NOVELTY - The process for fixing carbon dioxide (CO2) (106) and treating organic wastes (104) using a methanization system (102) and a phytoplankton microorganism production system, comprises treating microorganisms (105) obtained from a phytoplankton culture and organic wastes in a hydrolysis reactor, treating a part of resulting liquid effluent in a biogas reactor, treating a liquid phase and biogas to be purified in a unit (103) of phytoplankton microorganism culture, and injecting a gas stream containing CO2 in the phytoplankton microorganism culture unit.    USE - The process is useful for fixing carbon dioxide and treating organic wastes using a methanization system and a phytoplankton microorganism production system for the production of biogas or biofuels, and in food processing industry, where the organic wastes include sewage, urban or industrial wastewater loaded with biodegradable organic matter, organic products in solid form, sewage sludge, household waste, bio-wastes, industrial wastes, agro-food industrial wastes, and wastes from forests or energy crops.    ADVANTAGE - The process allows effective and complete removal of carbon dioxide without creating any harmful environment, allows energy efficient, easy and homogeneous treatment all kinds of organic wastes without any contamination in reactor units, and reduces the risk of inhibition of a methanization system and a phytoplankton microorganism production system.    DETAILED DESCRIPTION - The process for fixing CO2 (106) and treating organic wastes (104) using a methanization system (102) and a phytoplankton microorganism production system, comprises treating microorganisms (105) obtained from a phytoplankton culture and organic wastes in a hydrolysis reactor, treating a part of resulting liquid effluent in a biogas reactor, treating a liquid phase and biogas to be purified in a unit (103) of phytoplankton microorganism culture, injecting a gas stream containing CO2 in the phytoplankton microorganism culture unit, maintaining a concentration of ammonia of less than 0.5 g/L in methanization reactor, and recovering a biogas rich in methane coming out of the unit for phytoplankton microorganism culture. A carbon/nitrogen (C/N) ratio value of 10-35 is maintained in the hydrolysis reactor for modulating the fraction of organic wastes introduced in the hydrolysis reactor. The liquid effluent is treated using microorganisms of autotrophic species having a C/N ratio of greater than 10. An average C/N ratio of 10-35 is maintained in the phytoplankton microorganism culture unit by modulating the amount of liquid effluent and the incoming flow of the biogas for: inducing a nutrient reduction; modifying the composition of microorganisms in the culture unit; enhancing the accumulation of lipids and carbohydrates in the microorganisms; and controlling the pH of the culture unit and creating the conditions to increase the C/N ratio. A flow of carbon, which is 10 times the flow of nitrogen is provided in the reactor by modulating the incoming flow of gas effluent and maintaining an average C/N ratio in the microorganism culture unit. A rate of dilution of the microorganism culture less than maximum growth rate of the microorganisms is imposed for inducing a reduction by a nutrient element. The biogas rich in methane is filtered in an exchange column. A fraction of liquid effluent is directly injected into the culture unit. An INDEPENDENT CLAIM is included for a coupled system for fixing CO2 and treating organic wastes.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an installation for fixing carbon dioxide and treating organic wastes.    Methanization system (102)    Phytoplankton microorganism culture unit (103)    Organic wastes (104)    Microorganisms (105)    Carbon dioxide gas. (106)