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Four Reasons Why Traditional Metal Toxicity Testing with Aquatic Insects Is Irrelevant Monica D. Poteat and David B. Buchwalter* Environmental and Molecular Toxicology Program, Department of Biological Sciences, North Carolina State University, Raleigh, North Carolina 27695, United States
Figure 1. Modeled time (in days) required to attain steady state tissue concentrations of Cd in 34 EPT species. Each symbol represents a different species, and each line represents the median time.
water. Steady state times were determined by the following equations,
T
race metal contamination of freshwater ecosystems is a problem worldwide, and insects are typically the predominant invertebrate faunal group in these systems. Metals can shape community structure, as evidenced by reduced biodiversity in affected areas. Aquatic insects are often some of the first species to disappear from metal-contaminated sites, despite the fact that laboratory toxicity tests would suggest that aquatic insects are generally insensitive to metals. In fact, typical laboratory results would indicate that insects only respond to dissolved metals at concentrations orders of magnitude larger than those found in the most insect-depleted contaminated sites. Even with mounting evidence highlighting the obvious disconnect between laboratory toxicity tests and field observations regarding metal toxicity to aquatic insects, water quality criteria for metals continues to rely primarily on toxicity values derived from short term dissolved-only exposures. Below we discuss four key reasons as to why such tests don’t provide relevant data for this important faunal group, focusing upon recent advances in our understanding of bioaccumulation and mechanisms of toxicity.
Css =
ku *Cw ke
where Css = steady state concentration, ku = uptake rate constant, ke = efflux rate constant, and Cw = dissolved metal concentration, and Tss = Css(1 − e−ket )
where Tss = time to steady state concentration, Css = steady state concentration, ke = efflux rate constant, and t = time. Median times to steady state concentration from dissolved Cd exposures were 405, 70, and 50 days, respectively, for EPT taxa (Figure 1). These times to steady state tissue concentration far surpass the time allotted for most acute toxicity tests (96 h). Exposure duration in traditional toxicity testing is clearly insufficient from a bioaccumulation perspective.
(1). REACHING STEADY STATE TISSUE CONCENTRATIONS TAKES TIME, LOTS OF TIME
(2). TRADITIONAL UNDERSTANDING OF THE DISSOLVED ACUTE TOXICITY MECHANISMS DO NOT SEEM TO APPLY TO AQUATIC INSECTS Some might argue that the surface action of metals (rather than bioaccumulation) better predicts acute toxicity. In rainbow trout for
Here, we used previously published1−3 cadmium uptake and efflux rate constants to model the time it would take 34 species representing the Ephemeroptera, Plecoptera, and Trichoptera (EPT taxa) to reach steady state tissue concentrations from
Received: December 11, 2013 Accepted: December 20, 2013 Published: December 27, 2013
© 2013 American Chemical Society
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dx.doi.org/10.1021/es405529n | Environ. Sci. Technol. 2014, 48, 887−888
Environmental Science & Technology
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exist in the protectiveness of water quality criteria for metals. We further advocate that the development of relevant insect toxicity models will help bridge the gap that currently exists between ecological monitoring programs (that typically focus upon insect community structure) and toxicity based approaches for setting environmental standards.
example, acute metal toxicity is well predicted by the surface action of metals on gills, and associated osmoregulatory disturbance. For example, metal exposures (e.g., Cd, Cu) have been shown to reduce the influx rates of major ions (Ca and Na, respectively). Recent work with aquatic insects has demonstrated a lack of interactions between Ca and heavy metals Zn and Cd at the apical surface of aquatic insects in dissolved exposures.2,3 Increased Ca concentrations only moderately protects against Cd and Zn uptake,2 and the uptake of Cd and Zn at environmentally relevant concentrations occurs without impeding Ca uptake.3 In insects, the prevailing paradigm that Cd and Zn will out-compete Ca for apical entry and result in osmoregulatory disturbance does not appear to apply.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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(3). TISSUE BURDENS OF METALS ACQUIRED FROM DIET FAR SURPASS THOSE OBTAINED FROM DISSOLVED EXPOSURES IN AQUATIC INSECTS Current testing methodologies rely solely on dissolved metal exposures to derive water quality criteria. However, in every comparison of dietary vs dissolved acquisition of metals that we are aware, diet is the predominant route of exposure for aquatic insects. For example, Cain et al.4 compared site specific water chemistry and field measures of tissue Cu and Cd concentrations with lab based bioaccumulation studies to show that dissolved exposures could only account for
