An inducible CO₂ concentrating mechanism in cyanobacterium Anabaena sp. strain PCC7120

In order to define its characteristics of the photosynthetic utilization of CO₂ and HCO₃^- when the ambient inorganic carbon changed, HCG (High-CO₂-Growing Cells) of cyanobacterium Anabaena sp. strain PCC7120 were prepared. The growth rate of HCG was higher than that of LCG (low-CO₂-growing cells, i.e. air-growing cells). When the HCG cells were transferred from 5% CO₂ to air levels of CO₂, a series of changes took place: its carbonic anhydrase activity as well as its photosynthetic affinity to the external inorganic carbon significantly increased; the number of the carboxysomes, which is one of the most important components of CCM in cyanobacteria also increased. These facts indicated that the CCM activity of Anabaena PCC 7120 was induced. When the pH in the medium increased from 6 to 9, the photosynthetic affinity to external inorganic carbon of both HCG and LCG declined, while the apparent photosynthetic affinity to external CO₂ increased. In the light of these findings, this inducible CCM in cyanobacteria provided a good model for the study of the photosynthetic Ci utilization in the phototrophic microoganisms.