It is an obvious idea—in fact, it’s how nature disposes of trees after they die. Yet before researchers at the University of Michigan tried it, no one had paired bacterium with fungus to make cellulosic biofuel.
The team took Trichoderma reesei, a fungi widely known for its ability to efficiently decompose the non-edible parts of plants, plus a specially engineered strain of the bacteria Escherichia coli, and applied them both to a vat of dried cornhusks. After the fungi degraded the husks into sugars, the bacteria finished the job. The result was isobutanol, a colorless, flammable liquid that researchers hope could one day replace gasoline.
The idea behind all biofuels is that they suck the greenhouse gas carbon dioxide out of the environment before releasing it again when burned. The promise of cellulosic biofuels—those made from the inedible, structural parts of plants—is that food isn’t a necessary ingredient in the production process. Yet cellulosic biofuels have long been a technical challenge. Coaxing bacteria into breaking down plant matter into the stuff that powers cars is a complex, multi-step process that often requires multiple organisms and bioreactors. As a result cellulosic fuels have been prohibitively expensive. In the 1990s a new technique emerged that allowed scientists to streamline the operation by making powerful microbes, or “superbugs,” that could perform the necessary processes all on their own. Even so the method, known as consolidated bioprocessing, or CBP, is still too costly to achieve commercially viable product yields.
Rather than spend more time attempting to make the perfect superbug, chemical engineer Jeremy Minty decided to look to nature for an example. He divided the required tasks of fuel production between two specialist organisms, allowing him to do all the work in a single bioreactor.
When Minty first combined T. reesei and E. coli in the lab, he wasn’t sure what to expect. But he soon realized this fungus and bacterium were made for each other: T. reesei’s surface is covered with enzymes that help dissolve the plant matter into sugars, which the E. coli bacteria further simplify. “That was really important to make this system stable,” Minty says. “It gives T. reesei privileged access to the hydrolysis process.”
Often when scientists arbitrarily combine organisms in the laboratory, one will outgrow another, driving it to extinction. Yet T. reesei and E. coli exhibited the one characteristic necessary for any stable system: synergy. “We allowed the natural dynamics to emerge,” Minty says.
This interaction, which Minty and his team call a cooperator-cheater mechanism, allow the bacteria and fungi to maintain a state of balance. When the fungi degrade materials in the cornhusks into sugars, some of that action takes place on its surface. T. reesei thus gets the first crack at using them, preventing E. coli, which is far more efficient at snatching them up, from stealing all of the sugars and potentially starving out T. reesei.
Efforts to make isobutanol from bacteria alone have been underway since 2000 when the U.S. Department of Agriculture and the U.S. Department of Energy (DOE) began distributing grants to universities that could demonstrate successful production of liquid biofuels. The DOE designated isobutanol as a “drop-in” replacement for gasoline in 2011.
Joshua Gallaway, a chemical engineer of Columbia University who was not affiliated with the study, says this type of work is critical because of biofuel’s potential to reduce America’s carbon footprint and its dependency on non-renewable fossil fuels.
When fossil fuels such as gasoline are burned, the chains of carbon that make them up are broken and carbon dioxide is released into the environment. Conversely when bacteria make fuel, they suck up carbon from the atmosphere. “The difference between gasoline and isobutanol,” Gallaway says, “is that you’re burning something that you just took out of the atmosphere and putting it back. It’s a closed loop.” In addition the plants that are eventually used to produce biofuel pull carbon from the atmosphere as they grow, contributing to greener overall production process.
In their experiments the University of Michigan team, led by chemical engineer Xiaoxia Lin, achieved yields of up to 62 percent, the highest reached so far using CBP. For their team to industrialize the process they would need to achieve a much higher yield. While they haven’t yet conducted a detailed analysis, Minty says they would like to achieve roughly 80 to 90 percent. In addition, he says, they would need to improve the concentration of isobutanol they end up with after fermentation and speed up the production process. “It’s very promising,” Minty says, “but still needs further development to be viable.”