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A genetically modified strain of common gut bacteria
may lead to a new technology for making biofuels that does not compete
with food crops for arable acreage
Seaweed may well be an ideal plant to turn into biofuel. It grows in much of the two thirds of the planet that is underwater, so it wouldn't crowd out food crops the way corn for ethanol does. Because it draws its own nutrients and water from the sea, it requires no fertilizer or irrigation. Most importantly for would-be biofuel-makers, it contains no lignin—a strong strand of complex sugars that stiffens plant stalks and poses a big obstacle to turning land-based plants such as switchgrass into biofuel.
Researchers at Bio Architecture Lab, Inc., (BAL) and the University of Washington in Seattle have now taken the first step to exploit the natural advantages of seaweed. They have built a microbe capable of digesting it and converting it into ethanol or other fuels or chemicals. Synthetic biologist Yasuo Yoshikuni, a co-founder of BAL, and his colleagues took Escherichia coli, a gut bacterium most famous as a food contaminant, and made some genetic modifications that give it the ability to turn the sugars in an edible kelp called kombu into fuel. They report their findings in the January 20 issue of the journal Science.
To get his E. coli to digest kombu, Yoshikuni turned to nature—specifically, he looked into the genetics of natural microbes that can break down alginate, the predominant sugar molecule in the brown seaweed. "The form of the sugar inside the seaweed is very exotic," Yoshikuni told Scientific American. "There is no industrial microbe to break down alginate and convert it into fuels and chemical compounds."
Once he and his colleagues had isolated the genes that would confer the required traits, they used a fosmid—a carrier for a small chunk of genetic code—to place the DNA into the E. coli cells, where it took its place in the microbe's own genetic instruction set. To test the new genetically engineered bacterium, the researchers ground up some kombu, mixed it with water and added the altered E. coli. Before two days had gone by the solution contained about 5 percent ethanol and water. It also did this at (relatively) low temperatures between 25 and 30 degrees Celsius, both of which mean that the engineered microbe can turn seaweed to fuel without requiring the use of additional energy for the process.
An analysis from the Pacific Northwest National Laboratory (pdf) suggests that the U.S. could supply 1 percent of its annual gasoline needs by growing such seaweed for harvest in slightly less than 1 percent of the nation's territorial waters. Humans already grow and harvest some 15 million metric tons of kombu and other seaweeds to eat. And there's no reason to fear the newly engineered E. coli escaping into the wild and consuming the seaweed already out there, Yoshikuni argues. "E. coli loves the human gut, it doesn't like the ocean environment," he says. "I can hardly imagine it would do something. It would just be dead."
The microbe could turn out to be useful for making molecules other than ethanol, such as isobutanol or even the precursors of plastics, Yoshikuni says. "Consider the microbe as the chassis with engineered functional modules," or pathways to produce a specific molecule, Yoshikuni says. "If we integrate other pathways instead of the ethanol pathway, this microbe can be a platform for converting sugar into a variety of molecules."
The fact that such a one-stop industrial microbe can turn seaweed into a variety of molecules has attracted the attention of outfits such as the U.S. Department of Energy's Advanced Research Projects Agency–Energy, or ARPA–e, which has funded BAL work with DuPont to produce other molecules from such engineered microbes. "Because seaweed grows naturally in the ocean, it uses the two thirds of the planet that we don't use for agriculture," ARPA–e program director Jonathan Burbaum wrote in an e-mail. "ARPA–e is directing a small portion of the remaining funding toward an aquafarm experiment to measure area productivity and harvest efficiency."
