Bug Creates Butanol Directly from Cellulose

Butanol—a promising next-generation biofuel—packs more energy than ethanol and can be shipped via oil pipelines. But, like ethanol, biobutanol production is focused on using edible feedstocks such as beets, corn starch, and sugarcane.

 Super bug: Genetic engineers reprogrammed this microbe, Clostridium cellulolyticum, to turn cellulose into butanol.

Now James Liao, a biomolecular engineer at the University of California, Los Angeles, has developed two routes to liberate butanol from its dependence on food crops. Liao, who has a track record for commercializing innovative biofuels processes, has proven that microbes can produce the advanced biofuel directly from agricultural wastes, as well as from protein feedstocks such as algae. 

Liao's demonstration of direct cellulose-to-butanol conversion could bring down the cost of cellulosic biofuels, which is currently prohibitively high. His protein-based process provides the biofuels field with entirely novel feedstock options. 

While they're renewable, biofuels face attacks from environmental and food activists, and biobutanol is no exception: the first generation of biobutanol plants under development will run on corn-based sugar and starch. "Butanol has some technical benefits, but the real problem is the amount of food that goes into making a gallon of fuel," says Jeremy Martin, a senior scientist at the Union of Concerned Scientists, a Cambridge, Massachusetts-based advocacy group that is part of a broad coalition pushing Congress to end lucrative tax credits for corn ethanol. 

Liao's innovations could end biobutanol's association with corn—an association that, ironically, is partly of his making. In 2008, Liao developed a microbial pathway for converting sugar into isobutanol, a high-octane isomer of butanol. That innovation is now being commercialized by Gevo, an Englewood, Colorado-based startup that Liao cofounded. Gevo raised $107 million in an IPO last month to support its plans to retrofit corn ethanol plants to produce isobutanol instead.

Plans for a shift to biofuels production from biomass feedstocks such as switchgrass, corn stalks, and sugarcane bagasse (or plant residue) are, meanwhile, moving slowly because of higher costs. The U.S. Environmental Protection Agency mandated use of just 6.6 million gallons of cellulosic ethanol this year—less than 3 percent of the 250-million-gallon goal set by Congress four years ago. The holdup is from added processing steps required to break down these cellulosic feedstocks and thus generate sugars for fermentation; the processing boosts costs considerably, making production facilities difficult to finance.

Liao's direct cellulose-to-butanol process, developed in collaboration with researchers at Oak Ridge National Laboratory, promises to simplify things by expanding the capabilities of fermentation microbes. The key was adding Liao's sugar-to-isobutanol pathway to a microbe, Clostridium cellulolyticum, that likes chewing on biomass but does not normally make butanol. The microbe was originally isolated from composted grass, and two years ago, the U.S. Department of Energy's Joint Genome Institute completed a sequence of its genome.
The result of the genetic engineering, published this month in the journal Applied and Environmental Microbiology, is a single organism that takes in cellulose and cranks out isobutanol. Liao says the output and conversion rate are low, but says this "proof of principle" is likely the trickiest part of the development process. "The rest is relatively straightforward. Not trivial, but straightforward. It becomes a matter of funding and resources," says Liao. 

The next step is to move the genetic modifications to a faster-growing variant of Clostridium or some other microbe. Liao bets the technology could be production-ready in as little as two years.

One speed bump that could slow things down is litigation over rights to use Liao's technology. Gevo is being sued for patent infringement by competitor Butamax Advanced Biofuels, a joint venture between BP and DuPont that, like Gevo, plans to convert corn-based ethanol plants to isobutanol. Butamax alleges that Gevo's use of genetic engineering to make butanol violates a broad U.S. patent issued to Butamax in December 2010. 

Another obstacle is concern about the environmental impact of heavy biomass use. In January, the EPA issued a draft report to Congress on the environmental impacts from biofuels production. The report outlined several concerns with production of biomass-based fuels. It noted that using corn stover (the leaves and stalks left after harvest) to produce fuels, instead of plowing the stover back into farmlands, could result in soil degradation and choke streams and rivers with increased runoff. Environmental activists have raised concerns about the cultivation of marginal lands that have been set aside to boost biodiversity and provide protective barriers around water bodies.

Liao's demonstration of genetically engineered E. coli that can turn protein into isobutanol also provides a potential alternative to biomass feedstocks: fast-growing photosynthetic algae. Current R&D projects developing algae-based biofuels seek to convert algal-produced fats, which make up about a quarter of algal mass. Proteins, in contrast, make up roughly two-thirds. 

It would be possible, says Liao, to create a recycling production system in which isobutanol-producing microbes are sustained by algal protein as well as industrial fermentation residues recovered from prior rounds of butanol production. Like algae, fermentation residues are composed largely of proteins.                                                    
"These results show the feasibility of using proteins for biorefineries," Liao and UCLA colleagues wrote this month in the journal Nature Biotechnology.

Liao says protein-fed biorefineries cranking out isobutanol are probably five to 10 years from realization, so cellulosic isobutanol is likely to come first. He acknowledges that algae-based protein feedstocks may, like cellulosic biomass, turn out to have unforeseen costs. But one thing is certain, says Liao: "They're certainly much more sustainable than petroleum or coal or sugar." 

By Peter Fairley
From Technology Review


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