The key to producing environmentally friendly biofuels from agricultural waste may reside in the gut of Indian elephants, say researchers in the Netherlands.
Ethanol, a popular type of biofuel, is currently produced by fermenting sugars in plants, like corn and sugarcane, with yeast. Previously, the fuel could only be produced from a plant’s most edible material, which is rich in starch and certain types of sugars called hexoses.
However, Dutch researchers genetically engineered a special strain of yeast that can also convert leftover plant material into ethanol, starting with wastes like straw, woodchips, and cornhusks. The development opens a previously untapped source of plant-based energy production that doesn’t compete with consumable crops.
“You can have food production as well as feedstock for your bioethanol,” said Marko Kuyper, the leader of the project and a researcher at the Delft University of Technology. “[And] you can do both in the same area of land.”
Agriculture waste products typically contain a class of sugars called pentoses, which normal yeast cannot convert into ethanol because it lacks an enzyme necessary to complete the fermentation process.
Kuyper’s team made total fermentation possible by inserting a foreign gene into the yeast’s genome. The gene was isolated from a fungus that researchers from the University of Nijmegen, also in the Netherlands, found in the feces of Indian elephants in 1984. This gene helps animals to digest plant material by producing an enzyme that converts a specific pentose sugar, xylose, into another form called xylulose. While yeast cannot ferment xylose, it can process xylulose.
“This is the first yeast that can do that at an industrially interesting rate and efficiency,” Kuyper said.
This new method of ethanol production has taken a few years to perfect. Before employing the fungal gene, which codes for the enzyme xylose isomerase, Kuyper’s group attempted to use a bacterial version of the gene. But they were disappointed by its effectiveness. Even in their yeast-based solution, low growth rates posed a problem. The xylose isomerase gene comes from an anaerobic organism—one that grows in an oxygen-depleted environment—which caused the modified yeast to grow slowly when exposed to oxygen. To improve the growth rate, researchers used evolutionary engineering: They cultivated successive generations of the yeast strains in an oxygenated environment until it adapted to living in an aerobic environment.
“The end product of our efforts was a yeast strain that converted both hexose and pentose sugars at a high rate and with high efficiency,” said Harry Harhangi, another member of the team, in an e-mail correspondence. Harhangi added that since 5 to 25% of a plant’s biomass is composed of pentose sugars, the modified yeast strain could improve the yield of bioethanol considerably.
Still, according to Kuyper, before it can be used to produce ethanol industrially, the genetically modified yeast will need a few more improvements. Currently, the yeast doesn’t tolerate the acids and other compounds that appear when sugars are freed from plant material, so Kuyper’s team hopes to modify the yeast to make it resistant to these substances.
“There’s still a few glitches, but a few years back there was no organism available that could do these hexose and pentose fermentations together,” Kuyper said. “And now we do have one, so we’re a lot closer to application than we were five years ago.”
Ton van Maris, an industrial microbiologist at Delft who was not directly involved with the research, called Kuyper’s new process the “dominant technique” for breaking down the sugars that yeast could not previously ferment.
“I think it definitely will be applied within five years, probably sooner,” van Maris said.
Originally published June 25, 2006
