ATHENS, Ga.-- Much of the information on pollution world-wide is flawed at best and could be entirely wrong, according to a just-published study, led by a visiting scientist at the University of Georgia. The consequences of this oversight are beginning to threaten public health and the environment, according the paper's senior author, research microbiologist David Lewis.
The research specifically deals with chirality, a characteristic exhibited by chemicals with asymmetric molecules. This asymmetry causes molecules of the same chemical pollutant to exist as mirror images of one another. Since many of the building blocks of living organisms, including certain sugars, amino acids and proteins, are also chiral, the effects of chiral pollutants depends on how well the toxic portions of the pollutant fit together 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," said Lewis. "The good news is that many environmental pollutants including some DDT derivatives, PCBs and plasticizers 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 today 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 the University of Georgia; 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, MA. Until recently, Melillo served as associate director for the environment in the White House Office of Science and Technology Policy.
Lewis is employed by EPA as well but is temporarily assigned to the University of Georgia as part of a legal settlement with EPA over concerns he raised about EPA science in a 1996 Nature commentary. He is assigned to the department of marine sciences at UGA.
The entire issue revolves around molecular shapes and how the environment affects the persistence of pollutants. When molecular shapes do not permit a close fit, they cannot interact very well, and thus chemicals with these molecules pose a less-serious threat to living things.
"It's like trying to shake someone's right hand with your left hand," said Lewis.
The knowledge of which molecules are ill-fitting mirror-images or enantiomers, as scientists call them, can be extremely helpful. Lewis points out that 50 of the top 100 best-selling drugs (including barbiturates, Ritalin and ibuprofen) are marketed 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 also chiral, including some pesticides, plasticizers and polychlorinated biphenyls--chemicals once used as electrical insulators and which are now pervasive in the environment. The problem, according the study, is twofold: first, very few chemicals now considered major pollutants have been evaluated for their chirality at all, and second, environmental changes appear to alter which mirror-images persist in the environment by affecting soil microbes responsible for breaking 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," said 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, he said. The high cost of separating 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 enantioners 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 U.S.; and in an area near the city of Porto Velho in Brazil.
The scientists amended the soil samples with herbicides dichlorprop and methyl dichlorprop, and with ruelene, which is an insecticide banned in the U.S. but still sold in other countries.
In tests designed to study the effects of long-term increases in soil temperature, the addition of fertilizers and of tropical deforestation, the researchers found that different soils and their associated microorganisms reacted in different ways. For example, the addition of inorganic fertilizers to forest and pasture plots in Brazil had no measurable effect on enantioselectivity for methyl dichlorprop or to plots in North American fertilized since 1988.
On the other hand, laboratory enrichments with organic nutrients, such as those present in sewage, caused a strong shift in which mirror-image molecules were preferred by microorganisms in samples from North America and Brazil but did not have the same effect in samples from Norway.
The study also used buried electrical lines used for warming to assess the potential effect on chiral chemicals of global warming and studied the genetic diversity and biogeography of bacteria capable of changing chiral chemicals.
"It appears that different kinds of environmental changes turn on different genetically related groups of microorganisms," said Dr. Eric Wommack, 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, the authors said, indicates 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," said Lewis. "This sludge transforms pollutants enantioselectively. 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 said the uncertainly 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 said, 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 said. "I believe in some Scandanavian countries 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."
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