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. Key technical terms Key term Definition Metabolic engineering A method of optimising the regulatory processes within cells, used to produce high amounts of desirable compounds Oleaginous yeast Oil containing yeast, typically the triglyceride oil should be above 20% of the dry weight Advanced biofuels Fuels which are compatible with current fossil fuels, and tend to give higher performance than either bioethanol or biodiesel Pentoses / hexoses C5 and C6 sugars respectively Isoprenoids Diverse range of compounds, derived from isoprene (2-methyl-1,3butadiene) units Future perspective Within 10 years a number of key advances could potentially make advanced fuels derived from yeast a central component of the global energy mix. It seems probable that legislation will be enacted to increase the biofuel content in current transportation fuels, while at the same time ensuring more evidence-based green credentials for biofuels on the market. Strict fuel properties legislation is slowly being relaxed to allow alternatives to ethanol and biodiesel to enter the fuel market, this is likely to continue and within a decade it is probable that a larger range of fuels will certified for general use. From an engineering perspective, advances in the processing, enzyme production and the development of novel strains of yeast will continue to reduce the total costs of converting lignocellulose to fuel molecules. Finally, a range of genetic toolkits are being developed for nonsaccharomyces yeasts, expanding the range of fuels and increasing the sugar to product conversion ratio. It seems likely therefore that alternative, more robust yeast strains to Saccharomyces will become prevalent for industrial biofuel production.

Toward a microbial palm oil substitute: oleaginous yeasts cultured on lignocellulose

“…Since 1990, motivations from sustainability, energy price, and consequent government pressure have led to a re‐assessment of Ratledge's view. Now the most commonly cited potential use is for biodiesel . In addition, a waste product from the biodiesel manufacturing process, glycerol, can be transformed into triglycerides by oleaginous yeasts, leading to increased biodiesel yields and lower waste disposal costs.…”

“…One potential solution is to chemically upgrade other edible oils. However, despite being used heavily for biodiesel production in Europe, oil‐producing crops such as soy and rape compete with agricultural land, impacting food prices and are widely accepted to be unsustainable . Therefore, the chemical upgrading of these sources to produce a more viscous oil is not an effective solution.…”

Co-production of bio-oil and propylene through the hydrothermal liquefaction of polyhydroxybutyrate producing cyanobacteria

“…Consequently the carbon and hydrogen retention for this feedstock, 37.5 % and 34.0 % respectively, were much higher than the nitrogen retention of only 12.5 %, resulting in an increase in the HHV from 20.8 to 35.4 MJ kg --1 (all values for 360 °C). Despite this, the residual nitrogen content remains above the acceptable threshold for fuel use, and therefore further treatment of the oil would be required (Chuck et al, 2015). Increasing the reaction temperature from 300 to 360 °C resulted in a large increase in the HHVs obtained for corn flour (23.3 to 29.5 MJ kg --1 ) and PHB (15.2 to 27.2 MJ kg --1 ), likely as a result of an increased degree of deoxygenation, and therefore increased carbon and hydrogen concentration.…”

“…Despite this, a number of challenges remain: most notably, the upgrading of the resulting crude oils (which cannot be treated in conventional refineries due to their high nitrogen contents) and the utilisation of additional product streams to help subsidise fuel costs Chuck et al, 2015).…”

Fast microwave-assisted acidolysis: a new biorefinery approach for the zero-waste utilisation of lignocellulosic biomass to produce high quality lignin and fermentable saccharides

“…Conventional yeast based processes, such as the production of bioethanol using S. cerevisiae, require inhibitor concentrations below 10mM and high hexose sugar concentrations. 39 However, several oleaginous yeasts are known to be able to survive in highly inhibitory conditions and can even metabolise some of these types of carbon sources. For example, Cryptococcus curvatus has been demonstrated to have a high threshold for furfural inhibitors, 40 while being able to metabolise acetate and some oligosaccharides.…”