CO2 supply is a powerful tool to control homoacetogenesis, chain elongation and solventogenesis in ethanol and carboxylate fed reactor microbiomes

• Published in: Frontiers in bioengineering and biotechnology 2024-04, Vol.12, p.1329288-1329288
• Main Authors: de Leeuw, Kasper D., van Willigen, Marius J. W., Vrauwdeunt, Ton, Strik, David P. P. T. B.
• Format: Article
• Language: English
• Subjects: acetogenesis; Bioengineering and Biotechnology; chain elongation; CO2; ethanol; reduction; solventogenesis
• ISSN: 2296-4185; 2296-4185
• DOI: 10.3389/fbioe.2024.1329288

Anaerobic fermentation technology enables the production of medium chain carboxylates and alcohols through microbial chain elongation. This involves steering reactor microbiomes to yield desired products, with CO 2 supply playing a crucial role in controlling ethanol-based chain elongation and facilitating various bioprocesses simultaneously. In the absence of CO 2 supply (Phase I), chain elongation predominantly led to n-caproate with a high selectivity of 96 Cmol%, albeit leaving approximately 80% of ethanol unconverted. During this phase, C. kluyveri and Proteiniphilum -related species dominated the reactors. In Phase II, with low CO 2 input (2.0 NmL L −1  min −1 ), formation of n-butyrate, butanol, and hexanol was stimulated. Increasing CO 2 doses in Phase III (6 NmL L −1  min −1 ) led to CO 2 utilization via homoacetogenesis, coinciding with the enrichment of Clostridium luticellarii , a bacterium that can use CO 2 as an electron acceptor. Lowering CO 2 dose to 0.5 NmL L −1  min −1 led to a shift in microbiome composition, diminishing the dominance of C. luticellarii while increasing C. kluyveri abundance. Additionally, other Clostridia , Proteiniphilum , and Lactobacillus sakei -related species became prevalent. This decrease in CO 2 load from 6 to 0.5 NmL L −1  min −1 minimized excessive ethanol oxidation from 30%–50% to 0%–3%, restoring a microbiome favoring net n-butyrate consumption and n-caproate production. The decreased ethanol oxidation coincided with the resurgence of hydrogen formation at partial pressures above 1%. High concentrations of butyrate, caproate, and ethanol in the reactor, along with low acetate concentration, promoted the formation of butanol and hexanol. It is evident that CO 2 supply is indispensable for controlling chain elongation in an open culture and it can be harnessed to stimulate higher alcohol formation or induce CO 2 utilization as an electron acceptor.