Changes in carboxysome structure and grouping and in photosynthetic affinity for inorganic carbon in Anabaena strain PCC 7119 (cyanophyta) in response to modification of CO2 and Na+ supply

In Anabaena PCC 7119 a 4-fold decrease in the value of the apparent photosy nthetic affinity for external inorganic carbon [K-1/2 (C-i)] occurred betwe en 9 and 12 h after the transfer from high-CO2 (2% CO2-enriched air) to air -growing conditions. A slight increase in carboxysome frequency occurred, b ut during this transition their appearance and distribution remained unchan ged. Anabaena PCC 7119 did not improve its K-1/2 (C-i) beyond the above cit ed level of acclimation neither by culturing the cyanobacteria in Na+-defic ient medium in air nor by aeration with CO2-depleted air. In air-grown cult ures, Na+ deficiency induced a large increase in carboxysome frequency and an alteration of their appearance: the greatest proportion were electron-de nse whereas this type constituted a minority in high-CO2, and in air, Na+-s ufficient conditions. It also induced major changes in carboxysome distribu tion, whereby more than 60% were grouped, compared with only 10% in high-CO 2 and in air, Na+-sufficient conditions. These changes in carboxysome expre ssion were extremely rapid, occurring mainly during the first 2 h.