Bacteria-based Biofuel May One Day Replace Gasoline, Save Money at the Pump

New lines of engineered bacteria can tailor-make key precursors of high-octane biofuels that could replace gasoline, scientists report.

In a study published in the online edition of Proceedings of the National Academy of Sciences, researchers at Harvard University's Wyss Institute for Biologically Inspired Engineering and Department of Systems Biology explain that the same lines can also produce precursors of pharmaceuticals, bioplastics, herbicides and detergents, among other things.

"The big contribution is that we were able to program cells to make specific fuel precursors," said Pamela Silver, a Wyss Institute Core Faculty member, Professor of Systems Biology at Harvard Medical School and senior author of the study.

New biofuels are currently in demand for cars and other vehicles; however, ethanol, the most popular biofuel on the market, packs only two-thirds the energy of gasoline. Furthermore, ethanol-containing fuels corrode pipes, tanks and other infrastructure used to transport and store gasoline. Burning gasoline, on the other hand, adds huge amounts of carbon dioxide to the atmosphere, and relies on the world's ever-decreasing supply of oil

For this reason, Silver and her team are busy devising new ways to make gasoline-like biofuels capable of being stored at gas stations and used to fuel the cars already on the road today.

In order to do this, they used employed E. coli to help make gasoline precursors called fatty acids - energy-packed molecules containing chains of carbon atoms flanked with hydrogen atoms that can be easily converted into fuels.

Specifically, the researchers are focusing on medium-chain fatty acids, meaning those with chains between four and 12 carbons long. This is because fatty acids with shorter chains store little energy and tend to vaporize more easily. Meanwhile, those with chains longer than 12 carbons have consistently proven to be too waxy.

Medium-length fatty acids, however, are just the right length to be transformed into an energy-packed liquid fuel for internal-combustion engines, according to the researchers.

Today, oil refineries produce medium-chain-length compounds from crude oil, which Silver and her colleagues determined could be done with microbes or other organisms rather than petroleum products.

To accomplish this, the scientists tweaked an E. coli metabolic pathway that produces fatty acids, mass producing an eight-carbon fatty acid called octanoate capable of being converted into octane.

In this pathway, carbon from sugar, which the bacterium eats, flows through the pathway like a river, growing longer as it goes and exiting as a long-chain fatty acid.

The scientists then partially dammed the river, so to speak, and built an irrigation ditch using a drug that blocks enzymes capable of extending fatty-acid chains, resulting in a growing number of medium-chain fatty acids pooling behind the dam.

However, while the strategy helped to increase octanoate yields, the drug's high price prevented the method from becoming scalable.

To get around this, the scientists tried a second strategy, allowing the cells to grow up, and then damming the river using a genetic trick. They also found a way to genetically alter a second enzyme that normally builds long-chain fatty acids so that it extended fatty acids to exactly eight carbons.

This two-pronged strategy, in addition to some other genetic modifications designed to keep the river from being diverted in other ways, yielded the highest amount of octanoate yet reported.

"Sustainability is one of the biggest problems we face today, and developing potent biofuels to replace gasoline is a major challenge in the field," said Don Ingber, Wyss Institute Founding Director. "Using ingenious synthetic-biology strategies to engineer microbes so that they can produce octane, Pam's team has taken a giant step toward meeting this challenge."

Going forward, the scientists plan to engineer E. coli to convert octanoate and other fatty acids into alcohols, potential fuel molecules themselves, and just one chemical step away from octane.

The work was funded by the Department of Energy's Advanced Research Project Agency-Energy (ARPA-E) and by the National Science Foundation.