BioEnergy Science Center researchers have devised a new bioprocessing method combined with a transgenic microbe to save two steps on the way to producing biofuels.
Using consolidated bioprocessing, researchers at the
Department of Energy's BioEnergy Science Center for the first time produced
isobutanol directly from cellulose. The team, led by James Liao of the University
of California at Los Angeles, notes that the method represents an across-the-board
savings in processing costs and time, and isobutanol is a higher grade of
alcohol than ethanol.
"Unlike ethanol, isobutanol can be blended at any ratio
with gasoline and should eliminate the need for dedicated infrastructure in
tanks or vehicles," says Liao, chancellor's professor and vice chair of
Chemical and Biomolecular Engineering at the UCLA Henry Samueli School of
Engineering and Applied Science. "Plus, 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 says.
While cellulosic biomass like corn stover and switchgrass is
abundant and cheap, it is much more difficult to utilize than corn and sugar
cane. This is due in large part because of recalcitrance, or a plant's natural
defenses to being chemically dismantled. Adding to the complexity is the fact
biofuel production that involves several steps - pretreatment, enzyme treatment
and fermentation - is more costly than a method that combines biomass
utilization and the fermentation of sugars to biofuel into a single process.
To make the conversion possible, Liao and postdoctoral
researcher Wendy Higashide of UCLA and Yongchao Li and Yunfeng Yang of Oak
Ridge National Laboratory had to develop a strain of Clostridium
cellulolyticum, a native cellulose-degrading microbe, that could synthesize
isobutanol directly from cellulose. "This work is based on our earlier
work at UCLA in building a synthetic pathway for isobutanol production,"
While some Clostridium species produce butanol, these
organisms typically do not digest cellulose directly. Other Clostridium species
digest cellulose but do not produce butanol. None produce isobutanol, an isomer
"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," Li says.
While there were many possible microbial candidates, the
research team ultimately chose Clostridium cellulolyticum, which was originally
isolated from decayed grass. The researchers noted that their strategy exploits
the host's natural cellulolytic activity and the amino acid biosynthetic
pathway and diverts its intermediates to produce higher alcohol than ethanol.
The researchers also noted that Clostridium cellulolyticum
has been genetically engineered to improve ethanol production, and this has led
to additional more detailed research. Clostridium cellulolyticum has a
sequenced genome available via DOE's Joint Genome Institute. This proof of
concept research sets the stage for studies that will likely involve genetic
manipulation of other consolidated bioprocessing microorganisms.
The team will publish its work online inApplied
and Environmental Microbiology, in an article titled "Metabolic
Engineering of Clostridium Cellulolyticum for Isobutanol Production from
Isobutanol Produced Directly from Cellulose
June 29, 2011