A new microbe engineering trick could potentially make butanol, a promising biofuel, so cheaply that it could compete with ethanol. By tapping into a highly efficient metabolic pathway, scientists at Rice University engineered E. coli to convert sugars to butanol 10 times more efficiently than any other organism.
Butanol, which is typically made by fermenting sugar, packs more energy per gallon than ethanol and can be shipped via existing oil pipelines.
Several companies are now trying to commercialize biobutanol, including some seeking to retrofit existing ethanol facilities. However, the bugs normally used to produce butanol don’t tolerate it well and so produce only small quantities. “You can get over 10 percent ethanol in the fermentation process from corn,” says Jonathan Mielenz, a biofuels researcher at Oak Ridge National Laboratory. “Butanol is nothing near that level. It’s typically not much more than 1 or 2 percent.”
The new E. coli work faster than other fuel-producing microbes. They also produce five to 10 times more fuel from the same amount of sugar. That means they require less sugar feedstock and can be grown in smaller vessels, cutting capital and operational costs. Ramon Gonzalez, a chemical and biomolecular engineering professor who led the work, says that several companies have shown interest in the technology, and he expects to see it on the market within the next three years.
Cobalt Biofuels, a biobutanol startup based in Mountainview, California, uses Clostridium bacteria to break down plant matter and convert the resulting sugars into a mix of butanol, acetone, and ethanol. Gevo, a company based in Englewood, Colorado is working with E. coli that are altered to divert some of their metabolites, which would otherwise be involved in synthesizing amino acids, toward alcohol production. And Butamax, a joint venture between Dupont and BP, is using genetically modified yeast.
Gonzalez and his colleagues outlined their new approach in a paper published online in the journal Nature. The researchers tapped into a pathway that microbes use to break down fatty acids, which are hydrocarbon molecules, to generate energy. They modified about a dozen genes in E. coli to reverse this beta-oxidation pathway so that the microbes build fatty acids.
The method is more efficient than others because it adds two carbon atoms at a time, rather than one, to the hydrocarbon molecules being formed. “What makes it really efficient is that the mechanism by which those two carbon atoms are added to the chain doesn’t require [energy],” Gonzalez says.
By selectively manipulating genes, the researchers can program the microbes to synthesize many different fuels and chemicals. In addition to butanol, the bacteria can produce various useful fatty acids that existing processes derive from plant and animal oils.
Because the beta-oxidation pathway is found in nearly all organisms, it could also be engineered in yeast and algae, which might make it easier for many different companies to adopt the technology, Gonzalez says. He is looking at modifying various organisms with the goal of making the process even cheaper and more efficient. Yeast, for instance, are more tolerant to ethanol and butanol, he says—but E. coli grow faster.
However, in the end, cautions Mielenz, all biobutanol processes face the crucial challenge of requiring a switch from food sources such as corn, sugarcane or beets for their sugar feedstock to cellulosic biomass, which is more expensive to convert to fuels.