Science & Technology
PLASTICS AS HIGH AS AN ELEPHANT'S EYE?
Christopher Somerville has a radical notion of what a plastics factory should be. The Michigan State University professor would put his in a field. A potato or cornfield, to be exact--and its production lines would be rows of plastic-producing vegetation rising out of the earth.
It sounds like fiction, but it's science. Around the world, a growing group of researchers are studying tiny bacteria that make minute amounts of natural plastics--called biopolymers--as a food reserve. Scientists have isolated the genes that control this process, and, using the latest gene-swapping techniques, they've succeeded in turning vats of these microscopic bugs into veritable plastic-production dynamos.
Next, Sommerville and others hope to insert the plastic genes into yeast, potatoes, even corn--and "grow" large quantities of biopolymers. "The productivity of plants is unparalleled," says Somerville, who predicts that agricultural plastics could become a reality within a decade.
Plastics produced by bacteria currently cost several times more than the pennies-per-pound petrochemical versions, but they offer some big advantages. They're truly biodegradable, for one. They also could help reduce reliance on petroleum: By combining genes from several types of bacteria, researchers in the U. S., Japan, and Germany are trying to devise polymers--chains of repeating molecules--that would replace the oil-based variety. And by inserting custom-designed genes into bacteria, scientists say they might coax out entirely new plastics, such as slowly degradable materials for use inside the body or superflexible plastics to replace rubber.
Already, there is growing commercial interest in biopolymers. In June, Britain's Imperial Chemical Industries PLC opened a plant where plastic-producing bacteria are grown in large fermenters. The plant, in Billingham, England, can produce 300 tons of the stuff annually. The price is $15 a pound--well above the average 60~ a pound for oil-based plastics. But this hasn't deterred some: Wella Corp., for instance, uses the biodegradable plastic in shampoo bottles to appeal to "green" German shoppers. And researchers predict that manufacturing efficiencies alone may slash the price of biopolymers to about $3 a pound in the next few years.
Scientists have known about plastic-producing bacteria since 1925, but it was a research fluke that persuaded them that biopolymers could be commercially useful. In 1989, Douglas Dennis, a molecular biologist at James Madison University in Harrisonburg, Va., was studying Alcaligenes eutrophus, soil bacteria that, when starved of the nitrogen essential to its growth, produce minute amounts of a family of polymers called PHAs (for polyhydroxyalkanoates). Just as bears add a layer of fat before winter, these bacteria store plastic when they sense a scarcity of nitrogen or oxygen. When supplies improve, the plastic is converted into food.
GENETIC MAP. The Virginia researcher was looking for the genetic switch, called a promoter, that turns on the plastic-producing genes. In the process, he transferred a long piece of DNA into E. coli, bacteria found in the human digestive system that are widely used for research. The bacteria immediately began churning out a plastic. Dennis had inadvertently stumbled onto the entire, three-gene set that Alcaligenes bacteria use to make plastic. He had also proved that plastic-producing genes will turn on in a new host. Dennis' good fortune set off a modern-day gold rush: Within weeks, scientists were phoning from around the world. As a result of his work, researchers gained a vital map to the plastic-producing genes. "The technology is now available to express these genes in any organism you want," says Oliver P. Peoples, a molecular biologist at Massachusetts Institute of Technology who is studying the complex process by which bacteria convert carbon-rich foodstuffs into polymers.
Since Dennis' find, researchers have discovered a rich diversity of bacterial plastics and have begun to decipher the biological mechanisms that these tiny bugs use to control their production. Using this knowledge, researchers at the University of Massachusetts in Amherst recently engineered a superstrain of bacteria that bulge with plastic--up to 75% of their weight. Friedrich Srienc, an Austrian chemist now leading a team at the University of Minnesota, is working on polymers that decompose in set time periods. These could be key for delivering drugs to the body.
And in Japan, Terumi Saito, a biologist at Kanagawa University, is trying to move the plastic genes into microorganisms that can produce biopolymers from sunlight. "Our aim is to improve productivity to reduce the cost" of biopolymers, says Saito.
CARBON-RICH DIET. Tinkering with genes isn't the only way to influence biopolymer production. Some researchers are feeding plastic-producing bacteria exotic foods to come up with novel materials. For example, scientists in the Netherlands and at U. Mass. Amherst have devised a rubbery PHA by feeding bacteria carbon-rich liquid octane and butane in place of sugars.
The bacteria assemble new polymers with octane "side chains" attached along their length that would be impossible to duplicate with petrochemicals. Because these side chains affect the strength and flexibility of the plastic, this research suggests that PHAs could replace a wider range of petroleum-based plastics than once thought--and be useful in everything from plastic films to industrial thermoplastics.
Bacteria are just the starting point. Last year, Michigan's Somerville coaxed the three genes that produce PHA in bacteria into Arabidopsis, a tiny weed related to the mustard plant. In January he was able to switch them all on, producing a polymer in a plant for the first time. Somerville and others say a plastic-producing crop plant isn't far off--and that could revolutionize farming. "We'd like to grow corn with all these plastic ears," says Anthony J. Sinskey, an MIT microbiology professor.
The Corn Belt won't become the Plastic Belt soon. There are still some major puzzles to be solved. Scientists need to know more about the biological pathways that bacteria use to assemble polymers before they can custom-design many plastics. Yet, biopolymers are on the way: The world's oil reserves are shrinking every day. And there's still plenty of room on earth to plant Sommerville's field of dreams.Gary McWilliams in Cambridge, Mass.