This master's thesis aims to convert CO2 to value added chemicals such as Single Cell Protein or polyhydroxybutyrate (PHB) via gasfermentation. Alternatively, formic acid, previously converted starting from CO2, will be used as carbon and energy source. Hence, this thesis will be focused on comparing direct (using different gas mixtures such as H2 and CO2), and indirect gas fermentation (using formic acid) for the production of valuable chemicals. Comparing the performance of this strategy with direct gas fermentation is of great interest, as the latter strategy avoids the use of explosive gas mixtures. Therefore, the performance of different strains will be evaluated under a wide range of process conditions using design of experiments.