INORGANIC CARBON TRANSPORT IN MICROALGAE .1. LOCATION OF CARBONIC-ANHYDRASE AND HCO3-OH- EXCHANGE

The plasmalemma of Scenedesmus obliquus is capable of a counter-transport of HCO3- and OH- at very high rates (up to 200 nmol/cm2/s). Such an exchange takes place during in vivo carbonic anhydrase (CA) assays and because this enzyme can be shown to be situated inside the cell it may be concluded that its substrates cross the plasmalemma during assay. Diffusion of OH- within the cell is then the most likely rate limiting process. This conclusion is supported by the stimulating effect of phosphate on the in situ CA activity. The high HCO3-/OH- exchange capacity of the Scenedesmus plasmalemma is corrobrated by tracer exchange studies using H14CO3- at O°C and the silicone oil layer centrifugation technique. Under these conditions 14C uptake in exchange for internal unlabelled inorganic carbon is rate limited by internal isotope equilibration, as shown by the inhibition of the process by the CA inhibitor, diamox. No rate limitation by external HCO3-/CO2 interconversion or HCO3-/OH- exchange could be detected. © 1979.