Dynamic modeling of sublethal mixture toxicity in the nematode Caenorhabditis elegans

Application of toxicokinetic-toxicodynamic models in the aquatic ecological risk assessment of metals: A review

The issue of toxic metal pollution is a considerable environmental concern owing to its complex nature, spatial and temporal variability, and susceptibility to environmental factors. Current water quality criteria and ecological risk assessments of metals are based on single-metal toxicity data from short-term, simplified indoor exposure conditions, ignoring the complexity of actual environmental conditions. This results in increased uncertainty in predicting toxic metal toxicity and risk assessment. Using appropriate bioavailability and effect modeling of metals is critical for establishing environmental quality standards and performing risk assessments for metals. Traditional dose-effect models are based on a static statistical relationship and fall short of revealing the bioavailability and effect processes of metals and do not effectively assess ecological impacts under complex exposure conditions. This paper summarizes the toxicokinetic-toxicodynamic (TK-TD) model, which is gaining interest in environmental and ecotoxicological research. The key concepts, and theories of its construction theories, are discussed and the application of the TK-TD model in toxicity prediction and risk assessment of different metals in the aquatic environment, and trends in the development of the TK-TD model are highlighted. The findings of our review prove that the TK-TD model can effectively predict toxic metal toxicity in real time and under complex exposure conditions in the future.

Predicting the Metal Mixture Toxicity with a Toxicokinetic-Toxicodynamic Model Considering the Time-Dependent Adverse Outcome Pathways

Chemicals mainly exist in ecosystems as mixtures, and understanding and predicting their effects are major challenges in ecotoxicology. While the adverse outcome pathway (AOP) and toxicokinetic-toxicodynamic (TK-TD) models show promise as mechanistic approaches in chemical risk assessment, there is still a lack of methodology to incorporate the AOP into a TK-TD model. Here, we describe a novel approach that integrates the AOP and TK-TD models to predict mixture toxicity using metal mixtures (specifically Cd-Cu) as a case study. We preliminarily constructed an AOP of the metal mixture through temporal transcriptome analysis together with confirmatory bioassays. The AOP revealed that prolonged exposure time activated more key events and adverse outcomes, indicating different modes of action over time. We selected a potential key event as a proxy for damage and used it as a measurable parameter to replace the theoretical parameter (scaled damage) in the TK-TD model. This refined model, which connects molecular responses to organism outcomes, effectively predicts Cd-Cu mixture toxicity over time and can be extended to other metal mixtures and even multicomponent mixtures. Overall, our results contribute to a better understanding of metal mixture toxicity and provide insights for integrating the AOP and TK-TD models to improve risk assessment for chemical mixtures.

Sublethal effects of binary mixtures of Cu2+ and Cd2+ on Daphnia magna: Standard Dynamic Energy Budget (DEB) model analysis

Dynamic Energy Budget theory (DEB) describes mass and energy fluxes that occur in living organisms. DEB models were successfully used to assess the influence of stress, including toxic substances, and changes in pH and temperature, on different organisms. In this study, the Standard DEB model was used to evaluate the toxicity of copper and cadmium ions and their binary mixtures on Daphnia magna. Both metal ions have a significant influence on daphnia growth and reproduction. Different physiological modes of action (pMoA) were applied to primary DEB model parameters. Model predictions for chosen modes of interaction of mixture components were evaluated. The goodness of model fit and the model prediction was assessed to indicate the most likely pMoA and interaction mode. Copper and cadmium influence more than one DEB model primary parameter. Different pMoAs can result in similar model fits, and therefore it is difficult to identify pMoA only by evaluation of the goodness of fit of the model to the growth and reproduction data. Some critical discussion and ideas for model development are therefore provided.

