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.”
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