Engineered microbe singlehandedly transforms switchgrass to ethanol
Forest and Kim Starr/Flickr (CC by 2.0)
A lone bacterium, genetically tweaked, can demolish switchgrass and ferment the sugary rubble to ethanol in one fell swoop. The microbe’s one-step conversion of the crop eliminates the need for expensive plant-digesting treatments, offering the potential for cheaper biofuels.
Plucked from hot springs, the bacterium Caldicellulosiruptor bescii grows around 80° Celsius, and naturally wrecks tough, complex plant molecules such as cellulose. Breaking down such roughage into fermentable sugars is one of the trickiest feats for converting plants to ethanol fuel, says geneticist Janet Westpheling of the University of Georgia in Athens. Standard methods require extra steps or costly combinations of enzymes.
For years, researchers have sought a single-step solution, often engineering common microbes that naturally produce ethanol from sugar to become plant destroyers. Westpheling and colleagues tried the reverse: transforming an unusual plant-annihilating microbe into an ethanol factory.
Plants, she says, have evolved for millions of years to thwart microbial attacks. But some rare bacteria, such as C. bescii, can tear apart vegetation with enzymatic weaponry. “It already knows how to do the hard part,” she says of the bacterium. “It was relatively easy to teach it to make ethanol.”
The researchers first deleted the bacteria’s gene for making lactate, which is what the microbe would normally produce from the sweet spoils of plant wreckage. Next, the team armed C. bescii with genes for fermentation of sugars to make ethanol, swiped from another microbe. When the researchers gave their custom-made microbe raw switchgrass, a starter plant for ethanol production (SN: 8/1/09, p. 24), the bacteria converted around 60 percent of the sugars into ethanol — not enough for commercial production, but a good start, Westpheling says. The findings appear June 2 in the Proceedings of the National Academy of Sciences.
The simple method is “the ideal situation that everyone dreams of,” says Alex Berlin, director of biotechnology research and development at Novozymes in Davis, Calif. “Everything in one pot, in one step, no complications,” he says.
But the genetically enhanced microbe is far from ready for an industrial debut, Berlin says. Researchers need to make the bacteria convert more of the plant material into ethanol, and the process needs to be scaled up to industrial levels, he says, neither of which are trivial tasks.
D. Chung et al. Direct conversion of plant biomass to ethanol by engineered Caldicellulosiruptor bescii. Proceedings of the National Academy of Sciences. Published online June 2, 2014. doi: 10.1073/pnas.1402210111.