by Michael Keller

The possibility of using nonfood plants to cheaply and sustainably fuel our vehicles may have just veered into the fast lane.

Scientists report they have successfully genetically engineered bacteria to convert complex carbohydrates in tough grasses directly into ethanol, a type of alcohol that can fuel internal combustion engines.

“Making biofuel from plants is really important because it’s carbon neutral—the same CO2 you put in to grow it comes out when you burn it,” says Janet Westpheling, a University of Georgia genetics professor who led the research. “It’s one of the reasons why the future of energy in this country has to rely at least in part on plants.”

At the heart of the work conducted at UGA and Oak Ridge National Lab, is what Westpheling calls a paradigm shift in approaching a longstanding problem in producing biofuels.

Biofuel’s obstacle

The easiest way to make fuel from plants is to use food crops like corn, which produce lots of simple sugars that microorganisms can readily digest. When some species of mold, yeast and bacteria encounter sugars from these plants in an environment without oxygen, they break the molecules down into ethanol and carbon dioxide during a process called alcohol fermentation. But using these crops for fuel leads to biofuel competition with the food supply and rising prices.

To get around this problem, many researchers around the world are working to instead use grasses and other plants–what’s called lignocellulosic biomass. Abundant plants like the perennial switchgrass readily and quickly grow in many locations on unfertilized, marginal lands. But making fuel from these nonfood plants has proven to be challenging. It turns out that getting microbes to make ethanol from simple sugars is the easy part of biofuel production. The hard part is first breaking down the tough carbohydrates into sugars.

To deconstruct fibrous biomass so that alcohol-making microbes can go to town on them, biofuel makers have had to pretreat the materials with enzymes and chemicals. This leads to higher prices and unwanted waste.

The possibility of refining the process to cut out pretreatment and other processing steps is tantalizing to many as part of the answer to increasing energy demand. “Something like switchgrass isn’t just sustainable,” Westpheling says. “You can grow tons per acre, and that’s what we’ll need.”

A different approach

Taking another tack, Westpheling’s team went looking for a microbe that naturally consumes tough biomass instead of one that makes fuel. They focused on Caldicellulosiruptor bescii, a heat-loving organism found growing in hot springs. The organism doesn’t normally produce ethanol but does survive by eating cellulose, a complex carbohydrate that provides structure within the cells of green plants. C. bescii thrives in the absence of oxygen—a condition necessary for alcohol fermentation—eand when the temperature hovers around 176 degrees Fahrenheit.

“This is a paradigm shift in that we took this organism that already did the difficult thing of breaking down these carbohydrates,” Westpheling tells Txchnologist, “and taught it how to do the easier thing–turning that sugar into ethanol.”

She says the alteration process involved deleting C. bescii’s genetic code to make one type of enzyme that breaks down sugar into acetic acid and replacing it with another from a different bacterium that breaks down sugar into ethanol and CO2. After making that single switch, the team put the engineered organism into a mixture containing only ground switchgrass and growth medium. They then watched as the microbe started directly deconstructing the plant material into ethanol.

Their work is reported on June 2 in the journal Proceedings of National Academy of Sciences. In it, they describe how the product of wild, unaltered C. bescii’s consumption of biomass contains no ethanol. After inserting instructions for the new enzyme into the engineered C. bescii, 70 percent of the product from fermentation was ethanol.

“Direct conversion of biomass to ethanol represents a new paradigm for consolidated bioprocessing, offering the potential for carbon neutral, cost-effective, sustainable fuel production,” the authors write in their paper. “To our knowledge, this work is the first demonstration of metabolic engineering in an extreme thermophilic organism for the conversion of lignocellulosic biomass to a liquid fuel. Furthermore, ethanol has been produced directly from plant biomass without the use of harsh, expensive, chemical pretreatment.”

Westpheling says her team’s work is continuing along two paths. The first involves continuing to push C. bescii to produce more ethanol as a percentage of its fermentation products. This will involve more basic genetic research, which the team is optimistic about because C. bescii seems to tolerate living in an environment containing higher levels of ethanol. At the same time, they are moving out of the lab and into a pilot plant to see if the process scales up as expected.

They also think the engineered microbe can be taught to produce other chemicals and transportation fuel alcohols like butanol and isobutanol.

“By engineering these genetic pathways, we’re taming these organisms and making them do something useful,” Westpheling says. “This is one of the most exciting frontiers in science because our lives depend on finding better sources of energy. This isn’t just about climate change; it’s about health, the environment and the economic environment.”

Top Image: Ornamental switchgrass courtesy of Cornell University.