Optimizing biofuel production in yeast

By identifying and characterizing major pathways involved in lipid accumulation in a promising yeast species, a recent study advances efforts to engineer these organisms to maximize the yield of carbon-based products and successfully transition to a biofuel-based energy system.

The Science

With increasing emphasis on sustainable energy sources, lipid-derived biofuels have been proposed as a promising substitute for fossil fuels. In particular, the yeast species Yarrowia lipolytica has strong potential as a biofuel-producing organism because it accumulates large amounts of lipids, but little is known about the key biological processes involved. To address this gap in knowledge, a recent study identified and characterized major pathways involved in lipid accumulation from glucose in Ylipolytica.

The Impact

This study sets the stage for efforts to engineer yeast to optimize lipid accumulation and maximize the yield of carbon-based products. Because lipids from Ylipolytica have chemical properties similar to those of diesel fuel, they can be readily used as biodiesel using current vehicles and existing infrastructure at gas stations. For this reason, harnessing lipids from Ylipolytica could represent a practical approach for transitioning more quickly to a biofuel-based energy system.

Summary

Researchers from EMSL, the Environmental Molecular Sciences Laboratory, and Pacific Northwest National Laboratory (PNNL) used a comprehensive approach to characterize metabolic processes associated with lipid accumulation in Y. lipolytica. They profiled lipids using analytical chemistry techniques and monitored changes in cell shape using capabilities in the Quiet Wing at EMSL. EMSL is a Department of Energy national scientific user facility.

They found lipids accumulated rapidly and peaked at 48 hours during the five-day experiment, concurrent with altered amino acid metabolism. Accumulation of lipids to their maximum level occurred when cells began to synthesize amino acids in the presence of excess glucose—the major carbon source in the culture medium. However, the highest proportion of a biofuel-friendly lipid occurred at 24 hours, suggesting the biofuel quality of the lipids was highest prior to peak lipid accumulation.

By 72 hours, the cells with depleted glucose levels began to make thicker cell walls, possibly to protect themselves until they could find more favorable environmental conditions. From the perspective of a bioengineer interested in converting glucose input into lipid output, the development of a thicker cell wall under starvation conditions represents wasted carbon that would otherwise be used for lipid production. Therefore, genes involved in cell wall synthesis could be promising targets to improve the efficiency of lipid producing yeast strains.

Taken together, the findings provide important insights into optimal timing and nutrient conditions for harnessing lipids from yeast. Moreover, the new comprehensive dataset describing lipid accumulation in Ylipolytica will enable more detailed experiments to provide specific genetic targets for future metabolic engineering efforts.