Variability in the Dynamics of Mortality and Immob

Authors: Mascha N Rubach Steven J H Crum Paul J Van den Brink

Publish Date: 2010/08/15

Volume: 60, Issue: 4, Pages: 708-721

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Abstract

The species sensitivity distribution SSD concept is an important probabilistic tool for environmental risk assessment ERA and accounts for differences in species sensitivity to different chemicals The SSD model assumes that the sensitivity of the species included is randomly distributed If this assumption is violated indicator values such as the 50 hazardous concentration can potentially change dramatically Fundamental research however has discovered and described specific mechanisms and factors influencing toxicity and sensitivity for several model species and chemical combinations Further knowledge on how these mechanisms and factors relate to toxicologic standard end points would be beneficial for ERA For instance little is known about how the processes of toxicity relate to the dynamics of standard toxicity end points and how these may vary across species In this article we discuss the relevance of immobilization and mortality as end points for effects of the organophosphate insecticide chlorpyrifos on 14 freshwater arthropods in the context of ERA For this we compared the differences in response dynamics during 96 h of exposure with the two end points across species using dose response models and SSDs The investigated freshwater arthropods vary less in their immobility than in their mortality response However differences in observed immobility and mortality were surprisingly large for some species even after 96 h of exposure As expected immobility was consistently the more sensitive end point and less variable across the tested species and may therefore be considered as the relevant end point for population of SSDs and ERA although an immobile animal may still potentially recover This is even more relevant because an immobile animal is unlikely to survive for long periods under field conditions This and other such considerations relevant to the decisionmaking process for a particular end point are discussedDecades of ecotoxicologic testing have repeatedly showed large differences in the response of species toward toxicants but they have not resulted in the identification of a “most sensitive species” Cairns 1986 which is now widely accepted as nonexistent although some indications for generally more sensitive groups exist Dwyer et al 2005 Across chemicals it is primarily the mode and mechanism of action of a toxicant that determines an organism’s response to exposure Thurston et al 1985 Escher and Hermens 2002 Jager et al 2007 but even for a single toxic compound large differences in species sensitivity have been found Rubach et al 2010 Differences in sensitivity across species are a source of uncertainty for the process of ERA In the lowest tier of ERA this uncertainty is often accounted for by using safety factors to derive threshold values for acceptable environmental concentrations Van Leeuwen and Vermeire 2007 Despite their importance to the improvement of risk assessment surprisingly little is known about the underlying mechanisms driving differences in sensitivity Hence most higher tier interspecies extrapolations are performed using probabilistic approaches such as the species sensitivity distribution SSD concept Posthuma et al 2002 or by performing multispecies tests It is well known that differences in uptake and elimination of a compound into the body or organs cause differences in sensitivity and these can be decreased when risk assessment is based on internal concentrations McCarty and Mackay 1993 In addition numerous studies have indicated that differences in sensitivity can also be explained by physiologic factors such as differences in target enzyme constitution detoxification or compensation abilities eg Heckmann et al 2008 In this context the comparison of toxic effects measured with different end points in bioassays can hold useful information for ERA when interpreted with regard to the processes of toxicity such as toxicokinetics and toxicodynamics For instance time dependency of toxicity and differences in the toxicity response for different end points indicate that major differences in the processes of toxicity exist across species Verhaar et al 1999 Although suitable methods such as timetoevent analysis Newman and McCloskey 1996 timeindependent sensitivity values Mayer et al 2002 and the dynamic energy budget theory Kooijman and Bedaux 1996 have been developed dynamics of effects have been largely ignored in ERA Often lethal and effective concentration values for different time points are treated equally without distinction both for practical reasons and for lack of more specific data The consequences of this ignorance are difficult to estimate at this point but may lead to arbitrary conclusions For instance for an organophosphorous compound such as chlorpyrifos which affects the nervous system but does not lead to immediate mortality large differences in effect end points could exist between species which is also relevant to risk assessment of timevariable exposure scenarios Despite widespread activities to establish sublethal end points in risk assessment not only for chronic but also for acute toxicity the literature lacks publications discussing the relevance of particular end points for the ERA of pesticides Baas et al 2010 The only exception are endocrine disruptors for which the debate for the most relevant end point has developed further Rhind 2009This study aimed to evaluate how well two end points mortality and immobility reflect the toxicity of the organophosphorous insecticide chlorpyrifos in a variety of freshwater arthropods in terms of their effect dynamics and their variability across species Experimental data were collected by means of 96h toxicity tests with a range of exposure concentrations These data were used to calculate both 50effective and lethal concentrations LEC50s with which SSDs per investigated time point and end point were populated The influence of end point choice on risk indicators such as LEC50s and the 5 hazardous concentration HC5 is discussed Furthermore we discuss the context in which toxicity information on different end points can improve understanding of differences in toxic processes among investigated species and how this can lead to more mechanismbased risk assessmentFor the 96h toxicity experiments chlorpyrifos OOdiethyl O356trichloro2pyridyl phosphorothioate 99 purity CAS 29218820 lot no 51205 purchased from Dr Ehrenstorfer GmbH Augsburg Germany was used To avoid the use of a solvent carrier aqueous solutions of chlorpyrifos were prepared using the principle of generator columns Devoe et al 1981 as described in the supporting information of Rubach et al in press The effluent of the generator column delivered a stable concentration of parental chlorpyrifos of approximately 1250 μg/l Before use each such obtained stock solution was measured by liquid–liquid extraction of subsamples with nhexane followed by gas chromatography GC and electron capture detection ECD to determine its exact concentration of chlorpyrifos and dosing schemes were subsequently adapted

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