Scientists look at evolution to solve biofuels ‘bottleneck’

A team of scientists from Arizona State University are working to break through the innovation bottleneck for renewable bioproduction of fuels and chemicals, to create a renewable biofuel technology that would lead to a cheaper alternative to gasoline.

Reed Cartwright and Xuan Wang have focused on the manipulation of microbes as biocatalysts to convert biomass such as agricultural wastes and even carbon dioxide into bio-based products. In particular, they’re interested in “harnessing the trial and error power of evolution into revealing an answer.”

The scientists have grown generations of bacteria under special conditions in fermentation tanks, test tube evolving bacteria to better ferment sugars derived from biomass. Their results have appeared in the Proceedings of the National Academy of Sciences.

In particular, the scientists looked into non-food crop sources of biofuels, lignocellulosic biomasses like tall switchgrasses and the inedible parts of corn and sugarcane. Lignocellulosic biomasses are rich in xylose, a five-carbon, energy-rich sugar relative of glucose. Unfortunately, industrial E coli strains can’t use xylose because when glucose is available, it turns off the use of glucose. Consequently, it has typically been inefficient and expensive to fully harvest and convert xylose to biofuels.

Cartwright and Wang wanted to get more energy from xylose sugars. To do so, they challenged E coli that could thrive comfortably on glucose to grow solely on xylose, forcing the bacteria to adapt to the new food supply.

Faced with the challenge, the bacteria randomly mutated their DNA until it could adapt to the new conditions. The team set out to identify how the most beneficial mutations worked, and identified three single mutations that could enhance xylose fermentation by changing bacterial sugar metabolism.

"This suggests that there are potentially multiple evolutionary solutions for the same problem, and a bacterium's genetic background may predetermine its evolutionary trajectories," said Cartwright, a researcher at ASU's Biodesign Institute and assistant professor in the School of Life Sciences.

Focusing on a mutation in a regulatory protein called XyIR, where just two amino acid switches enhanced xylose utilisation and released the glucose repression, the team managed to achieve impressive increases in fermentation efficiency. When the XyIR mutant strain was placed in a “wild type” strain or an industrial E. coli biocatalyst, it massively improved the yield, by up to 50% over four days of fermentation, according to the team’s study.

"With these new results, I believe we've solved one big, persistent bottleneck in this field," concluded Wang. Arizona Technology Enterprises (AzTE) is now filling a non-provisional patent for their discovery.