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Capturing and utilising carbon dioxide (CO2) is a major challenge for developing a low- carbon economy. In addition, the reduction of CO2 allows the synthesis of platform molecules for the chemical and energy industry. Anaerobic mixed cultures contain homoacetogenic microorganisms (HAC) capable of reducing CO2 to acetate. However, one of the obstacles to their use is the understanding and control of their functional diversity. In particular, managing the competition between HAC and hydrogenotrophic methanogens (HM) that convert CO2 into methane is crucial to selectively produce acetate.This study contributes to bring new knowledge on the competition between HAC and HM. In this sense, mass transfer between the gas phase where the substrates are located, and the liquid phase which contains the microbial catalysts, as well as kinetic and thermodynamic aspects of biological reactions have been integrated in this work. The microbial competition between HM and HAC was studied in successive batches. The effect of temperature between 25 °C and 35 °C was investigated, as well as the time of each batch, leading to different states of mass transfer limitation in the system. A clear effect of temperature between 25 °C and 35 °C on the outcome of the competition between HM and HAC was highlighted, as well as the effect of mass transfer limitation. This study contributes to identify specific process parameters influencing the selection of HAC over HM and should help in the design of experiments depending on the target product from H2/CO2.In successive batches, lifting of mass transfer limitation, as well as lowering of the temperature to 25 °C made it possible to select HAC in mixed anaerobic cultures for acetate production by eliminating the methanogens, after four successive batches of 4 to 6 days each.

International audience; This study aimed at understanding how growth conditions influence the biofilm structural properties and the competition between microbial aggregates (biofilms, streamers, suspended biomass). The biological aggregates were developed during more than one month under contrasted conditions in terms of shear stress, hydraulic retention time (HRT), apparent surface organic loading rate (SOLR) and COD/N ratio (fourteen conditions). At high HRT (absence of streamers) an increase of shear stress led to a decrease of the average biofilm thickness whereas it increased with the apparent SOLR. In addition, biofilm density increased with shear stress and with the apparent SOLR at high shear stress. The presence of streamers was observed only at low HRT (< 3h). This presence has been attributed to the residual concentration in the bulk and to the hydrodynamic conditions of growth. Turbulent conditions and low substrate concentration considerably favor the development of streamers which benefit from a higher external mass transfer compared to biofilms due to their flapping movement. In some 2 conditions, the mass of streamers in the reactor were close to the biofilm one's, highlighting the necessity to consider such morphotypes in further studies since they can have a significant impact on the global microbial activity.