Selective butanol production from carbon monoxide by an enriched anaerobic culture

An anaerobic mixed culture able to grow on pure carbon monoxide (CO) as well as syngas (CO, CO₂ and H₂), that produced unusual high concentrations of butanol, was enriched in a bioreactor with intermittent CO gas feeding. At pH 6.2, it mainly produced acids, generally acetic and butyric acid. After adaptation, under stress conditions of CO exposure at a partial pressure of 1.8 bar and low pH (e.g., 5.7), the enrichment accumulated ethanol, but also high amounts of butanol, up to 6.8 g/L, never reported before, with a high butanol/butyric acid molar ratio of 12.6, highlighting the high level of acid to alcohol conversion. At the end of the assay, both the acetic acid and ethanol concentrations decreased, with concomitant butyric acid production, suggesting C₂ to C₄ acid bioconversion, though this was not a dominant bioconversion process. The reverse reaction of ethanol oxidation to acetic acid was observed in the presence of CO₂ produced during CO fermentation. Interestingly, butanol oxidation with simultaneous butyric acid production occurred upon production of CO₂ from CO, which has to the best of our knowledge never been reported. Although the sludge inoculum contained a few known solventogenic Clostridia, the relative taxonomic abundance of the enriched sludge was diverse in Clostridia and Bacilli classes, containing known solventogens, e.g., Clostridium ljungdhalii, Clostridium ragsdalei and Clostridium coskatii, confirming their efficient enrichment. The relative abundance of unassigned Clostridium species amounted to 27% with presumably novel ethanol/butanol producers.