In the context of the energy transition, the use of organic waste in environmental biorefineries is an attractive option due to its low cost and potential to replace fossil fuels. However, this approach presents technological challenges due to the heterogeneity and variability over time of organic waste, increasing the complexity of purification treatments for the molecules of interest and resulting in higher final costs. Microbial electrochemical technologies are emerging as promising solutions to overcome these problems, by allowing the physical separation of oxidation from contaminated waste streams used as raw materials (bioanode) from the synthesis of bio-based chemical molecules (biocathode). Recent scientific studies have focused on the production of methane and acetic acid at the cathode, associated with the electrolysis of water at the anode (abiotic). The objective of this study is to combine a bioanode with a biocathode, in order to reduce carbon dioxide to carboxylates, by the selection of homoacetogenic bacterial communities, at the cathode. The electrons generated at the anode, by oxidation of the organic matter by electrogenic microorganisms, are transferred to the cathode, for the reduction of carbon dioxide into multicarbon molecules by electrotrophic microorganisms. In addition to the two main compartments, an intermediate compartment, isolated by ion exchange membranes, is integrated in order to extract and concentrate the carboxylates produced in a sterile solution. This research aims to explore the diversity of carboxylates produced at the biocathode as a function of the selection and dispersion conditions of the microbial assembly. Both compartments are inoculated with the same mixture of biowaste hydrolysate (Tian et al., 2023) and salt marsh sediments, as hypersaline inocula have been identified as particularly suitable for microbial electrochemical applications. These inoculation conditions make it possible to enrich microbial diversity and study the selection process that takes place at the anode and cathode on the same reservoir of diversity. Only the mixture intended for cathode inoculation is heat-treated in order to select homoacetogenic bacterial communities capable of sporulation, from the class Clostridia (Diallo et al., 2021). Anaerobic conditions are ensured by N2 bubbles and the cathode is supplied daily with CO2, which is the only source of external carbon. In addition, the two electrodes have distinct geometrics: the anode is a carbon cloth while the cathode is a carbon brush with granules of the same material. Thus, the same inoculum is subject to different selection processes, including oxidation or reduction capacity, different carbon sources, the ability to form a biofilm and to carry out electron exchanges with a 2D electrode or 3D, etc. The main objective of this first triplicata experiment is to evaluate the efficiency of the use of a hypersaline inoculum, both for the oxidation of organic matter at the anode, and for the reduction of carbon dioxide to carboxylates at the cathode, as well as the efficiency of the extraction of these carboxylate ions in an intermediate compartment. We also want to determine all the carboxylates produced, in order to evaluate the possibility of diversifying or, on the contrary, of specializing the synthesis of these molecules by modifying the microbial assembly through selection and dispersion. Ultimately, this project aims to guide the production of dyes by a model microorganism, froms carboxylates formed in the biocathode and concentrated within the intermediate compartment. These dyes will have specific applications in the textile industry, demonstrating the potential of this innovative approach in the development of sustainable processes for industrial applications. References: Diallo, M., Kengen, S. W. M., & López-Contreras, A. M. (2021). Sporulation in solventogenic and acetogenic clostridia. Applied Microbiology and Biotechnology, 105(9), 3533-3557. https://doi.org/10.1007/s00253-021-11289-9 Tian, J.-H., Lacroix, R., Yaqoob, A. A., Bureau, C., Midoux, C., Desmond-Le Quéméner, E., & Bouchez, T. (2023). Study of a Pilot Scale Microbial Electrosynthesis Reactor for Organic Waste Biorefinery. Energies, 16(2), 591. https://doi.org/10.3390/en16020591