生質燃料新幫手 基因轉殖微生物快速轉化纖維素

In the quest for inexpensive biofuels, U.S. scientists have created a genetically engineered microbe and used it to convert plant matter directly into isobutanol.

Isobutanol can be burned in regular car engines with a heat value higher than ethanol and similar to gasoline.

Using consolidated bioprocessing, a team led by James Liao of the University of California at Los Angeles for the first time produced isobutanol directly from cellulose.

The team's work, published online in "Applied and Environmental Microbiology," represents savings in processing costs and time.

"Unlike ethanol, isobutanol can be blended at any ratio with gasoline and should eliminate the need for dedicated infrastructure in tanks or vehicles," said Liao, chancellor's professor and vice chair of chemical and biomolecular engineering at the UCLA Henry Samueli School of Engineering and Applied Science.

"Plus," he said, "it may be possible to use isobutanol directly in current engines without modification."

Compared to ethanol, higher alcohols such as isobutanol are better candidates for gasoline replacement because they have an energy density, octane value and Reid vapor pressure - a measurement of volatility - that is much closer to gasoline, Liao said.

While cellulosic biomass like corn stover and switchgrass is abundant and cheap, it is much more difficult to utilize for biofuel than corn and sugar cane. This is due to a plant's natural defenses to being chemically dismantled, Liao explained.

To make the conversion possible, Liao and researcher Wendy Higashide of UCLA and Yongchao Li and Yunfeng Yang of Oak Ridge National Laboratory genetically engineered a strain of Clostridium cellulolyticum, a native cellulose-degrading microbe, that could synthesize isobutanol directly from cellulose.

"In nature, no microorganisms have been identified that possess all of the characteristics necessary for the ideal consolidated bioprocessing strain, so we knew we had to genetically engineer a strain for this purpose," Liao said.

While there were many possible microbial candidates, the research team ultimately chose Clostridium cellulolyticum, which was originally isolated from decayed grass to improve ethanol production.

The team worked with a sequenced genome of Clostridium cellulolyticum available through the Energy Department's Joint Genome Institute.

Liao said this proof of concept research sets the stage for studies that will likely involve genetic manipulation of other consolidated bioprocessing microorganisms.

Isobutanol, sometimes called biobutanol, is being produced by Butamax Advanced Biofuels, a joint venture between BP and DuPont formed in 2009. It has not been produced by the action of transgenic microbes but by a production process similar to ethanol. The first commercial plant is expected to be operational by 2013.

"Global climate change has heightened the need to reduce carbon dioxide emissions. Jim's work will provide cleaner energy sources and change the world for the better," said Vijay Dhir, dean of UCLA Engineering. "We are proud of his significant accomplishments."