The combustion of fossil fuels has created many environmental problems, the major one, the greenhouse effect. Thus, we need solutions in order to replace fossil fuels and recycle the CO₂ in the atmosphere. Renewable energies have created attention the last decades but electricity is the main energy form obtained. Photosynthetic organisms (including cyanobacteria) can be used as cell factories since they can convert solar energy to chemical energy. In addition, the requisites to grow them are few; light water, CO₂ and inorganic nutrients. Cyanobacteria have been genetically engineered in order to produce numerous chemicals and fuels of human interest in direct processes. However, the amount of product obtained is still low. Increased carbon fixation in cyanobacteria results in higher production of carbon-based substances. This thesis focuses on the effects of overexpressing the native phosphoenolpyruvate carboxylase (PEPc) in the model cyanobacterium Synechocystis PCC 6803. PEPc is an essential enzyme and provides oxaloacetate, an intermediate of the tricarboxylic acid cycle (TCA cycle). The TCA cycle is involved in connecting the carbon and nitrogen metabolism in cyanobacteria. The strains were further engineered to produce ethylene and succinate, two examples of interests for the chemical and fuel industry. Strains with additional PEPc produced significantly more ethylene and succinate. Moreover, an in vitro characterization of PEPc from the cyanobacterium Synechococcus PCC 7002 was performed. The focus was on oligomerization state, kinetics and the structure of the carboxylase. This thesis demonstrates that increasing carbon fixation and discovering the bottlenecks in chemical production can lead to higher yields and gives us hope that cyanobacteria can be commercialized.