Microalgae are among the most productive biological systems for converting sunlight into chemical energy which is used to capture and transform inorganic carbon into biomass. The efficiency of CO₂ capture depends on the cultivation system configuration (photobioreactors or open systems) and can vary according to the state of the algal physiology, the chemical composition of the nutrient medium and environmental factors, such as irradiance, temperature and pH. This mini-review is focused on some of the most important environmental factors determining photosynthetic activity, CO₂ biofixation, cell growth rates and biomass productivities by microalgae. These include CO₂ and O₂ concentrations, light intensity, cultivation temperature, nutrients, etc. Finally, a review on operation of microalgae cultivation systems in outdoors is presented as an example of the impact of environmental conditions on biomass productivity and CO₂ fixation.