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Synechocystis PCC 6803 strains overexpressing pepc , gene encoding the carbon fixing enzyme phosphoenolpyruvate carboxylase (PEPc), were constructed and characterized for growth, PEPc protein content and in vitro PEPc activities. Synechocystis strains WT + Km r – one (native) copy of pepc (control), WT + 2xPEPc – native copy of pepc and two additional native copies of pepc (in total three copies of pepc ), and WT + PPM — native copies of ppsa (encoding phosphoenolpyruvate synthase), pepc and mdh (encoding malate dehydrogenase) and one additional copy of each gene (in total two copies each of ppsa , pepc and mdh ) were analyzed for growth under normal and low light intensities, and in darkness (no growth). No significant differences in the growth rates were observed when the cells were grown under normal light intensity. However, growth under low light intensity (3 μmol photons·m − 2 ·sec − 1 ) resulted in increased growth rate, in particular in the strain with 3 copies of pepc . SDS-PAGE/Western immunoblots using antibodies directed against PEPc demonstrated an increased level of PEPc protein with increasing number of copies of pepc . This was followed by increased levels of in vitro PEPc activities. A less efficient ribulose 1,5-bisphosphate carboxylase/oxygenase in combination with reduced levels of NADPH and ATP under low light condition may make the relatively more efficient carbon fixing enzyme PEPc the limiting step for growth under this condition.

Cyanobacteria, gram-negative prokaryotic microorganisms, perform oxygenic photosynthesis with a photosynthetic machinery similar to higher plants which includes ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) as the main CO2-fixing enzyme. Currently, there is a growing interest to use cyanobacteria as photosynthetic microbial cell factories for the direct production of solar fuels or other compounds of human interest. However, rates and efficiencies to produce e.g. biofuels are still very low. The amount of available fixed carbon for the synthesis of desired product(s) may be one of the limiting steps. This contribution reviews CO2-fixation in cyanobacteria with focus on CO2-concentrating mechanisms, RuBisCO, phosphoenolpyruvate carboxylase and other carboxylases, engineering approaches for increased carbon fixation, and finally the synthetic malonyl-CoA-oxaloacetate-glyoxylate pathways.