Model for the Relationships betweenCO2-Concentrating Mechanism, CO2 Fixation, and Glycolate Synthesis during Photosynthesis in Chlamydomonas reinhardtii

We constructed a mathematical model for simulating the relationshipsof extracellular concentration of dissolved inorganic carbon(DIC), the rates of photosynthetic CO₂ fixation and glycolatesynthesis, and the concentrations of intrachloroplast CO₂ andO₂ in Chlamydomonas reinhardtii. When we compared the photosyntheticrates of I0W-CO₂ (air)-grown C. reinhardtii measured experimentallyand the rates simulated with the incubation conditions in themodel, the model was found to function well. The calculatedrates for glycolate synthesis also matched the measured ratesbetween 80 to 200 µM extracellular DIC, found in the presenceof 1 mM aminooxyacetate. The conformity of the calculated ratesto the measured ones of the glycolate synthesis encouraged usto estimate the O₂ concentration at the active site of ribulosebisphosphate carboxylase/oxygenase; the results were 0.36 and0.40 mM at 80 and 200 µM extracellular DIC, respectively.These high concentrations of O₂ were due to stimulation of photosyntheticCO₂ fixation and further O₂ evolution by a CO₂- concentratingmechanism in the low-CO₂-grown cells. These cells were calculatedto consume 43% of ATP formed photosynthetically for CO₂ concentrationat 200 µM extracellular DIC. The model modified to simulatethese relationships in high-CO₂ (3 to 5% CO₂)-grown C. reinhardtiipredicted O₂ concentration in chloroplasts to be 0.36 mM ina 1% CO₂ atmosphere. This high concentration of O₂ caused activeglycolate synthesis at the measured rate in the high-CO₂-growncells even in the presence of 1% CO₂. The comparisons of themeasured and simulated rates of photosynthesis in low- and high-CO₂-grownC. reinhardtii indicated that no matter how the CO₂ accumulatedin the chloroplasts, it increased the O₂ concentration in theorganelles, and consequently enhanced glycolate synthesis. ¹This paper is the twenty-first in a series on glycolate metabolismin Euglena gracilis. (Received March 11, 1987; Accepted August 17, 1987)