Harnessing an adapted strain of Clostridium carboxidivorans to unlock hexanol production from carbon dioxide and hydrogen in elevated-pressure stirred tank reactors

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• Process development with Clostridium carboxidivorans in elevated-pressure reactor. • Definition of crucial gas feeding strategies influencing the product metabolism. • Unprecedented hexanol carbon selectivity (60%) • Highest hexanol titer (3.7 gL −1 ) and productivity (0.9 gg CDW −1 d −1 ) from H 2 and CO 2. • Enhanced caproate production (1.6 gL −1 ) from Clostridium carboxidivorans. To successfully scale-up the production of bio-based building blocks through CO 2 and H 2 -based gas fermentation, it is crucial to deeply understand and control the microbial catalyst response to the bioreactor environment. This study investigates the effects of key process parameters, such as CO 2 and H 2 partial pressures, gas feeding strategies, and mixture composition, on the production pathways of an evolved Clostridium carboxidivorans strain. The ultimate goal is to optimize 1-hexanol production in elevated-pressure stirred-tank reactors. Continuous gas feeding enhanced acetogenic and solventogenic metabolisms, while gas-limited conditions promoted chain elongation to caproic acid. An optimized process, combining an initial gas-limited step followed by a continuous gas phase, increased 1-hexanol production, achieving a maximum biomass-specific productivity of 0.9 g g CDW −1 day −1 . In-situ product extraction improved 1-hexanol carbon selectivity to an unprecedented 60 %. These findings demonstrate the potential of CO 2 and H 2 -fed fermentation to produce high-value chemicals other than ethanol and acetate.