Deletion of aldehyde:ferredoxin oxidoreductase-encoding genes in Clostridium ljungdahlii results in changes in product spectrum with various carbon sources

Biofuels, such as ethanol, can be produced by the microbial fermentation of waste gases that contain carbon dioxide (CO₂) and carbon monoxide (CO). The acetogenic model microbe Clostridium ljungdahlii converts those substrates into acetyl-CoA with the Wood-Ljungdahl pathway. During autotrophic conditions, acetyl-CoA can be reduced further to ethanol via acetic acid by the enzymes aldehyde:ferredoxin oxidoreductase (AOR) and alcohol dehydrogenase. Here, the genes encoding both tungsten-dependent AORs (aor1, CLJU_c20110 and aor2, CLJU_c20210) were deleted from the genome of C. ljungdahlii. The effects on the product spectrum of the individual and double deletion strains were investigated. Most pronounced, ethanol formation was enhanced for C. ljungdahlii Δaor1 with different carbon sources, that is, fructose, hydrogen (H₂) and CO₂, and CO. The lowest and highest ethanol:acetic acid ratio was detected during growth with H₂/CO₂ and CO, respectively. Oscillating patterns were observed during growth with CO, underpinning the importance of a balanced redox metabolism.