Syngas fermentation: What’s important for production of ethanol?

Syngas fermentation with carboxydotrophic bacteria, such as Clostridium ljungdahlii and C. autoethanogenum, is a pertinent topic within the bioenergy and renewable fuels and chemicals sector. The respective organisms that are employed are referred to as homoacetogens. As the name suggests, they produce acetic acid rather than ethanol. However, even without any genetic engineering, it is possible to shift the metabolic pathway towards solvent production. Mechanisms that lead to formation of ethanol by C. ljungdahlii will be discussed. Specifically, we present the results of a proteomics/metabolomics study comparing acidogenesis and solventogenesis, and discuss the thermodynamic and metabolic implications of the use of various substrate gases (hydrogen plus carbon dioxide vs. carbon monoxide), and the supply of specific micronutrients. An outlook will be given how to apply the lessons learned towards optimization of yields and productivity for ethanol and, in the broader context, towards production of higher alcohols.