Removing a highly toxic chemical from the environment will be easier thanks to recent research by SIUC microbiologists. Benzene, widely used in chemical manufacturing and a major component of gasoline and other petroleum-based fuels, is one of the most common contaminants found in groundwater supplies. Long-term exposure to it can lead to cancer in humans.

Although benzene is readily biodegraded when exposed to the atmosphere, removing the compound from airless environments is much more difficult. But that may soon change: A team led by John Coates, an assistant professor of microbiology, has isolated two microorganisms that break down benzene without the aid of oxygen. The researchers reported their discovery in the June 28, 2001, issue of Nature, a leading scientific journal.

"Not only is benzene a problem in underground water supplies, it can be found in soils and sediments," Coates says. "Fuel spills and leaking underground storage tanks are the main culprits."

The newly discovered microbes, bacteria known as Dechloromonas strain RCB and Dechloromonas strain JJ, degrade benzene to carbon dioxide anaerobically—that is, without oxygen. (Most bacteria are aerobic, needing oxygen to live.)

"One of the big problems in contaminated environments is a lack of oxygen," Coates says. "Without oxygen, the biodegradation of benzene essentially slows to a crawl. These organisms are able to speed up the process. What once took 70 days now takes place in seven days."

When contamination occurs, such as an oil spill or chemical tank leak, aerobic bacteria start breaking down the pollutants—but they often get stopped in their tracks because they use up all the available oxygen. Coates's lab previously had isolated several Dechloromonas species capable of breaking down chlorite (another pollutant) and giving off oxygen, a rare feat for microbes. That oxygen production can fuel aerobic bacteria to continue their cleanup work.

Initially Coates's group was simply looking for Dechloromonas bacteria that could give an assist to benzene biodegradation in much the same way. Instead, they found two strains that could do the job directly. 

Both use nitrate instead of oxygen to break down benzene. (RCB also can use perchlorate, which is itself a pollutant.) Adding nitrate, commonly used in fertilizers, to a contaminated site would fuel the activity of both strains.

Although Dechloromonas strains appear to be ubiquitous—found everywhere from Illinois to the Antarctic—they’re not easy to ferret out from soil samples. Romy Chakraborty, a doctoral student in microbiology, was first to isolate a benzene-eating Dechloromonas strain, from sediments taken from the Potomac River. "She deserves a lot of credit," says Coates. "Romy overcame the difficulties by using some novel approaches." 

Kimberly Cole, a research technician in Coates’s lab, isolated the second strain from samples taken from SIUC’s own Campus Lake. Joseph Lack, a master’s student in microbiology, and Susan O’Connor, another research technician, helped Chakraborty and Cole characterize both strains of bacteria by studying their metabolism and the conditions required for growth. 

"They proved conclusively that these organisms could use benzene anaerobically," says Coates. "It was a real team effort."

Meanwhile, SIUC microbiologist Laurie Achenbach and Kelly Bender, a doctoral student in her lab, handled the genetic analysis, identifying where the bacteria fit in the tree of life.

While the team’s discovery could have major implications for environmental remediation, it’s also of considerable interest to many laboratory scientists.

Because of its ring structure, benzene is a very stable molecule. Before 1996, when it was proven that benzene could sometimes break down in oxygen-depleted environments, many scientists had assumed that oxygen was needed to weaken that structure. Now that Coates’s group has identified the first benzene-busting organisms, much can be learned from them.

"They offer a model to study how organisms attack a stable structure like benzene," says Coates. "How organisms do this is a longstanding question in environmental microbiology. It will give us more insight into the workings of nature."
 

Coates's research has been  funded by the U.S. Office of Naval Research, the U.S. Department of Defense, the U.S. Department of Energy, and SIUC. He is now with the University of California at Berkeley.

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