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ABSTRACT Chlorella emersonii Shihira et Krauss var. emersonii exhibits ‘C 4 ‐like’ gas exchange characteristics when grown at air levels of CO 2 , but is ‘C 3 ‐like’ when grown with extra CO 2 . The total inorganic carbon concentration, and the free CO 2 concentration, averaged over the cell interior are higher in air‐adapted cells than can be accounted for by passive CO 2 equilibration from the medium and the mean intracellular pH value. The ‘extra’ inorganic C in the air‐grown cells probably cannot all be accounted for in terms of binding to proteins and requires an active transport process to account for it. The electrical potential of the cell interior becomes more negative when the ‘CO 2 concentrating mechanism’ is operative; this is most readily explained if the active step in inorganic C accumulation is primary active uniport of HCO 3 − . Since the ‘CO 2 concentrating mechanism’ can operate when CO 2 is the species crossing the outer permeation barrier, it is suggested that the site of active HCO 3 − transport in Chlorella (and other eukaryotes) is the chloroplast envelope, and the plasmalemma in cyanobacteria. This scheme explains the obligatory role of the de‐repressed carbonic anhydrase in C 4 ‐like photosynthesis in algae, but some other data support an explanation of C 4 ‐like photosynthesis in terms of special properties of carbonic anhydrase as a carbon donor to RuBP carboxylase‐oxygenase.
Beardall et al. (Mon,) studied this question.