Model for the Relationships betweenCO<sub>2</sub>-Concentrating Mechanism, CO<sub>2</sub> Fixation, and Glycolate Synthesis during Photosynthesis in <italic>Chlamydomonas reinhardtii</italic><xref ref-type="fn" rid="fn1"><sup>1</sup></xref>

Abstract

We constructed a mathematical model for simulating the relationships of extracellular concentration of dissolved inorganic carbon (DIC), the rates of photosynthetic CO2 fixation and glycolate synthesis, and the concentrations of intrachloroplast CO2 and O2 in Chlamydomonas reinhardtii. When we compared the photosynthetic rates of I0W-CO2 (air)-grown C. reinhardtii measured experimentally and the rates simulated with the incubation conditions in the model, the model was found to function well. The calculated rates for glycolate synthesis also matched the measured rates between 80 to 200 μM extracellular DIC, found in the presence of 1 mM aminooxyacetate. The conformity of the calculated rates to the measured ones of the glycolate synthesis encouraged us to estimate the O2 concentration at the active site of ribulose bisphosphate carboxylase/oxygenase; the results were 0.36 and 0.40 mM at 80 and 200 μM extracellular DIC, respectively. These high concentrations of O2 were due to stimulation of photosynthetic CO2 fixation and further O2 evolution by a CO2- concentrating mechanism in the low-CO2-grown cells. These cells were calculated to consume 43% of ATP formed photosynthetically for CO2 concentration at 200 μM extracellular DIC. The model modified to simulate these relationships in high-CO2 (3 to 5% CO2)-grown C. reinhardtii predicted O2 concentration in chloroplasts to be 0.36 mM in a 1% CO2 atmosphere. This high concentration of O2 caused active glycolate synthesis at the measured rate in the high-CO2-grown cells even in the presence of 1% CO2. The comparisons of the measured and simulated rates of photosynthesis in low- and high-CO2-grown C. reinhardtii indicated that no matter how the CO2 accumulated in the chloroplasts, it increased the O2 concentration in the organelles, and consequently enhanced glycolate synthesis.