|
By Phil Williams
Much of the information on world-wide pollution is flawed at best and could be entirely wrong, according to a recent study led by a visiting scientist at the university. The consequences of this  oversight are beginning to threaten public health and the environment, according the paper’s senior author, research microbiologist David Lewis of UGA’s department of marine sciences.
The research specifically deals with chirality, a characteristic exhibited by chemicals with asymmetric molecules. The asymmetry means that molecules of a single chemical pollutant can exist as mirror images. Since many of the building blocks of living organisms, including certain sugars, amino acids and proteins, are also chiral, the effects of chiral pollutants depend on how well the toxic portions of the pollutant fit with molecules of living things.
“Our study emphasizes the fact that much of the historical environmental data collected on pollutants is unreliable because so many of the chemicals are chiral, and the data do not distinguish which mirror images of certain chemicals were present and which were harmful,” says Lewis. “The good news is that many environmental pollutants--including some DDT derivatives and PCBs--aren’t as bad as previously thought. On the other hand, steps taken to protect the environment, such as using treated sewage sludge as a commercial fertilizer, will likely increase the persistence of the more toxic forms of some pesticides.”
The study was published last month in the British journal Nature.
Lewis’s co-authors are A. Wayne Garrison of the U.S. Environmental Protection Agency Ecosystems Research Division in Athens, along with Eric Wommack of the National Research Council Fellowship Program and UGA, Alton Whittemore of the Senior Environment Employment Program at EPA, and Paul Steudler and Jerry Melillo of the Ecosystems Center at the Marine  Biological Laboratory at Woods Hole, Mass.
When molecular shapes do not permit a close fit, chemicals cannot interact very well, and thus pose a less-serious threat to living things.
“It’s like trying to shake someone’s right hand with your left hand,” says Lewis.
Knowing which molecules are ill-fitting mirror images--or enantiomers--can be extremely helpful. Lewis points out that 50 of the top 100 best-selling drugs (including barbiturates, Ritalin and ibuprofen) are marketed only after separating out the enantiomers with harmful side-effects, such as the birth defects found three decades ago with the drug thalidomide.
Many pollutants are chiral, including some pesticides, plasticizers and polychlorinated biphenyls--chemicals once used as electrical insulators and now pervasive in the environment. The problem, according the study, is twofold: first, very few chemicals now considered major pollutants have been evaluated for chirality and, second, environmental changes appear to alter which mirror images persist in the environment, by affecting soil microbes that break down the chemicals.
“In general we concluded that global environmental changes--such as tropical deforestation, nutrient pollution and global warming--will significantly alter the risks posed by many pollutants, making the effects of some worse and some less harmful,” says Lewis. “Without knowing how chiral pollutants will be affected, environmental measures aimed at reducing the effects of pollution are being formulated virtually in the dark.”
Since Lewis estimates that about one-fourth of all pesticides are chiral, the scope of the problem could be large. The high cost of separating out mirror images is currently prohibitive for most pesticides and industrial chemicals, though there are two notable exceptions--the herbicides dichlorprop and mecoprop, in which a limited portion of the total worldwide production includes single enantiomers.
To assess the persistence of different pollutant enantiomers and the possible influence of large-scale environmental change on that persistence, the research team studied soils collected in three areas: an upland plateau in Norway, an 80-year-old mixed deciduous forest in the United States, and an area near the city of Porto Velho in Brazil.
The scientists amended the soil samples with the herbicides dichlor-prop and methyl dichlorprop and with ruelene, an insecticide banned in the United States but still available in other countries.
In tests to study the effects of long-term increases in soil temperature, of the addition of fertilizers and of tropical deforestation, the researchers found that different soils and their associated microorganisms reacted differently.
“It appears that different kinds of environmental changes turn on different genetically related groups of microorganisms,” says Wom-mack, who carried out the genetic experiments in the study. “This may explain why environmental changes affect which mirror-image molecules persist in the environment.”
The results indicate a complexity that means current methods of determining which chemicals pose threats to the environment may be worthless in many cases.
“The differing effects of inorganic and organic nutrients on enantioselectivity, for example, raise new questions with respect to land application of processed sewage sludge,” says Lewis. “This sludge transforms pollutants enantioselec-tively. Thus, the sludge may increase microbial transformation rates of some of the enantiomers of pesticides and diminish their effectiveness in the field. On the other hand, some enantiomers may persist longer under these conditions, posing a risk to public health and the environment.”
Lewis says the uncertainty about the mirror-image effects of pollutants could also raise questions about so-called endocrine-disrupters, chemicals in the environment that allegedly interfere with the endocrine systems of animals.
Meanwhile, Lewis says, changes in understanding the chirality of pollutants must occur if global environmental policies are to have proper scientific weight.
“I don’t think it’s so expensive to create single-enantiomer pesticides that it couldn’t be done,” he says. “In some Scandinavian countries I believe that they are marketing them already. More and more, it will become technically feasible, but first we must understand the toxicity associated with them, and right now we don’t even know that.”
|
|
|
