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The CO-concentrating mechanism (CCM) was induced in the green unicellular alga Chlorella when cells were transferred from high (5% CO) to low (0.03%) CO concentrations. The induction of the CCM correlated with the formation of a starch sheath specifically around the pyrenoid in the chloroplast. With the aim of clarifying whether the starch sheath was involved in the operation of the CCM, we isolated and physiologically characterized a starchless mutant of Chlorella pyrenoidosa, designated as IAA-36. The mutant strain grew as vigorously as the wild type under high and low CO concentrations, continuous light and a 12 h light/12 h dark photoperiod. The CO requirement for half-maximal rates of photosynthesis [K(CO)] decreased from 40 μM to 2-3 μM of CO when both wild type and mutant were switched from high to low CO. The high affinity for inorganic carbon indicates that the IAA-36 mutant is able to induce a fully active CCM. Since the mutant does not have the pyrenoid starch sheath, we conclude that the sheath is not involved in the operation of the CCM in Chlorella cells. [ABSTRACT FROM AUTHOR]