Wood chips like these could be food for new ethanol-producing bacteria. [Credit: Brett Israel]
Tiny microbial factories, operating under sweatshop conditions, may end up producing the next generation of biofuels, a new study suggests.
The key is a new strain of bacteria engineered to turn organic waste into ethanol. The microbes create the fuel much more efficiently than current methods, which rely on corn and other food crops as the starting material. The process may cut the cost of producing ethanol to a quarter of conventional corn-to-ethanol technology, according to the study’s authors, based at Dartmouth College. Ethanol can be used as transportation fuel, but current methods that use food crops to make ethanol have negative side effects that cast doubt on the technology’s sustainability. Bacteria-driven ethanol production has given new hope to ethanol’s future.
“[Ethanol from organic waste] is our only option for the sustainable production of liquid fuels that we so overwhelmingly favor for transportation,” said Charles Wyman, an engineering professor at Dartmouth and co-founder of Mascoma Corporation, an ethanol company where the study’s technology is being further refined. The Department of Energy and the state of Michigan recently awarded Mascoma $49.5 million for a new facility to scale up the technology. Lee Lynd, an author of the study, is also a co-founder of Mascoma.
The newly engineered bacteria, Thermoanaerobacterium saccharolyticum, is a potential energy breakthrough because it produces ethanol at temperatures up to 140 degrees Fahrenheit, or 60 degrees Centigrade—the equivalent of a piping hot cup of tea. Yeast, the current workhorse of corn-to-ethanol production, ferment at lukewarm temperatures and would die under such scalding conditions. But heat speeds up the conversion process, and expensive enzymes are required to jump-start this cooler method. The extra cost has prevented the ethanol biofuel industry from scaling up and is “the only barrier to low cost production,” according to Wyman.
“If you were to look at costs going in you’d say that enzymes would make up the highest costs,” said A. Joe Shaw, a biochemist at Dartmouth and co-author of the study, which was published this fall in the Proceedings of the National Academy of Sciences. Despite the breakthrough, Shaw said researchers must still figure out how to maximize the bacteria’s production of ethanol before it is ready to start producing for the gas pumps.
To tailor the bacteria to make ethanol, Shaw and his Dartmouth colleagues rewired the bacteria’s metabolic circuitry. Under normal conditions, the bacteria produce a wide range of organic wastes. But the researchers turned off specific genes not involved in making ethanol. Once those genes are turned off, the bacteria’s metabolism is almost entirely devoted to ethanol production.
Ethanol burns as well as gasoline in conventional engines, but unlike gasoline and other petroleum products, it is a renewable resource because it’s produced from plants. Most new research into ethanol production is focused on so-called ‘second-generation biofuels’ that rely on non-food sources like wood chips, switch grass and other sources of cellulose, the carbohydrate that gives structure to plants. Unlike the current method of making ethanol from cornstarch, cellulose-derived fuels would not affect the prices or harvests of food crops. Bacteria can turn the cellulose contained in organic waste into ethanol, instead of using food.
The researchers still have a few hurdles remaining. While the bacteria produce the best ratio of ethanol to date—37 grams of ethanol per liter of bacteria grown—they can potentially manufacture even more. At a high yield, ethanol will kill bacteria, but the cells are only making half that amount currently. Also, to avoid contamination, the ethanol must be cleanly separated from the bacteria after production. Some ethanol is lost during this process, so the researchers are looking for ways to eliminate the waste. Despite the challenges ahead, the authors remain optimistic, pointing out that similar problems have already been solved in other strains of thermophiles, or heat-loving bacteria.
It’s still too early to declare the Dartmouth group the front-runner in the race to develop second-generation biofuels, but their engineered bacteria holds promise, according to Nathanael Greene, a biofuels expert at the Natural Resources Defense Council, an environmental advocacy group.
The Dartmouth group’s approach is called ‘consolidated bioprocessing’ because it uses a single organism to make ethanol. It presents a rich opportunity for success, said Greene, since they are manipulating the bacteria specifically to produce ethanol. No one knows, however, when any of the second-generation biofuels now being engineered in laboratories will be available to customers.
“The reality is . . . there’s not going to be one single technology to win the day,” said Greene. “Hopefully sooner rather than later some amount of ethanol from consolidated bioprocessing will be available to consumers.”
The study’s authors could not comment on when their technology will be available at the pump. At Mascoma, Charles Wyman is confident they can make the leap from bench science to transport fuel.
“Cellulosic fuels better be successful if we hope to reduce our addiction to imported oil,” said Wyman.
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