The paper deals with a review of the data available in literature on the regulation of the metabolic processes in microorganisms with CO2 fixation. The main function of CO2 is traced as exemplified by different types of microorganism nutrition. The carboxylation reactions in their main forms are observed in all heterotrophic microorganisms. The experimental data relative to the heterotrophic fixation of CO2 in yeast assimilating oil carbohydrates are presented as an example. The degree of CO2 fixation under these conditions may reach 30%. Carbon of CO2 is utilized for the biomass biosynthesis.