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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]

In Chlamydomonas reinhardtii the formation of a starch sheath surrounding the pyrenoid is observed when cells grown under high CO2 (5% CO2 in air) are transferred to low CO2 (0.03%) conditions. Formation of the starch sheath occurs coincidentally with induction of the CO2 concentrating mechanism and with de novo synthesis of 5 polypeptides with molecular masses of 21, 36, 37, 42-44 kDa. We studied the effect of CO2 concentrations on photosynthesis, ultrastructure and protein synthesis in Chlamydomonas reinhardtii cw-15 (wild phenotype for photosynthesis) and in the starch-less mutant BAFJ-6, with the aim to clarify the role of the pyrenoid starch sheath in the operation of the CO2 concentrating mechanism and whether these low CO2-inducible polypeptides are involved in the formation of starch sheath. When wild type and starch-less mutant cells were transferred from high to low CO2 the CO2 requirement for half-maximal rates of photosynthesis decreased from 40 μM to 2 μM CO2. 35SO42- labeling analyses showed that the starch-less mutant induced the 5 low CO2-inducible polypeptides. These observations suggest that the starch-less mutant was able to induce a fully active CO2 concentrating mechanism. Since the starch-less mutant did not form a pyrenoid starch sheath, we suggest that the starch sheath is not involved in the operation of the CO2 concentrating mechanism and that none of these 5 low CO2-inducible proteins is involved in the formation of the starch sheath in Chlamydomonas.