetics of 14 C-incorporation into the major intermediates of the C2 photorespiratory carbon oxidation (C2PCO) and the C3 photosynthetic carbon reduction (C3PCR) cycles, and into C4 compounds were determined in Chlamydomonas segnis during photosynthesis at sub-optimal concentration of inorganic carbon. These and the level of carbonic anhydrase (CA) activity were compared in zoospores, at mid G 1 -phase of the cell cycle, adapting (i.e. for 4h) and adapted (i.e. for many generations) to air and 5 % CO 2 . The results indicated that the suppression of the C2PCO cycle was an early event in the process of zoospore adaptation to air and was accompanied by a limited 14 C-incorporation into malate and aspartate. The 14 C in these acids did not exceed 14 % of the total 14 C recovered in early photosynthates. This and the lack of 14 C flow from C4 acids to the intermediates of the C3PCR cycle provided evidence that the C4 pathway was absent in zoospores capable of suppressing the C2PCO cycle. However, the ratio of 14 C in malate and aspartate to that in glycolate and glycine after 5–10 s photosynthesis was higher (2.26–2.72) in zoospores adapting or adapted to 5 % CO 2 than to air (1.35–1.46). The high C4/C2 ratio in such zoospores with an active C2PCO cycle could be explained in terms of their ability to refix photorespiratory CO 2 via β-carboxylation. In zoospores adapting to air, the acquisition of a relatively high level fo CA activity appeared to be unnecessary for the stimulation of the C3PCR cycle.