Latest Issue of Science News


Feature

On the Rebound

Scientists revive search for new rubber sources

Magazine issue: 
Sponsor Message

In the summer of 2008, Jan Kirschner led an expedition to the highlands of southeastern Kazakhstan in search of a dandelion. Not just any dandelion: He was hunting kok-saghyz, a flower much like the common roadside weed that flourishes all over the world. But kok-saghyz (pronounced “coke-suh-GEEZ”) grows only in remote valleys of the Tien Shan Mountains.

Kirschner, a Czech taxonomist, was not the only dandelion hunter to visit Central Asia around that time. An expedition from the U.S. Department of Agriculture scoured the same valleys just a few weeks behind him. Two years earlier, an Israeli-Kazakh team had visited the area and concluded that kok-saghyz was worryingly rare.

Scientists from around the world aren’t traveling to remote corners of Central Asia out of concern for the region’s dandelion biodiversity. Their motivation is a curious substance in the flowers’ roots: rubber.

Bendable, stretchable and waterproof, rubber is one of the essential materials of the modern world. Each year, manufacturers transform more than 25 million tons of rubber into things like tires, shoes, adhesives, paint, hoses, gaskets, gloves, condoms, machine belts, phone protectors and yoga mats. Without rubber, trucks would never drive and planes could never land.

But for all its importance, there are only two economically important sources of rubber. The first is Hevea brasiliensis, a tree that is grown at commercial scale in only a few countries. The second source is fossil fuels, which have their own well-known supply issues.

Over the last century, the precarious nature of the global rubber supply has spawned repeated efforts to find a substitute source of the material. These campaigns have almost always started — and ended — with supply crises like the Arab oil embargo or World War II, when the Japanese occupation of Asian plantations cut off supplies of natural rubber to the Allies and synthetic rubber saved the day.

Now, with various economic and environmental forces threatening to squeeze rubber supplies again, researchers like Kirschner are attempting to revive defunct research programs while other scientists try to harness biotechnology to find alternative sources of rubber.

Natural rubber comes from latex, a sticky, milky goop made by some plants as a defense against herbivores. When an insect (or rubber harvester) pierces the plant tissue, latex oozes out, delivering natural pesticides, trapping bugs and sealing the wound.

Only once latex dries does it become rubbery. Long, flexible chains of polyisoprene, which is made up of many individual units of the hydrocarbon isoprene, form an interlocked tangle that allows rubber to compress, stretch and bend without breaking.

Fermenting rubber

Rubber trees aren’t the only plants that make isoprene. On hot, windless days in the Blue Ridge Mountains stretching from Georgia to Pennsylvania, a bluish haze of isoprene gas often settles over the forest canopy.

Although scientists don’t know exactly why some plants produce isoprene, the chemical seems to protect from the damaging effects of high temperatures. In a single day, the world’s plants emit more isoprene than manufacturers use in a year.

When biologist Anastasios Melis of the University of California, Berkeley first heard these figures, he was struck by the enormous potential of plant-produced isoprene as both a hydrocarbon fuel and a raw material for rubber synthesis. But he also realized that collecting isoprene from the air is not as simple as draining rubber from a tree. “Covering the canopy of the forest is not going to be the most practical thing,” Melis says.

A better approach, he thought, would be to engineer a microorganism to produce isoprene. Introducing the gene for the plant enzyme isoprene synthase into an easy-to-work-with microorganism could produce the raw material for rubber in a convenient form. “It’s a very straightforward approach,” Melis says.

He is not the only one who thinks so. Several research groups are currently working on microbial production of isoprene. One of the processes closest to commercial viability comes from a biotech company that is now part of DuPont Industrial Biosciences. The idea was developed in response to a 2007 request from the Goodyear Tire & Rubber Co. — the consumer of 13 percent of the world’s isoprene supply — which was under pressure due to spiking oil prices.

In 2010 in Industrial Biotechnology, the DuPont group described how to use engineered Escherichia coli bacteria to ferment isoprene from sugar. The isoprene bubbles out of the fermenting E. coli soup as a gas, making it simple to collect in the very pure form needed for rubber production. From this microbial isoprene, Goodyear made “green” rubber that was indistinguishable from standard synthetic rubber.

One drawback of DuPont’s approach is that it relies on a fuel — sugar — that is also a food. The same oil price spike that prompted Goodyear to look for new sources of isoprene also sparked a global food price crisis that in some cases culminated in riots. Some people blamed part of the crisis on the diversion of food resources into biofuel production. To avoid any such competition with food, many companies, including DuPont, are now trying to come up with ways to ferment fuel and chemicals from inedible woody plant waste.

