Today, almost all ethanol—at least in the United States—comes from converting corn kernels into fuel. But because farming corn requires lots of energy and fertilizer, corn ethanol doesn't actually do much to reduce petroleum use or greenhouse gas emissions. Several companies are working to convert agricultural waste—known as cellulosic biomass—into ethanol. But they've had a hard time making it as cheaply as corn ethanol, because it's costly to break down biomass into sugars that microbes can ferment. Now, researchers in the United States have engineered a microbe that both breaks down cellulose into sugar and ferments it to produce ethanol.
The newly engineered microbes aren't yet as proficient at making ethanol as yeast, which converts corn kernels to fuel. But if the researchers can boost the bugs' ethanol output by just another 20%, it could give cellulosic ethanol producers a new way to drop their costs and displace ethanol made from corn.
Corn ethanol is already big business. In the United States alone, ethanol producers make nearly 53 billion liters of ethanol from corn annually. That's partly because it's relatively easy to do. Corn kernels are made from starch, a simple chainlike polymer of glucose. Ethanolmakers need only add enzymes eager to chop the starch into individual glucose molecules. Yeast then does the rest in a fermentation tank, converting the glucose to ethanol.
Making ethanol from cellulosic biomass is harder. Cellulose makes up the cell walls of plant leaves and stems. It's also made from long chains of glucose molecules linked together into fibers. But those fibers are wrapped in a woody material called lignin which protects the sugars from hungry pests, and would-be cellulosic ethanol producers. So to convert cellulosic biomass to ethanol, engineers must first grind it up and add acids or bases to degrade the lignin. Next, they add an enzyme to cut the cellulose into individual sugar molecules so that yeast can ferment them to ethanol. But these extra steps add cost, which has put cellulosic ethanol at a commercial disadvantage compared with corn ethanol.
One ray of hope came with the discovery in the 1980s of microbes native to hot springs in Yellowstone National Park in the United States and elsewhere that are naturally able to break down the lignin in cellulosic biomass. These organisms don't make ethanol, but researchers hoped they could manipulate their genes to give them that ability. "If you start with organisms that can do the hard part, teaching them to make ethanol is relatively easy," says Janet Westpheling, a geneticist at the University of Georgia (UGA), Athens.
Westpheling and her colleagues decided to use bacteria called Caldicellulosiruptor bescii, or Caldi for short. The trouble was, Caldi is such an exotic microbe that the conventional genetic tools for cutting and splicing genes, which were developed for industrial organisms such as Escherichia coli and yeast, didn't work. So over the past 2 years, Westpheling and her colleagues developed a new set of gene manipulation tools to insert and delete genes in Caldi. That set the stage for their current project, giving Caldi its new talent.
Westpheling and colleagues from UGA and the BioEnergy Science Center at Oak Ridge National Laboratory added a gene called acetaldehyde/alcohol dehydrogenase that enables the organisms to ferment simple sugars into ethanol. And they deleted another gene that the organism uses to make a product called lactate, a compound that the microbes can live without but that normally takes up much of their metabolic activity. The result, which they report online today in the Proceedings of the National Academy of Sciences, was a batch of Caldi that when applied to unprocessed switchgrass—a form of cellulosic biomass that can grow almost anywhere—was able to break down the plant matter and convert 70% of the sugars to ethanol.
That's still well below the 100% sugar to ethanol conversion of yeast. But Westpheling believes that if they can boost that number to around 90%, making ethanol from switchgrass using Caldi could be done more cheaply than doing so from corn. "The idea of processing cellulosic biomass without pretreatment is an important idea," says Lee Lynd, a cellulosic ethanol expert at Dartmouth College who was not associated with the work. He adds that the strategy of trying to turn exotic microbes into industrial powerhouses rather than tweaking current industrial microbes is "an emerging trend that is exciting." And it’s one that's likely to get better, as researchers hone their tools for manipulating Caldi's metabolism. "This bug isn't long out of the mud," Lynd says. And already it's on the cusp of changing the world.