Better Bug to Make Cellulosic Ethanol
A new strain of bacteria could make cellulosic ethanol cheaper.
New genetically modified bacteria could slash the costs of producing ethanol from cellulosic biomass, such as corn cobs and leaves, switchgrass, and paper pulp. The microbes produce ethanol at higher temperatures than are possible using yeast, which is currently employed to ferment sugar into the biofuel. The higher temperature more than halves the quantity of the costly enzymes needed to split cellulose into the sugars that the microbes can ferment. What’s more, while yeast can only ferment glucose, “this microorganism is good at using all the different sugars in biomass and can use them simultaneously and rapidly,” says Lee Lynd, an engineering professor at Dartmouth College, who led the microbe’s development.
Most of the ethanol produced in the United States is made from corn. But making the biofuel from corn takes a lot of energy and competes with agricultural uses of the crop. Making fuel from cellulosic plant matter has the potential to be much more sustainable. However, cellulosic-ethanol production is still too expensive to be commercially competitive with corn ethanol.
Turning cellulose into ethanol involves two steps: using enzymes to break complex cellulose into simple sugars such as glucose, and then using yeast to ferment the sugar into ethanol. Both steps add to the price of ethanol. Enzymes can add about 50 cents to a gallon of ethanol. And the second step is relatively expensive because conventional yeast ferments only glucose, although biomass contains five different sugars, linked to form cellulose and hemicellulose in plant cell walls. (Cellulose is a long chain of glucose molecules, while hemicellulose contains all five sugars.) “You really need to be able to convert [all] these sugars into ethanol in order to make it economical, to get a good enough yield,” says Bruce Dien, a biochemical engineer doing ethanol research at the USDA’s Agricultural Research Service.
Lynd wants to create microbes that would do it all: efficiently break down the cellulose and hemicellulose, and then ferment all the resulting sugars. Lynd, a cofounder of Mascoma, is working with colleagues at the startup, based in Cambridge, MA, to develop a simple one-step process for making cellulosic ethanol. In the combined process, a mixture of biomass and the microbes would go into a tank, and ethanol would come out.
The new microbe, presented in this week’s PNAS, is a crucial step toward such a combined process. The bacteria can break down hemicellulose into its five constituent sugars, which they ferment efficiently. To increase the bacteria’s ethanol yield, Lynd and his colleagues knocked out the gene that results in organic acid formation.
However, the genetically engineered bacteria cannot break down cellulose. In their laboratory experiments, Lynd and his coworkers needed to add enzymes to free the glucose from crystals of cellulose. Still, the bacteria offer an advantage because they are thermophilic–that is, they naturally grow at temperatures of 50 to 60 ºC. This is much higher than the 37 degrees at which yeast ferments sugars, and thus the bacteria require less fewer enzymes. “Because enzymes are more active at higher temperatures, using these bacteria would mean you have to add less enzyme,” Lynd says.
In the experiments, the bacteria fermented sugar mixtures at 50 ºC to give 4 percent ethanol concentration. “It’s the highest concentration of ethanol that’s been produced by thermophilic bacteria,” Lynd says.
Conventional yeast can give higher ethanol concentrations of 10 to 12 percent, says Harvey Blanch, a chemical-engineering professor at the University of California, Berkeley. Nevertheless, he says that the new work is a “nice proof of concept” for a combined approach to make cellulosic ethanol. While the researchers use cellulose crystals in their lab experiment, the challenge will be to see if the microbes can produce similar results with cellulosic biomass such as wood chips and switchgrass, says Blanch. “If this can be successfully accomplished, it will be a significant advance,” he says.
Lynd’s team is also trying to increase ethanol yield in a thermophilic bacteria that breaks down cellulose. The group wants to team it with the bacteria that are good at breaking down hemicellulose and using all sugars. That would give an all-in-one microbe system that breaks down biomass and converts all of its sugars into ethanol.
“Using one microbe or community of microbes for essentially the whole conversion process would be a major cost breakthrough,” says Anna Palmisano, associate director of science at the DOE’s Office of Biological and Environmental Research. “It’s one of the ways really fundamental biology could transform the equation and help pave the way to commercially viable cellulosic biofuels.”