Effects of triadimefon fungicide on Daphnia magna: Multigenerational effect and population-level ecological risk

Triadimefon is ubiquitous in various environmental media. Although toxicity of triadimefon to individual of aquatic organisms has been confirmed, its effect on organisms at population level remain poorly understood. In this study the long-term effect of triadimefon on individual and population of Daphnia magna were studied using multi-generational experiments and matrix model. Development and reproduction of three generations of F1 and F2 were significantly inhibited with the triadimefon concentration of 0.1 mg/L (p < 0.01). Toxicity of triadimefon to the offspring was stronger than to the parent (p < 0.05). When triadimefon concentration was higher than 0.1 mg/L, both population number and intrinsic rate of increase showed a decreasing trend with the increasing exposure concentration. Age structure of the population also tended to decline. Toxicity threshold derived on population-level was between mortality-based LC₅₀ and reproduction-based NOEC of Daphnia magna, and also between acute toxicity and chronic toxicity derived from species sensitivity distribution (SSD). The risk of population level derived from risk quotient was low for most areas, and the results derived from probability risk showed that the expected loss of intrinsic rate of increase of population was 0.0039 without considering other factors. Compared to the individual-level, the ecological risks at the population level were closer to the actual situation of the ecosystem response to the chemical pollution.

Co-contaminants of ethinylestradiol and sulfamethoxazole in groundwater exacerbate ecotoxicity and ecological risk and compromise the energy budget of C. elegans

Ethinylestradiol (EE2) and sulfamethoxazole (SMX) are among pharmaceuticals and personal care products (PPCPs) and regarded as emerging contaminants in groundwater worldwide. However, the ecotoxicity and potential risk of these co-contaminants remain unknown. We investigated the effects of early-life long-term co-exposure to EE2 and SMX in groundwater on life-history traits of Caenorhabditis elegans and determined potential ecological risks in groundwater. L1 larvae of wild-type N2 C. elegans were exposed to measured concentrations of EE2 (0.001, 0.75, 5.1, 11.8 mg/L) or SMX (0.001, 1, 10, 100 mg/L) or co-exposed to EE2 (0.75 mg/L, no observed adverse effect level derived from its reproductive toxicity) and SMX (0.001, 1, 10, 100 mg/L) in groundwater. Growth and reproduction were monitored on days 0 - 6 of the exposure period. Toxicological data were analyzed using DEBtox modeling to determine the physiological modes of action (pMoAs) and the predicted no-effect concentrations (PNECs) to estimate ecological risks posed by EE2 and SMX in global groundwater. Early-life EE2 exposure significantly inhibited the growth and reproduction of C. elegans, with lowest observed adverse effect levels (LOAELs) of 11.8 and 5.1 mg/L, respectively. SMX exposure impaired the reproductive capacity of C. elegans (LOAEL = 0.001 mg/L). Co-exposure to EE2 and SMX exacerbated ecotoxicity (LOAELs of 1 mg/L SMX for growth, and 0.001 mg/L SMX for reproduction). DEBtox modeling showed that the pMoAs were increased growth and reproduction costs for EE2 and increased reproduction costs for SMX. The derived PNEC falls within the range of detected environmental levels of EE2 and SMX in groundwater worldwide. The pMoAs for EE2 and SMX combined were increased growth and reproduction costs, resulting in lower energy threshold values than single exposure. Based on global groundwater contamination data and energy threshold values, we calculated risk quotients for EE2 (0.1 - 123.0), SMX (0.2 - 91.3), and combination of EE2 and SMX (0.4 - 341.1). Our findings found that co-contamination by EE2 and SMX exacerbates toxicity and ecological risk to non-target organisms, suggesting that the ecotoxicity and ecological risk of co-contaminants of pharmaceuticals should be considered to sustainably manage groundwater and aquatic ecosystems.

