Biobutanol: A better biofuel?

Global energy demand and concerns over greenhouse gas emissions have increased interest in producing fuels which are compatible with current transportation infrastructure, from renewable sources. In recent years the profile of bio-butanol has been raised due to its perceived advantages over the traditionally considered gasoline substitute, bio-ethanol. In this review, traditional acetone-butanol-ethanol production by solventogenic clostridia is discussed. Different strategies have been employed to increase butanol productivity and overcome problems with toxicity; however a poor understanding of the regulatory networks governing butanol fermentation and lack of efficient genetic tools for clostridia has hampered progress in improving butanol titres. The butanol biosynthetic pathway from clostridia has been expressed in a range of alternative hosts; however reported butanol yields achieved are currently not comparable to those produced from clostridial fermentation. Nevertheless, microorganisms able to tolerate substantially higher concentrations of butanol than clostridia have been isolated, indicating that this may ultimately be the preferred route. Problems with toxicity can be alleviated by the use of an efficient butanol recovery system.