bicarbonate-carbon dioxide exchange catalyzed by extracellular carbonic anhydrase in Chlamydomonas reinhardtii

Acetate assimilation in C. reinhardtii leads to bicarbonate and CO2 aq formation in heterotrophic growth condition. Bicarbonate and CO2 aq thus formed under this condition remain in equilibrium with the action of carbonic anhydrases. Carbonic anhydrase catalyzes reversible hydration of carbon dioxide and dehydration of bicarbonate. In this article we report that the rapid exchange catalyzed by extracellular carbonic anhydrase causes a large magnetization (saturation) transfer effect on the C signal of bicarbonate at 161.01 ppm when the resonance of the carbon dioxide (aq) at 125.48 ppm is irradiated with RF pulses. In C. reinhardtii extracellular space the unidirectional, pseudo first-order rate constant of this exchange in the dehydration direction was determined to be 0.011 ± 0.005 sec. The presence of highly specific carbonic anhydrase inhibitor acetazolamide, was also shown to drastically attenuate the observed C magnetization transfer effect of the carbon dioxide–bicarbonate exchange in C. reinhardtii. We have demonstrated the utility of C saturation transfer for determining the exchange rate between bicarbonate and carbon dioxide catalyzed by extracellular carbonic anhydrase in C. reinhardtii extracellular space.This study for the first time reports the dehydration rate of bicarbonate to CO2 in live C. reinhardtii cells.