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The use of photosynthetic CO₂ reduction capacity of microalgae can be used for biogas upgrading. Such process would convert CO₂ contained in the biogas into microalgal biomass, generating two products: upgraded biogas and biomass. Growth rate of Nannochloropsis gaditana was determined in atmospheres containing different levels of CH₄ and CO₂. Results showed no effect of CH₄ rich atmosphere over microalgal development. CO₂ inhibition was observed only when microalgae culture was exposed to atmospheres containing 9% of CO₂. Direct contact of the biogas and the microalgal culture is not a feasible way to upgrade biogas, due to oxygen desorption to the gas phase. A two-stage process, involving a photobioreactor connected with a gas/liquid mass transfer unit showed to be an efficient way to remove CO₂ from the biogas, keeping low levels of oxygen in the upgraded biogas.