Nanoplastic exposure in soil compromises the energy budget of the soil nematode C. elegans and decreases reproductive fitness

Environmental nanoplastics (NPs) can accumulate in soils, posing a potential risk to soil ecosystems. However, the ecotoxicity of NPs for soil organisms has received little research attention. This study investigated whether NP exposure in soil leads to reproductive decline in the soil nematode Caenorhabditis elegans and sought to determine the mechanisms by which it may occur. Wild-type N2 C. elegans L1 larvae were exposed to various concentrations of nano-sized polystyrene (100 nm) in soil (0, 1, 10, 100, and 1000 mg/kg dry weight) for 96 h. We show that nano-sized polystyrene (100 nm) labeled with red fluorescence significantly accumulated in the intestine of C. elegans in a dose-dependent fashion via soil exposure (8%-47% increase). In addition, NP soil exposure led to 7%-33% decline in the number of eggs in utero and 2.6%-4.4% decline in the egg hatching percentage. We also find that the number of germ cell corpses (31%-55% increase) and the mRNA levels of germline apoptosis marker gene ced-3 (14%-31% increase) were significantly higher with greater NP soil exposure (10, 100, and 1000 mg/kg), while intracellular ATP levels were significantly reduced. Finally, the DEBtox model, which is based on the dynamic energy budget theory, was applied to show that the increased reproductive costs for C. elegans caused by NPs in soil are associated with energy depletion and reproductive decline. The threshold value (4.18 × 10^-6 mg/kg) for the energy budget also highlighted the potential high reproductive risk posed by NPs in terrestrial ecosystems. Our study provides new insights into how soil organisms interact with NPs in soil ecosystems.

Elucidating the Differences in Metal Toxicity by Quantitative Adverse Outcome Pathways

Numerous studies have reported that the toxicity differences among metals are widespread; however, little is known about the mechanism of differences in metal toxicity to aquatic organisms due to the lack of quantitative understanding of their adverse outcome pathway. Here, we investigated the effects of Cd and Cu on bioaccumulation, gene expression, physiological responses, and apical effects in zebrafish larvae. RNA sequencing was conducted to provide supplementary mechanistic information for the effects of Cd and Cu exposure. On this basis, we proposed a quantitative adverse outcome pathway (qAOP) suitable for metal risk assessment of aquatic organisms. Our work provides a mechanistic explanation for the differences in metal toxicity where the strong bioaccumulation of Cu enables the newly accumulated Cu to reach the threshold that causes different adverse effects faster than Cd in zebrafish larvae, resulting in a higher toxicity of Cu than that of Cd. Furthermore, we proposed a parameter C_IT/BCF (the ratio of internal threshold concentration and bioaccumulation factor) that helps to understand the toxicity differences by combining the information of bioaccumulation and internal threshold of adverse effects. This work demonstrated that qAOP is an effective quantitative tool for understanding the toxicity mechanism and highlight the importance of toxicokinetics and toxicodynamics at different biological levels in determining the metal toxicity.

The application and limitations of exposure multiplication factors in sublethal effect modelling

Thanks to growing interest and research in the field, toxicokinetic–toxicodynamic (TKTD) models are close to realising their potential in environmental risk assessment (ERA) of chemicals such as plant protection products. A fundamental application is to find a multiplicative scale factor which—when applied to an exposure profile—results in some specified effect relative to a control. The approach is similar to applying assessment factors to experimental results, common in regulatory frameworks. It also relies on the same core assumption: that increasing the scaling always produces more extreme effects. Unlike experimental approaches, TKTD models offer an opportunity to interrogate this assumption in a mathematically rigorous manner. For four well-known TKTD models we seek to prove that the approach guarantees a unique scale factor for any percentage effect. Somewhat surprisingly, certain model configurations may have multiple scale factors which result in the same percentage effect. These cases require a more cautious regulatory approach and generate open biological and mathematical questions. We provide examples of the violations and suggest how to deal with them. Mathematical proofs provide the strongest possible backing for TKTD modelling approaches in ERA, since the applicability of the models can be determined exactly.

