Correspondence/Rebuttal pubs.acs.org/est
Comment on “Assessing Aromatic-Hydrocarbon Toxicity to Fish Early Life Stages Using Passive-Dosing Methods and Target-Lipid and Chemical-Activity Models” from Butler and co-workers1 against log Kow. Generic regressions for subcooled liquid solubility (SL) served as visual reference for chemical activity of unity, they were published by Mackay et al.8 (log SL = 3.26−1.00 log Kow; Figure 1A) and Di Toro et al.9 (log SL = 3.54−1.10 log Kow; Figure 1B). All LC50 values were well within the reported chemical activity range for baseline toxicity in Figure 1A, and a little closer to the upper limit in Figure 1B. The two compounds without acute toxicity had high melting points and a maximum chemical activity near the lower limit of the baseline toxic range. The absence of toxicity was thus consistent with the melting point cutoff in toxicity.4 We then fitted a linear regression to the LC50 values from Butler et al.:1 log LC50 = 2.34−1.10 log Kow (slope 95% CI: −1.43 to −0.76, r2 = 0.95, n = 6). Importantly, the slope was very close to and statistically not significantly different from the slopes of either generic regression for SL, in excellent agreement with an earlier study that included chemicals with higher log Kow values4 (see Figure 1B in reference 4). The experimental results from Butler et al.1 do thus neither indicate that LC50 values approach SL nor indicate a hydrophobicity cutoff in toxicity at or near the tested range. Eventually, some of the reasons for the apparent disagreement between the studies were found. Butler and co-workers plotted their data after adjustment by the target lipid model (TLM) rather than plotting actual experimental LC50 values. The TLM adjusted LC50 values were markedly higher than the experimental LC50 values, which partly explains the divergence in the results. Another potential cause for differences is
R
ecently, Butler and co-workers reported a comprehensive zebrafish early life stage toxicity study of aromatic hydrocarbons:1 Passive dosing was applied to tightly control the exposure of several hydrophobic organic chemicals in 30 day chronic toxicity tests, and constant exposure concentrations were carefully confirmed by analytical measurements. The embryo acute toxicity of phenanthrene in this recent study1 (LC50 of 334 μg/L) agreed very well with another recent shortterm study with the same test organism, passive dosing system and exposure time (LC50 of 310 μg/L2). While experimental results agreed well, the subsequent data analysis and the conclusions drawn were different between the studies. In our passive dosing studies,2,3 baseline toxicity initiated typically within the reported chemical activity range for baseline toxicity of 0.01−0.1.4,5 We have also observed that several solid compounds with high melting points did not reach sufficient chemical activity for exerting baseline toxicity even at their solubility limit.3,4 We explained this by a thermodynamic melting point cutoff in toxicity,4 and showed that such nontoxic compounds still can contribute to mixture toxicity,6 because of the solubility additivity for solid chemicals.7 Two main results highlighted in the TOC Figure in Butler et al.1 were in conflict with our studies: (1) lethality appeared to require higher chemical activities (chemical activity >0.1) and (2) the distance between LC50 and subcooled liquid solubility appeared to decrease with increasing hydrophobicity, possibly indicating a toxicity cutoff. In order to shed light on this apparent discrepancy between the studies, we replotted the 5 day embryo mortality data (Table 1)
Figure 1. Regression of subcooled liquid solubility (SL, mmol L−1, a = 1) as a function of Kow from (A) Mackay et al.8 and (B) Di Toro et al.9 and lines representing chemical activity of 0.1, 0.01, and 0.001. Non-adjusted LC50 values and solubility limits of compounds not exerting acute toxicity were taken from Table 1 in Butler et al.1 The red lines are the linear regression of the LC50 values, and the gray areas indicate the reported chemical activity range for baseline toxicity.4,5
© XXXX American Chemical Society
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DOI: 10.1021/acs.est.6b06416 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
toxicity of saturated mixtures. Environ. Toxicol. Chem. 2007, 26 (1), 24−36. (10) Kwon, J. H.; Lee, S. Y.; Kang, H. J.; Mayer, P.; Escher, B. I. Including Bioconcentration Kinetics for the Prioritization and Interpretation of Regulatory Aquatic Toxicity Tests of Highly Hydrophobic Chemicals. Environ. Sci. Technol. 2016, 50, 12004− 12011.
