Liquid transport fuels from microbial yeasts – current and future perspectives

Abstract

Global transportation is one of the major contributors to GHG emissions. It is essential, therefore, that renewable, carbon neutral fuels are developed to reduce the impact of this sector on the environment. Yeasts, especially Saccharomyces cerevisiae, are key to transforming renewable bioresources to fuels that can be used with little adaption to the current transport infrastructure. Yeasts demonstrate a large diversity that produces a great metabolic plasticity; as such, yeasts are able to produce a range of fuel-like molecules including alcohols, lipids and hydrocarbons. In this article the current and potential fuels produced through fermentation, the latest advances in metabolic engineering and the production of lipids suitable for biodiesel production are all reviewed.

Acknowledgements

The authors would like to extend their thanks to the University of Bath and the EPSRC for partially funding this work through the Doctoral Training Centre at the CSCT and to R. Whorrod and S. Whorrod for their kind endowment to the University, resulting in the Whorrod Fellowship in Sustainable Chemical Technologies held by C. Chuck.

Executive summary

Introduction: Yeasts are capable of converting highly functionalised carbohydrate feedstocks into a range of potential fuel molecules such as alcohols, lipids and hydrocarbons. Over 1300 species of yeasts have been identified although it is Saccharomyces cerevisiae that remains one of the most widely used organisms for biotechnological applications.

Feedstocks for yeast culture: Central to the economic production of fuels from yeasts is a renewable source of feedstock to culture the yeasts. Lignocellulosic feedstocks are difficult to breakdown and offer a number of challenges that have limited the scale up of this technology. Current industrial processes generally use a pretreatment stage, followed by enzymatic hydrolysis, fermentation and separation. Developments over the last decade have reduced the costs of the processing substantially, and a number of demonstration plants are in operation that can produce lignocellulosic ethanol.

Advanced biofuels through metabolic engineering: While bioethanol is currently the most prevalent biofuel produced globally a range of alternative fuels with superior fuel properties are being developed using a number of yeasts. These include, longer chain alcohols and unsaturated hydrocarbon precursors that can be chemically upgraded to suitable fuel molecules. In general, bacteria have faster growth rates than yeasts, a higher metabolic plasticity and a wider range of available genetic tools. However, yeasts, in particular S. cerevisiae, have a higher tolerance to solvents and conditions often encountered on an industrial scale, have wider optimal pH ranges and a natural resistance to bacteriophages. These key factors have promoted a wealth of research into using S. cerevisiae, in addition to other suitable yeasts, to produce advanced biofuels.

Oleaginous yeasts: An alternative to fermentation fuels are lipid-derived fuels such as biodiesel or hydrogenated fatty acids. While algal lipids have been heavily researched, a range of oleaginous yeasts are also capable of producing lipids. Over 20 species of oleaginous yeast have been identified and unlike algae, the lipid profile is simple and predominantly made up of palmitic acid, oleic acid and linoleic acid. The amount of lipid and its profile is highly dependent on the species and the growth conditions, though most yeast lipid will also contain other soluble components, such as sterols that can cause issues in the processing to suitable biofuels.