Towards product diversification in carbon negative bioprocesses with Clostridium ljungdahlii The way to sustainable chemical production will require carbon capture technologies that allow abundant and cheap carbon dioxide to be available to be used in carbon fixation processes. Among the non photosynthetic biological carbon fixation processes the ones catalyzed by acetogenic bacteria such as gas fermentation and microbial electrosynthesis stand out. Both processes are limited in terms of the range of products they can offer and although gas fermentation has already reached the industrial scale microbial electrosynthesis is a process with great potential but still new and not yet developed. The deepening of our knowledge about the metabolism of acetogenic bacteria such as Clostridium ljungdahlii as well as the development of molecular tools is crucial for the expansion of the portfolio of products that acetogens can offer. In this thesis the mode of extracellular electron transfer and the role of hydrogen in microbial electrosynthesis with C. ljungdahlii is unveiled leading to highly productive planktonic systems that decouple activity from biofilm formation. Furthermore it is proposed that two putative glycine based carbon fixation pathways are activated under redox stress conditions leading to the production in significant quantities of metabolites that are rarely found in microbial electrosynthesis or gas fermentation. The in situ formation of electrocatalysts promotes the formation of formate on carbon based cathodes likely boosting C. ljungdahlii growth and it represents a promising finding for the future development of catalytic cathodes. The development of molecular tools allowed to establish in C. ljungdahlii an anaerobic fluorescent marker CRISPR Cas systems for the generation of deletions and a heterologous expression system to produce ethyl acetate from carbon dioxide and hydrogen. All these advances will be used synergistically in the future to further expand the product range and improve the selectivity for target metabolites.