Concerted changes in the transcriptional pattern and physiological traits that result from long-term (here defined as up to 24 h) limitation of inorganic carbon (C-i) have been investigated for the cyanobacterium Synechocystis sp. strain PCC 6803. Results from reverse transcription-polymerase chain reaction and genome-wide DNA microarray analyses indicated stable up-regulation of genes for inducible CO2 and HCO3- uptake systems and of the rfb cluster that encodes enzymes involved in outer cell wall polysaccharide synthesis. Coordinated up-regulation of photosystem I genes was further found and supported by a higher photosystem I content and activity under low C-i (LC) conditions. Bacterial-type glycerate pathway genes were induced by LC conditions, in contrast to the genes for the plant-like photorespiratory C-2 cycle. Down-regulation was observed for nitrate assimilation genes and surprisingly also for almost all carboxysomal proteins. However, for the latter the observed elongation of the half-life time of the large subunit of Rubisco protein may render compensation. Mutants defective in glycolate turnover (Delta glcD and Delta gcvT) showed some transcriptional changes under high C-i conditions that are characteristic for LC conditions in wild-type cells, like a modest down-regulation of carboxysomal genes. Properties under LC conditions were comparable to LC wild type, including the strong response of genes encoding inducible high-affinity C-i uptake systems. Electron microscopy revealed a conspicuous increase in number of carboxysomes per cell in mutant Delta glcD already under high C-i conditions. These data indicate that an increased level of photorespiratory intermediates may affect carboxysomal components but does not intervene with the expression of majority of LC inducible genes.