Bacteria Thrive on Styrofoam

Cheap, durable and totally resistant to decay, Styrofoam is a dream for people on the go and an environmentalist's nightmare. Americans plow through 3 million tons of the stuff every year, with 2.3 million tons winding up in landfills. In the ground, Styrofoam, a manmade plastic derived from petroleum, will endure for centuries, but to date, there is no other safe, economical way of disposing of it on a large scale. Now, however, researchers at the National University of Ireland have come up with a possible solution to the messy problem of Styrofoam trash. They found a bacterium, Pseudomonas putida, that can turn Styrofoam waste products into a useful, biodegradable plastic. If it can make the leap from laboratory bench to factory floor, this bio-disposal technique could launch a recycling revolution.
Green Plastic

Plastic is a general term for materials made of organic polymers-chains of small, carbon-based molecular units linked end to end. Most of the plastic we encounter every day, in soda bottles, grocery bags, keyboards, telephones, and myriad other products, is made by chemically altering petroleum. While we think of plastic as the ultimate synthetic (artificial) material, a form of plastic does occur in nature. In 1925, researchers at the Pasteur Institute discovered that many bacteria produce a polymer called poly-B-hydroxyalkanoate (PHA). PHA is a bona fide plastic, made of linked carbon subunits, with properties similar to polypropylene, a common synthetic plastic used to make bottle caps. But unlike petroleum-based plastics, because it is of biological origin, PHA is completely biodegradable. Microbes make the substance, after all, as a fuel source, so they must have mechanisms for breaking it down. In fact, bacteria metabolize PHA completely into carbon dioxide and water.

Not all bacteria make PHA, but for the ones that do, it serves as an energy-storage molecule of last resort. Microbiologists compare PHA to fat tissue in humans, a reserve the organism can call upon when times are tough. When they sense uninviting conditions, such as scarcity of an important nutrient, bacteria will kick into PHA-producing mode and stash away large quantities of it in their cells.

Although it was discovered decades earlier, scientists paid little attention to PHA until the 1970s, when the international oil crisis got people worried about their extreme reliance on petroleum-based products. Suddenly, alternative sources of plastic seemed very appealing, and researchers at Imperial Chemical Industries (ICI), a British company, began investigating the properties of PHA. They found that, when harvested in large amounts, PHA formed a stiff, brittle plastic that could substitute for synthetic plastic in many applications. Making bioplastic in bulk, however, was (and still is) much more expensive than producing synthetic plastic, so it never really took off. Today, ICI sells a paltry 600 tons a year of "Biopol," a PHA plastic, for use in shampoo bottles and other packaging materials. (For comparison, the total plastic output of the U.S. in 2004 was over 50 million tons.)

Kevin O'Connor and colleagues at University College Dublin, a division of the National University of Ireland, decided to investigate whether PHA-producing bacteria could be enlisted in the fight against burgeoning Styrofoam waste. Styrofoam, after all, is carbon-based, so the researchers theorized that a bacterium might be able to use it as a carbon source for making PHA. No bacterium can "eat" a Styrofoam cup, but when it is heated in the absence of oxygen, Styrofoam degrades into a liquid called styrene oil. Styrene oil consists mostly of styrene, the monomer (single unit) that is the building block of Styrofoam, along with low levels of other organic byproducts. This liquid, the researchers hoped, might work as bacterial chow.

With at least one bacterial species, styrene oil cuisine was a hit. Pseudomonas putida, a PHA-producing microbe, flourished on a pure diet of styrene oil. When fed styrene oil as its sole source of carbon, the bacteria turned 16 grams of styrene oil into 1.6 grams of PHA and 2.8 grams of new bacterial biomass. That amounts to a 10% conversion rate-not tremendous, but when the starting material is garbage, efficiency isn't so crucial. To get the bacteria to churn out PHA, the researchers kept the level of nitrogen, an important nutrient, very low. Nitrogen deprivation acted as a signal to the bacteria to go into starvation-preparation mode, in which they stockpile PHA.

The beauty of the technique is that it eliminates waste while creating a highly useful product. Combining waste management with plastic production gives the research a huge leg up in terms of economic viability. While earlier attempts to produce eco-friendly "green" plastic were thwarted by the cost of raw materials (usually various kinds of sugars that were fed to the bacteria), O'Connor's team proposes using trash-which should cost nothing at all. (The process is not a complete free lunch: turning Styrofoam into styrene oil requires energy, but nevertheless the advantages are potentially quite significant.) Instead of dumping tons of Styrofoam into landfills, we may be able to turn at least some of it into a useful material, and the researchers also think that the technique might easily be extended to other kinds of petroleum-based, non-degradable plastics. When PHA products themselves become trash, they break down in soil, rather than sticking around as unwanted souvenirs for countless generations.

In addition to bottle caps, PHA can be fashioned into plastic kitchenware, plastic wrap, and other disposable items. It is also uniquely suited, because of the way it breaks down, for a number of medical applications. Doctors use plates made of PHA to hold fractured bones in place; once the bones heal, the PHA gradually breaks down and exits the body.

O'Connor is confident that his research is part of a growing trend. "I think we'll see a lot more of this type of technology in the future," he told Wired News. Bacteria are great at cleaning up messes-some have been used in the past to digest petroleum to clean up oil spills. Now, O'Connor said, they may be used "to help prevent the mess in the first place." The wonderful world of microbes, where Styrofoam can be supper, could soon make the human world a cleaner place.

Sources

Biello, David. "Bacteria Turn Styrofoam into Biodegradable Plastic." Scientific American, (February 27, 2006) www.sciam.com/ article. cfm? articleID= 0007B0AE- 88AF- 13FF- 88AF83414B7F0000.

Hooper, Rowan. "Bacteria Turn Toxins Into Plastic." Wired News, (September 9, 2004) www.wired.com/ news/ technology/ 0,1282, 64862, 00.html.

"Microbes Convert Styrofoam into Biodegradable Plastic." Press release form the American Chemical Society. (February 23, 2006) www.eurekalert.org/ pub_ releases/ 2006- 02/ acs- mc022306.php.

Ward, Patrick et al. "A Two Step Chemo-biotechnological Conversion of Polystyrene to a Biodegradable Thermoplastic." Environmental Science and Technology, published online February 15, 2006: pubs.acs.org/ cgi- bin/ abstract. cgi/ esthag/ asap/ abs/ es0517668.html.