If ever there was a commercial chemical from hell, it would have to be an industrial solvent called 1,1,1,-trichloroethane - TCA for short.
"It's been used for many years as a metal degreasing agent, for dry cleaning - all this kind of stuff," observed microbiologist Baolin Sun, a post-doctoral research associate at Michigan State University in East Lansing. "It's found in glue, paint and aerosol sprays, for example."
If inhaled in large amounts, TCA vapor can cause consciousness blackout, low blood pressure and loss of heartbeat. Evaporated into the air from TCA-contaminated lakes and rivers, it may last in the atmosphere for about six years.
"TCA was manufactured everywhere since the 1940s or '50s," Sun said. "But the last time it was produced in the U.S.," he recounted, "was some years ago. Then people realized by the Protocol of Montrose, Montreal, that 1,1,1,-trichloroethane was an ozone-depleting substance. In 1987, many nations, including the U.S., decided that this compound therefore should not be produced any more. However," he noted, "TCA is still being imported into the U.S. from other countries, so it's still being used, although now it's known that sunlight can break it down into chemicals that destroy the ozone layer."
FDA Banned Chloroform, TCA Derivative
In fact, the once-popular general anesthetic chloroform (trichloromethane) was outlawed by the FDA from use in drugs, cosmetics and food packaging as a carcinogen and hazardous inhalant. Chloroform is still used as an insecticide, fumigant, solvent and in plastics.
Sun is first author of a paper in today's Science, dated Nov. 1, 2002. Its title: "Microbial dehalorespiration with 1,1,1,-trichloroethane." The article's senior author is James Tiedje, distinguished professor of microbiology and molecular genetics at Michigan State.
Their paper reports discovery of a microorganism that derives energy by degrading the toxic worldwide pollutant. "This is the first bacterium we know," Sun told BioWorld Today, "that can grow on this compound. So far," he noted, "the new-found bacterium's only taxonomic name is a strain designated TCA1. It's a Gram-negative motile short rod with a diameter of 0.4 to 0.6 micrometers and a length of 1.0 to 2.0 micrometers.
"The closest phylogenetic relative of TCA1," Sun pointed out, "is the Dehalobacter restrictus strain, which shares a 99 percent sequence similarity with our Michigan microorganism. D. restrictus, isolated not quite a decade ago by pioneer bacteriologist Christopher Holliger, was the first bacterium discovered that coupled growth with the reduction of a chlorinated aliphatic solvent acting as a respiratory electron acceptor," Sun observed.
"TCA is present in at least 696 of the 1,430 National Priorities List sites identified by the U.S. Environmental Protection Agency," Sun continued. "Because of its adverse effects on human health, EPA has set a maximum contaminant level of 200 micrograms per liter in drinking water."
From Ozone Layer To Groundwater
"Even when released to soil or leached to groundwater," he pointed out, "TCA's primary environmental fate is volatilization to the atmosphere, where it interacts with ozone. It also contaminates subsurface aquifers." Michigan harbors at least two large contaminated aquifer plumes,' which, Sun explained, "are like lakes or rivers under the ground. TCA's density is higher than water, so it always stays underneath the water. The traditional method to pump and treat the aquifer's content," he related, "cannot work for this kind of contaminant. The TCA1 bug we have found is anaerobic, so it's ideal for this sort of bioremediation. We have a plan right now to use it for some field studies here in Michigan, now that we know these plumes are contaminated with TCA. Actually, we've got a project grant from EPA, part of which is included in our aquifer plan. So that's one part we're going to work on next.
"This TCA and other pollutant compounds were produced half a century ago - a relatively short time compared with microbial evolution. So how has the bug evolved to get this remediation against TCA is something we want to research in the near future. We have some information now, but it's not enough. So in genetics, biochemistry, evolution, all this, that's something interesting and we're going to work on it. We'll be glad to provide this TCA1 bug to other investigators," Sun concluded, "for field studies and bioremediation to help solve these polluting problems."