Bioenergetic constraints for conversion of syngas to biofuels in acetogenic bacteria

Synthesis gas (syngas) is a gas mixture consisting mainly of H-2, CO, and CO2 and can be derived from different sources, including renewable materials like lignocellulose. The fermentation of syngas to certain biofuels, using acetogenic bacteria, has attracted more and more interest over the last years. However, this technology is limited by two things: (1) the lack of complete knowledge of the energy metabolism of acetogenic bacteria, and (2) the lack of sophisticated genetic tools for the modification of acetogens. In this review, we discuss the bioenergetic constraints for the conversion of syngas to different biofuels. We will mainly focus on Acetobacterium woodii, which is the best understood acetogen in terms of energy conservation. Syngas fermentation with Clostridium autoethanogenum will also be discussed, since this organism is well suited to convert syngas to certain products and already used in large-scale industrial processes.