ATHENS, Ga. -- Scientists have known for several decades that bivalves such as freshwater mussels readily accumulate many classes of environmental pollutants. This ability has made these shellfish an effective early-warning system for contamination by heavy metals, petroleum residues and even radioactive deposits.
Research by environmental toxicologists at the University of Georgia, however, finds that a new method of studying the DNA of freshwater mussels could make them an even more effective tool in limiting the effects of pollution.
"We've known for some time that these bivalves make excellent biomonitors," said Dr. Marsha Black, "but what we have needed are new tests -- such as one called the eletrophoretic DNA strand breakage assay -- that allow us to examine sublethal effects of pollutants. This test allows us to detect changes in an organism before there is significant accumulation of toxic substances."
The research by Black at the University of Georgia and colleagues from Oklahoma State University was published earlier this year in the journal Environmental Toxicology and Chemistry.
Black used a technique called gel electrophoresis to detect DNA strand breaks in freshwater mussels (Anodonta grandis) exposed to lead in acute (laboratory) and chronic (field) conditions. Although past studies have largely focused on the physical accumulation of measurable pollutants in shellfish, Black and others have recently begun to study how pollutants affect DNA -- a much more sensitive way to detect genotoxicity.
Black and her colleagues found a clear difference in DNA response to acute, short-term pollution and long-term chronic pollution with lead. She found significant strand breakage in the DNA of the mussels after a 28-day lab exposure to lead, while mussels experiencing lifetime exposure to low levels of lead in sediments showed no DNA breakage in any of the tissues examined. Lead was chosen as the test contaminant because of its stability in aqueous solution, ease in handing and disposal and the availability of sensitive techniques to analyze it in the tissues of mussels.
The differences between acute and chronic exposure present a perplexing problem. The mussels (which can live in some cases for longer than 30 years) clearly have the ability to "get well" or repair their DNA over time.
"Enhanced tolerance, which is often heritable, has been observed in organisms chronically exposed to lead and other heavy metals," said Black. "The results of our study emphasize the importance of exposure length and concentration to the progression of damage to double-stranded DNA and also to possible repair following sublethal exposure to lead."
Black chose to sample the foot, mantle and adductor muscles of the mussels used in the test to look for DNA breakage because of their physiological functions, physical location and ease of extraction. While gills, which filter water flow, might seem to be prime candidates for analysis, they were not used because extraction of them is quite difficult.
Of crucial importance in understanding how mussels might be used as biomarkers is discovering and understanding the enzyme that mussels use to repair genetic damage from environmental pollutants such as lead. Knowledge of how the enzyme works will make it far easier to know why there is such a difference in DNA reaction to chronic and acute exposures to pollutants.
Another problem remains for heavy metals such as lead. In mammals, lead is deposited and ultimately stored in bone; shellfish the comparable storage location is the shell. This is helpful in fighting further effects of pollution, because once heavy metals are trapped in the shell matrix, little metal release is possible. On the other hand, it may also make it more difficult to assess damage by examing DNA breakage in muscle tissues.
Black believes that a two-tiered approach to DNA damage may give the best indication of exposure to pollutants in freshwater mussels.
"In this paper we proposed that perhaps strand breakage determinations should be coupled with indicators of DNA repair to yield a more complete picture of how an organism is coping with genotoxic stressors," she said.
Several factors remain unclear, including determination of whether high incidence of strand breakages occur at necrotic zones in the tissue and distinguishing the damage from altered living cells. Despite the limitations, however, Black believes there is increasing evidence that freshwater mussels are effective biomarkers for many kinds of pollution.
(Editors/writers: Copies of the paper are available from Phil Williams. See above for e-mail and phone number.)