Superfund Research Program
A Comparative Toxicology Study of Metal Mixtures
Background: The SBRP-funded research of Drs. Carol Folt and Celia Chen examines the movement of metals through aquatic food webs and their effects on natural lake populations. While aquatic organisms are often exposed to multiple metals, water quality criteria are established and ecological risk assessments performed largely on tests conducted on single metals using single species (and in many cases single clones) of test organisms. Since exposure to metal mixtures can result in noninteractive (additive) or interactive (greater or less than additive) responses, their effects cannot be predicted based on single metal toxicity tests. Moreover, different species vary in their tolerance to environmental contaminants.
Drs. Folt and Chen and their colleague, Dr. Joseph Shaw, investigated the combined effects of cadmium and zinc, two metals commonly found in contaminated systems on four species from the Daphniidae (waterflea) family. These invertebrates have long been used as standard organisms in aquatic toxicity tests because they play a central role in freshwater ecology; serve as a key conduit for contaminant transfer; are sensitive to a wide spectrum of chemical stressors; and are amenable to laboratory culture. However, few studies have investigated toxicological responses across this diverse family of organisms – and few toxicology studies assess the impact of exposure to mixtures of contaminants.
Advances: The investigators studied four species (Daphnia magna, D. ambigua, D. pulex and Ceriodaphnia dubia) that represent a diversity of sizes, habitat types, and phylogenetic and geographical distributions. They selected cadmium and zinc because antagonistic, additive, and synergistic interactions between cadmium and zinc have all been reported in aquatic invertebrates.
First, the research group conducted toxicity tests with the single metals to determine lethal effects concentrations (LCx values where x equals percent mortality).
To span the range of effects, they calculated LC15, LC50, and LC85 values for each metal and species. Their findings indicate:
- There are interspecies differences in sensitivities to both metals.
For both metals, D. ambigua was most sensitive and D. magna was most tolerant. LC50 values ranged from 0.09 to 0.9 μmol/L for cadmium and 4.0 to 12.5 μmol/L for zinc.
- D. magna responds differently to cadmium and zinc.
It is generally accepted that chemicals acting via common modes of action share similar slopes of their concentration-response curves. The slopes of the concentration-response curves for D. magna for both cadmium and zinc are different than for the other species, suggesting different mechanisms of toxicity in D. magna.
To assess toxicity of exposures to both metals, the research group conducted a series of tests that exposed each species to a low level of one metal (LC15) + LC15, LC50 or LC85 levels of the other metal. LC50 cadmium + LC50 zinc values were also tested in combination. They identified metal interactions (antagonistic or synergistic) by comparing observed mortality in the combined metal exposures with that predicted on the basis of simple additivity from single-metal tests.
Following treatments with LC15 cadmium + LC15 zinc, no interactions were observed (i.e., additive effects were observed). This trend continued with few exceptions when LC15 concentrations of cadmium were combined with LC50 or LC85 values for zinc. However, when LC15 concentrations of zinc were combined with LC50 or LC85 values for cadmium, responses of all species except D. magna indicated less-than-additive (protective) effects. For C. dubia, a near complete reduction in toxicity was observed when the LC15 for zinc was combined with LC85 for cadmium.
The research group determined that the mixture toxicity in this treatment regime is governed by zinc, which was always present in molar excess of cadmium. If zinc was introduced in the effects range (LC50, LC85), combined effects were additive. However, if zinc was present in low concentrations (LC15), combined toxicity was reduced, and less-than-additive effects were observed.
Significance: Despite their common occurrence, biological effects of metal contaminant mixtures are poorly understood and difficult to predict. Drs. Folt and Chen have brought together an interdisciplinary team (ecologists, geochemists, toxicologists and molecular biologists) to investigate natural processes regulating toxic effects of metals across several orders of biological complexity.
This study revealed interspecies variation in toxicologic responses to cadmium and zinc. The work demonstrated that D. magna is the most tolerant daphniid species to both cadmium and zinc and that these metals have different mechanisms of toxicity in D. magna than in the other daphniid species tested. These findings suggest that D. magna, which is the most common species used in aquatic toxicity tests, may not be representative of this important family of zooplankton, particularly as the differences in response typically were in the direction of greater tolerance. Consequently, it is possible that regulations and risk management decisions derived from tests with D. magna may not be representative of zooplankton communities in the field, at least within this ecologically important family.
For More Information Contact:
To learn more about this research, please refer to the following sources:
- Shaw JR, Dempsey TD, Chen CY, Hamilton JW, Folt CL. 2006. Comparative toxicity of cadmium, zinc, and mixtures of cadmium and zinc to daphnids. Environ Toxicol Chem 25(1):182-189. doi:10.1897/05-243R.1 PMID:16494240
To receive monthly mailings of the Research Briefs, send your email address to firstname.lastname@example.org.