bioconcentration kinetics,10 as some of the figures in Butler et al.1 were based on lethality after 2 rather than 5 days. Decreasing bioconcentration kinetics with increasing hydrophobicity beyond log Kow of 6 will eventually lead to insufficient body burdens for exerting acute toxicity,10 which in turn asks for chronic toxicity testing as conducted by Butler et al.1 Overall, we find the comparative assessment of experimental toxicity data with the TLM and the chemical activity framework very useful and meaningful. However, experimental data should not be adjusted by the TLM approach before making the comparison between the TLM and chemical activity approach. The excellent agreement of the experimental results between the two recent zebrafish early life stage studies1,2 demonstrates how exposure control greatly can improve data quality and facilitate comparisons between studies. We hope that this comment clarifies that the non-adjusted experimental results from Butler and co-workers are in excellent agreement with previous studies showing that the baseline toxicity of nonpolar organics (MOA 1) typically initiates at a chemical activity of 0.01−0.1. The target lipid model is certainly a very strong and versatile approach for assessing and predicting toxicity, but using TLM adjusted LC50 values for assessing toxicity within the chemical activity framework seems not justified.
Philipp Mayer* Stine Nørgaard Schmidt
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Technical University of Denmark, Department of Environmental Engineering, Building 115, 2800 Kgs. Lyngby, Denmark
AUTHOR INFORMATION
Corresponding Author
*phone - + 45 45 25 15 69; e-mail -
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Butler, J. D.; Parkerton, T. F.; Redman, A. D.; Letinski, D. J.; Cooper, K. R. Assessing Aromatic-Hydrocarbon Toxicity to Fish Early Life Stages Using Passive-Dosing Methods and Target-Lipid and Chemical- Activity Models. Environ. Sci. Technol. 2016, 50, 8305− 8315. (2) Vergauwen, L.; Schmidt, S. N.; Stinckens, E.; Maho, W.; Blust, R.; Mayer, P.; Covaci, A.; Knapen, D. A high throughput passive dosing format for the Fish Embryo Acute Toxicity test. Chemosphere 2015, 139, 9−17. (3) Mayer, P.; Holmstrup, M. Passive dosing of soil invertebrates with polycyclic aromatic hydrocarbons: limited chemical activity explains toxicity cutoff. Environ. Sci. Technol. 2008, 42, 7516−7521. (4) Mayer, P.; Reichenberg, F. Can highly hydrophobic organic substances cause aquatic baseline toxicity and can they contribute to mixture toxicity? Environ. Toxicol. Chem. 2006, 25, 2639−2644. (5) Reichenberg, F.; Mayer, P. Two complementary sides of bioavailability: accessibility and chemical activity of organic contaminants. Environ. Toxicol. Chem. 2006, 25, 1239−1245. (6) Smith, K. E. C.; Schmidt, S. N.; Dom, N.; Blust, R.; Holmstrup, M.; Mayer, P. Baseline Toxic Mixtures of Non-Toxic Chemicals: “Solubility Addition” Increases Exposure for Solid Hydrophobic Chemicals. Environ. Sci. Technol. 2013, 47 (4), 2026−2033. (7) Banerjee, S. Solubility of organic mixtures in water. Environ. Sci. Technol. 1984, 18, 587−591. (8) Mackay, D.; Bobra, A.; Shiu, W. Y.; Yalkowsky, S. H. Relationships between aqueous solubility and octanol−water partition coefficients. Chemosphere 1980, 9, 701−711. (9) Di Toro, D. M.; McGrath, J. A.; Stubblefield, W. A. Predicting the toxicity of neat and weathered crude oil: toxic potential and the B
DOI: 10.1021/acs.est.6b06416 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