But Melis argues that rather than try to convert the energy in plants to isoprene via bacteria, it would be even greener to cut out the middleman. Engineered photosynthetic micro­organisms, such as cyanobacteria and microalgae, could directly convert the energy of the sun into isoprene. This would eliminate the need to grow, harvest, dry and process plant material to feed the microbes. Last year in Biotechnology and Bioengineering, Melis and UC Berkeley researcher Fiona Bentley described a system for producing isoprene using the cyanobacterium Synechocystis engineered with an isoprene synthase gene from the kudzu vine. Though yields are too low for commercial production, Melis hopes his team can re-engineer the microbes’ metabolism to force more of its resources into synthesizing isoprene.

Several companies are also working on ways to coax microbes into making large quantities of 2,3-butanediol, which can be chemically converted to butadiene, the basis of another kind of synthetic rubber. For the most part, these companies feed their fermentations with plant material. But the company LanzaTech is trying to feed its microbes pollution emitted by steel mills.

Steel smelting produces a mixture of waste gases — including carbon monoxide, carbon dioxide and hydrogen — that are often burned off in huge flares. LanzaTech takes these waste gases and converts them into 2,3-butanediol with the help of a species of bacteria isolated from rabbit feces.

That bacterium, Clostridium autoethanogenum, taps gases animals emit as waste — carbon monoxide, carbon dioxide and hydrogen — for energy. In the process, it makes acetic acid and ethanol. LanzaTech researchers are trying to harness C. autoethanogenum and its relatives to create biofuels from the steel mill waste.

But like many other startups working on biofuels, LanzaTech is also investigating whether its process can be adapted to produce more profitable chemical products like rubber. In 2011, LanzaTech researchers reported in Applied and Environmental Microbiology that C. autoethanogenum produces respectable amounts of 2,3-butanediol when grown on carbon monoxide–containing waste gas.

Dandelion rubber

Biotechnologists are confident that renewable chemicals will one day feed into the growing stream of synthetic rubber products. But even after a century of research, synthetic mimics can’t completely replace the sticky stuff produced by wounded trees.

Part of the reason is that tree latex also contains traces of other plant biochemicals that make it more resilient than synthetic rubber and better at dispersing heat. That’s why high-performance tires, like those used for trucks and planes, must contain a very high percentage of natural rubber.

Though native to the Amazon, the rubber tree can’t be grown for commercial operation there because a fungal disease called South American leaf blight quickly infests any closely spaced plantations. Over 90 percent of natural rubber comes from the leaf blight–free plantations of Southeast Asia.

Thanks to strict import regulations, leaf blight has never made it to Asia. But if the fungus ever snuck through quarantine and spread there, a shortage of tires for trucks, planes and construction equipment would hit the global economy hard.

Fortunately, almost 2,000 plants besides rubber trees produce rubber. It may not be as easy as drilling a hole in a tree trunk, but with enough cultivation the sticky stuff could be harvested from some of them.

During World War II, the United States created an enormous research program to develop emergency rubber supplies from two plants: guayule (pronounced “gway-OO-lay”), a desert shrub native to northern Mexico, and Taraxacum koksaghyz, the kok-saghyz dandelion.

Kok-saghyz produces rubber-rich latex in its roots, which can be harvested only by uprooting the whole plant. But the dandelion can be harvested the same year it is planted, rather than after the five to seven years it takes a rubber tree to mature. So, though fiddly to cultivate, kok-saghyz had obvious advantages for a nation seeking an immediate source of emergency rubber.

Soviet scientists were the first to pursue kok-saghyz as a rubber crop, creating a huge network of experimental farms and rubber factories. After the Nazis captured Soviet rubber research facilities in 1941, German scientists began their own kok-saghyz projects, including a major cultivation facility staffed by prisoners from Auschwitz.

The U.S. government negotiated in 1942 with the Soviet Union for kok-saghyz seeds to kick-start its own dandelion research. Around 200 scientists worked on the project, planting test plots in 41 U.S. states and Canada.

But the U.S. scientists were unable to coax much rubber from the dandelions, and within two years concluded that the Soviets had exaggerated their successes. In the summer of 1944, the United States ceased all kok-saghyz research, plowing most of that year’s crop back into the soil. Much of the wartime knowledge of kok-saghyz eventually evaporated.

In the mid-2000s, the European Union became interested in alternative rubber plants as part of a push toward sustainable plant-based replacements for fossil fuel products. But their kok-saghyz project stumbled at the first hurdle. All the seeds they could get their hands on, from collections all over the world, proved useless. “Everything available was crap,” says Peter van Dijk, a geneticist with the Dutch company Keygene, which led the EUkok-saghyz breeding program.

