Abstract
Caenorhabditis elegans is used for assessing the toxicity of chemicals in aqueous medium. However, chemicals can absorb to the bacterial food, which reduces the freely dissolved concentrations of the tested compounds. Thus, based on total or nominal concentrations, toxicity is underestimated, resulting in misleading assumptions on toxicity mechanisms or comparisons to other test organisms. As the verification of freely dissolved exposure concentrations (Cfree) is challenging in small test systems, simple partitioning models might by a good option for estimating Cfree. Therefore, C. elegans was exposed to seven differently acting organic chemicals with varying hydrophobicities, thus also different affinities to bind to the food of C. elegans. Measured concentrations of the dissolved aqueous and the bacterial-bound fraction allowed the calculation of binding constants (Kb). Experimental Kb were comparable to literature data of hydrophobic chemicals and correlated well with their hydrophobicity, expressed as log KOW. The chronic toxicity of the various compounds on C. elegans’ reproduction, based on their aqueous concentration, was weakly related to their log KOW. Toxicity expressed based on chemical activity and comparisons with a baseline toxicity model, nevertheless, suggested a narcotic mode of action for most hydrophobic compounds (except methylisothiazolinone and trichlorocarbanilide). Although revealing a similar toxicity ranking than Daphnia magna, C. elegans was less sensitive, probably due to its ability to reduce its internal concentrations by means of its very impermeable cuticle or by efficient detoxification mechanisms. It could be shown that measured aqueous concentrations in the nematode test system corresponded well with freely dissolved concentrations that were modeled using simple mass-balance models from nominal concentrations. This offers the possibility to estimate freely dissolved concentrations of chemicals from nominal concentrations, making routine testing of chemicals and their comparison to other species more accurate.
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Raw data are available on request (hoess@ecossa.de).
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Acknowledgements
This study was funded by Unilever (Project: CH-2011-0953). The gifts of Caenorhabditis elegans (strains N2) and Escherichia coli (strain OP50) from the Caenorhabditis Genetic Center, which is supported by the National Institutes of Health, are gratefully acknowledged. We want to thank Jayne Roberts and Geoff Hodges for helpful comments on an earlier version of the manuscript and for their help with MechoA profiling. Moreover, we want to thank two anonymous reviewers for their valuable comments on an earlier version of the manuscript.
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This study was funded by Unilever (Project: CH-2011-0953)
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Sebastian Höss and Roger van Egmont contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Sebastian Höss and David Sanders. The first draft of the manuscript was written by Sebastian Höss, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
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Höss, ., Sanders, D. & van Egmond, R. Determining the toxicity of organic compounds to the nematode Caenorhabditis elegans based on aqueous concentrations. Environ Sci Pollut Res 30, 96290–96300 (2023). https://doi.org/10.1007/s11356-023-29193-2
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DOI: https://doi.org/10.1007/s11356-023-29193-2