Photoassimilation of Inorganic Carbon in Chlamydomonas segnis during Cell Development at Low and High Carbon Dioxide Tension

Abstract

Summary Rates of photoassimilation of inorganic carbon at non-limiting (4.8 mM) and limiting (0.14 mM) concentrations of NaHCO 3 were compared in air- and 5% CO 2 -adapted cells of Chlamydomonas segnis during development in continuously illuminated synchronous cultures. At the limiting concentration of inorganic carbon and depending on the algal developmental stage, 5% CO 2 -adapted cells exhibited only 1–6% of the rates shown at the non-limiting concentration, compared to 30–90% in air-adapted cells. As indicated by these low and high rates, adaptation of 5% CO 2 zoospores to air occurred faster (4 h) than that of air zoospores to 5% CO 2 (24 h). The longer period required by the latter (i.e. the completion of one cell cycle) could be explained in terms of a requirement for structural changes. Analysis of the patterns of 14 C distribution among the ethanol-water soluble products of short-term photosynthesis (1 min) at the limiting concentration of inorganic carbon revealed significant differences between air- and 5% CO 2 -adapted cells. At early stages of development, the flow of carbon to the C2 photorespiratory carbon oxidation cycle appeared to be suppressed in the former, but not in the latter cells which were characterized by comparatively high rates of carbon flow into C4 compounds. At later stages of development (early to mid S-phase), the major difference was seen in the greater ability of 5% CO 2 -adapted cells to fix inorganic carbon via phosphoenolpyruvate (PEP) carboxylation. It was argued that PEP derived from the degradation of carbohydrate reserves in these cells, rather than PEP recently produced from the C3 photosynthetic carbon reduction cycle, would be responsible for the enhancement of β-carboxylation reaction(s) at S-phase.