Aerobic autotrophic isopropanol production under low H₂ atmosphere in Hollow Fiber Membrane Bioreactor

One of the challenges of aerobic gas fermentation for the production of biomolecules is to control the gas environment in order to operate outside the explosive zone, i.e., with low O₂ (< 6%) and/or H₂ (< 4%) contents. In this study, a bioreactor was to a hollow fiber membrane contactor to supply hydrogen to the microbial suspension. This technology allows the transfer of H₂ directly in soluble form into the culture medium without generating bubbles, responsible for the enrichment of the gas mixture with hydrogen by bursting in the gas headspace. Coupling a Hollow Fiber Membrane Contactor to the gas bioreactor in C. necator cultures resulted in either 10 g/L of biomass or 1.3 g/L of isopropanol, while minimizing the H₂ content in the gas headspace to a level very close to, or even below, the safety threshold of 4 mol% H₂. Isopropanol production performance was compared with that obtained in a gas reactor without HFMC (6.8 g/L of isopropanol). Although the production performance was lower, the coupling still achieved it under low H₂ atmosphere, providing proof of concept that it is possible to perform aerobic autotrophic process under low hydrogen atmosphere, thus allowing the exit from the explosive zone regardless of the oxygen content of the reactor. This advancement should help make aerobic gas fermentation much safer and extend these processes on a larger scale.