The seeds proved to be from a different dandelion species that produces very little rubber. Deciding that it would be better off starting from scratch, the consortium sent Kirschner, a taxonomist from the Institute of Botany of the Czech Academy of Sciences, to retrace the steps of Soviet seed hunters of the 1930s.

In an article published this year in Genetic Resources and Crop Evolution, Kirschner and van Dijk’s team reported that although other researchers had previously found wild kok-saghyz to be rare in Kazakhstan, if you knew where and when to look, the dandelion flowered in abundance. In the same valleys where kok-saghyz grew, Kirschner and his team also found the weedy, rubber-poor cousin that had contaminated many seed bank stocks.

Ironically, both the Soviet and U.S. kok-saghyz projects of the 1940s were well acquainted with these rubber-poor imitators. The Soviets described the dandelion T. brevicorniculatum as a common weed of kok-saghyz plantations and had studied the contaminant’s genetics and its breeding compatibility with kok-saghyz. The U.S. scientists struggled to control “rogue” dandelion species in the imported seed samples and had quantified the ability of the clonally reproducing rogues to take over their kok-saghyz plantings.

Now that they have the right species, scientists like van Dijk are just starting to achieve the dandelion rubber successes already reported by their predecessors over 50 years ago.

Lost knowledge

The darling of the American World War II emergency rubber project was not kok-saghyz but guayule (Parthenium argentatum). Wild guayule was the center of a thriving international rubber industry until the Mexican Revolution intervened, and the plant seemed a good choice for a domestic supply. But despite the successes and hard work of more than 1,000 scientists, including many Japanese-Americans working in internment camps, guayule research was almost completely abandoned after the war. Although the dream of guayule rubber was briefly revived during the oil crisis, it wasn’t until the ’90s, when concern about latex allergies began to surface among health care workers, that guayule research rebounded.

The latex allergy crisis brought guayule back into the spotlight because its rubber lacks the proteins that cause the most severe allergic reactions. It was not long before a boutique industry had sprouted up in Arizona to produce hypoallergenic rubber for gloves and medical devices.

Today, the promise of the guayule industry has reignited hopes that the plant could be used for many other products. Several of the largest tire companies have been pursuing guayule research in recent years. Researchers are rapidly breeding guayule plants that yield more rubber and mature earlier, says Dennis Ray, a crop scientist at the University of Arizona in Tucson.

But historically, slow-moving plant experiments haven’t been able to match the pace of political interest. Government funding has typically come in two- or three-year increments. “But breeding is a long term thing. You need 10, 20 years in a row for consistency,” Ray says. He thinks the constancy of the rubber plantation industry’s breeding efforts explains why Asia is the commercial epicenter of natural rubber, while guayule is still an experimental crop. “They’ve had 85 years of breeding that have improved the yield,” he says.

For all of that time, guayule has been considered the next big source of natural rubber. “It’s a crop that has had many futures,” says rubber historian Mark Finlay of Armstrong Atlantic State University in Savannah, Ga. “In 1910 it was the crop of the future, and in 1940 it was the crop of the future.”

But when guayule research was abandoned after the war, 23,000 acres of the rubber crop were destroyed. Despite the huge scale of wartime guayule research, the only seeds available to scientists restarting the program in the 1970s came from just 26 breeding lines.

“We have a culture in which it’s possible for knowledge to be dismissed and tossed aside,” Finlay says. “There was no foresight that the next crisis might be around the corner.”

Back to the future

Today’s researchers exude optimism about the future of rubber from both new natural sources and renewable synthetic ones. One sign that their optimism is warranted is that tire companies — the largest consumers of all types of rubber — are investing in alternatives to rubber trees and oil.

“This will come to fruition,” says Chuck Yurkovich, vice president of global research and development at Cooper Tire & Rubber Co., which is working on guayule tires. “The technology is here. You have a number of companies competing to try to get there first. I do believe it will happen.”

Yet the supply squeezes that drove the latest research surge are already starting to dissipate. Thanks to an economic recession and the spread of rubber plantations to more parts of Asia, rubber prices have been plummeting for two years. Butadiene prices are also slumping to new lows. Despite the recent advances in rubber research, the reality is that none of the proposed techniques can yet compete with either trees or oil.

The situation seems not much different than it was 83 years ago, when a Science News article concluded that “the low market offers no promise of profit in guayule rubber, and commercial prospects are very much those of the future.”

Note: To comment, Science News subscribing members must now establish a separate login relationship with Disqus. Click the Disqus icon below, enter your e-mail and click “forgot password” to reset your password. You may also log into Disqus using Facebook, Twitter or Google.

X