Baur, S. T.; Schulz, S.; McCluskey, J. B.; Velazquez Gomez, J. A.; Angenent, L. T.; Molitor, B.

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. Ethanol formation was enhanced for C. ljungdahlii {Delta}aor1 with different carbon sources, that is, fructose, a mixture of hydrogen (H2) and CO2, and CO. The highest and lowest 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.