Jun
26
New Engineered Bacteria Make a Biofuel Precursor
June 26, 2013 | 1 Comment
In a major effort at Harvard University scientists at the Wyss Institute for Biologically Inspired Engineering and the Department of Systems Biology at Harvard Medical School scientists have engineered new lines of bacteria that can tailor-make key precursors of high-octane biofuels that could one day replace gasoline.
The new bacteria lines report has been published in the June 24 online edition of Proceedings of the National Academy of Sciences. The same lines can also produce precursors of pharmaceuticals, bioplastics, herbicides, detergents, and more.
E. coli Bacteria Engineered to Make Octanoate. Image Credit: Wyss Institute, Harvard University/
Pamela Silver, Ph.D., a Wyss Institute Core Faculty member, Professor of Systems Biology at Harvard Medical School, and senior author of the study said, “The big contribution is that we were able to program cells to make specific fuel precursors.”
Many believe new biofuels are needed for cars and other vehicles. Ethanol, the most popular biofuel on the market, packs only two-thirds the energy of gasoline. Oxygen rich and water miscible ethanol containing fuels used in old technology systems will corrode pipes, tanks, and other infrastructure used to transport and store gasoline. Burning gasoline alone adds huge amounts of carbon dioxide to the atmosphere, and relies on the world’s dwindling supply of oil. With the ethanol added the oil content is reduced by about 10% and the octane and gasoline potency is increased.
Gasoline owns the market now. Gasoline produces more energy than the current biofuel selections when burned in an internal combustion engine, and it remains liquid in temperatures ranging from a Texas heat wave to a North Dakota cold snap. Moreover, hundreds of millions of cars worldwide are built to run on it.
One cannot change too much or the world’s fleet isn’t part of the market.
Silver and her team are seeking new ways to make gasoline-like biofuels that could be stored at gas stations and used to fuel the cars we already have. To develop these, they enlisted the iconic and old reliable laboratory bacterium E. coli to help make gasoline precursors called fatty acids – those energy-packed molecules containing chains of carbon atoms flanked with hydrogen atoms that can be easily converted into fuels.
The Harvard group is specifically focusing on medium-chain fatty acids – those with chains between four and 12 carbon atoms long. Fatty acids with shorter chains do not store enough energy to be main fuel components and they tend to vaporize easily, while those with chains longer than 12 carbons are too waxy. But medium-length fatty acids are just the right length to be transformed into an energy-packed liquid fuel for internal-combustion engines.
Silver points out that today oil refineries produce medium-chain-length compounds from crude oil. But “instead of using petroleum products, you can have microbes or other living organisms do it for you,” she said.
To accomplish that, Joe Torella, Ph.D., and Tyler Ford, Harvard Medical School Systems Biology graduate students in Silver’s laboratory and the paper’s lead coauthors, tweaked an E. coli metabolic pathway that produces fatty acids. Specifically, they mass-produced an eight-carbon fatty acid called octanoate that can be converted into octane.
In this pathway, carbon from sugar, which the bacterium eats, flows through the pathway like a river, growing longer as it flows. Downstream, it exits as a long-chain fatty acid.
Torella and Ford first partially dammed the river and built an irrigation ditch using a drug that blocks enzymes that extend fatty-acid chains. This caused medium-chain fatty acids to pool behind the dam, while still allowing enough of the river to flow by for the bacteria to build their membranes and stay alive. The strategy increased octanoate yields, but the drug is too expensive for the process to be scaled up.
With that lesson in hand the pair of young scientists tried a second strategy that could be scaled up more readily. They let the cells grow to maturity, and then dammed the river using a genetic trick. They also genetically altered a second enzyme that normally builds long-chain fatty acids such that it extends fatty acids to eight carbons and no longer.
This two-pronged strategy – plus some other genetic nips and tucks to keep the river from being diverted in other ways – gave the scientists the highest yields of octanoate yet reported.
Torella said, “We found if we stop up the river – if we slow fatty acid elongation – we encouraged the creation of medium-chain fatty acids.”
Don Ingber, M.D., Ph.D., Wyss Institute Founding Director offers us a deeper explanation, “Sustainability is one of the biggest problems we face today, and developing potent biofuels to replace gasoline is a major challenge in the field. 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.”
Next up Silver’s scientists plan to engineer E. coli to convert octanoate and other fatty acids into alcohols, potential fuel molecules themselves, and that gets them just one chemical step away from octane – aka – a major component of gasoline.
Sounds good, looks great. But today the ethanol market is full up and uses 40% of the U.S. corn crop to no ill effect. Say the octanoate would be double the ethanol market. Then the issue would be where to get all the sugar to make it and at that kind of production the land requirements are going to be substantial.
Meanwhile the Harvard looks a lot like the Joule effort. Then there are other commercial biofuel efforts well underway. As the university patent office gets involved much more will become evident as the prior art is examined.
The Harvard effort may well have a major impact, where it might fit is still in the air. It’s going to be hard to gain traction though if the CO2 to CO and then on to recycled synthetic fuels find a low economic cost.
An eight carbon product will be welcome. Remember the EPA has some 30+ gasoline recipes for various regions and times of year and a clean eight carbon molecule will have a place at the table.
Still, oil isn’t in short supply for now. The crude prices are desperately hanging in the high zone and will surely crack someday. The industry is in attack mode on ethanol in a big way – again.
For this technology and many others the foundation and where the money will be made is in coming up with the mountains of cheap sugar.
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One cannot change too much or the world’s fleet isn’t part of the market.