A role for mitochondrial carbonic anhydrase in limiting CO₂ leakage from low CO₂‐grown cells of Chlamydomonas reinhardtii

A model is presented which quantifies a possible role for the carbonic anhydrase in the mitochondrial matrix of Chlamydomonas reinhardtii which incorporates the observation that the expression of this enzyme is increased under growth conditions in which the expression of the carbon dioxide‐concentrating mechanism is increased. It is assumed that the inorganic carbon enters the cytosol from the medium, and leaves the cytosol to the plastids, as HCO 3 − and that there is negligible carbonic anhydrase activity in the cytosol. The role of the mitochondrial carbonic anhydrase is suggested to be the conversion to HCO 3 – of the CO 2 produced in the mitochondria in the light from tricarboxylic acid cycle activity and from decarboxylation of glycine in any photorespiratory carbon oxidation cycle activity which is not suppressed by the carbon concentrating mechanism. If there is a HCO 3 − channel in the inner mitochondrial membrane then almost all of the inorganic carbon leaves the mitochondria as HCO 3 − , thus limiting the potential for CO 2 leakage through the plasmalemma. This mechanism could increase inorganic C supply to ribulose bisphosphate carboxylase‐oxygenase by some 10% at the energetic expense of less than 1% of the total ATP generation by plastids plus mitochondria.