The response of Synechococcus PCC 7942 (Cyanophyta) to changes in CO2supply in relation to the acclimation of the CO2-concentrating mechanism. II. carboxysomes

Distinct types of carboxysomes were distinguished in Synechococcus PCC 7942: electron-clear, electron-intermediate, carboxysomes with internal electron-clear areas, typical electron-dense and bar-shaped carboxysomes. Immunogold location with antibodies against the Rubisco large subunit showed specific label in all carboxysomes. The positive correlation between electron-density, the density of immunogold label, and the percentage of labeled structures within each type support a model of carboxysome biogenesis whereby electron-clear evolve to electron-intermediate and then to electron-dense carboxysomes by the progressive sequestering of Rubisco molecules. Cells responded to limitation in CO2 supply by increasing carboxysome frequency and the proportion of typical electron-dense carboxysomes, the extent of the response depending on the degree of limitation. The time course of carboxysome expression during transfers between different conditions of CO2 supply indicated that, under our experimental conditions, there were different levels of response, depending on the degree of limitation. The first level occured at atmospheric levels of CO2 and involved changes in the affinity of the CCM and in carboxysome, which occurred simultaneously. More severe limitation of CO2 supply affected carboxysomes exclusively, without further improvement in the affinity of the CCM.