Early developmental nanoplastics exposure disturbs circadian rhythms associated with stress resistance decline and modulated by DAF-16 and PRDX-2 in C. elegans

Plastics pollution is an emerging environmental problem and nanoplastics (NPs) toxicity has received great concern. This study investigated whether early developmental exposure to polystyrene NPs influence the circadian rhythms and the possible underlying mechanisms in C. elegans. We show that early developmental NPs exposure disturbs circadian rhythms in C. elegans and ASH neurons and G protein-coupled receptor kinase (GRK-2) are involved in the level of chemotaxis response. A higher bioconcentration factor in entrained worms was observed, suggesting that circadian interference results in increased NPs bioaccumulation in C. elegans. In addition, we show that reactive oxygen species produced by NPs exposure and peroxiredoxin-2 (PRDX-2) are related to the disturbed circadian rhythms. We further show that the NPs-induced circadian rhythms disruption is associated with stress resistance decline and modulated by transcription DAF-16/FOXO signaling. Because circadian rhythms are found in most living organisms and the fact that DAF-16 and PRDX-2 are evolutionarily conserved, our findings suggest a possible negative impact of NPs on circadian rhythms and stress resistance in higher organisms including humans.

Prediction on the combined toxicities of stimulation-only and inhibition-only contaminants using improved inverse distance weighted interpolation

The evaluation of ecological risks of contaminant mixtures to organisms is very challenging due to the non-linear response of organisms to each component, especially under the co-existence of both stimulators and inhibitors. Whether the stimulatory effect can reduce or even offset the inhibitory effect would be critical to the risk assessment and the treatment measures of mixed pollutants. Here, the combined toxicity of sodium fluoride (NaF), a stimulator with stimulation rate >100%, and six compounds that cannot induce hormesis (four ionic liquids (ILs) and two pesticides) were studied. The time-dependent toxicity of each toxicant on Vibrio qinghaiensis sp.-Q67 was investigated at 0.25, 2, 4, 6, 8, 10 and 12 h. Results showed that four ILs and two pesticides failed to induce hormesis, while NaF induced hormesis from 2 to 6 h and induced stimulation only after 6 h and reached its maximum (650%) at 12 h. All mixture rays with NaF induced hormesis at different times. In the four NaF-IL mixture systems, the absolute value of maximum stimulation demonstrated an upwards and then a downwards trend with the increasing of mixture ratio of IL. In two NaF-pesticide systems, the maximum stimulation effect declined with the increasing of the mixture ratio of pesticide. The toxicities of the mixture were successfully predicted by the improved inverse distance weighted interpolation, which are not able to be predicted by the commonly used concentration addition or independent action models. This paper shed lights on evaluating the hormesis of mixtures and the ecological risk of fluoride.

Dynamic interaction processes of rare earth metal mixtures in terrestrial organisms interpreted by toxicokinetic and toxicodynamic model

Despite the progress in explanation of mixture toxicity of rare earth elements (REEs), a large knowledge gap still exists in interpreting their mixed effects from a dynamic perspective. Here, we investigated the effects of La-Ce mixtures in Enchytraeus crypticus at different exposure times. The single and mixture toxicity of La and Ce increased with time, as reflected by the reduced LC50/MT50 values. With concentration addition as the reference model, the interactions between La and Ce were quantified by MIXTOX modelling tool, showing a time-dependent pattern with antagonistic effect after 1 and 2 d but additive effects afterwards. The dynamic accumulation and toxicity of La/Ce in organisms exposed to REE mixtures was fitted using a process-based toxicokinetic and toxicodynamic (TK-TD) model to unravel how the elements interacted. Generally, the estimated uptake, elimination, and damage rate constants of La/Ce declined with increasing level of each other, suggesting inhibited uptake and subsequently reduced toxicity of La/Ce due to competition effect. The interplay of La and Ce in TK and TD processes seemed responsible for the observed antagonism. Our study showed that mixture toxicity and interaction of REEs are time-dependent processes and application of TK-TD model may provide more insight into this dynamic effect.

Temperature dependence of population responses to competition and metabolic stress: An agent-based model to inform ecological risk assessment in a changing climate

Understanding the interactions among multiple stressors is a crucial issue for ecological risk assessment and ecosystem management. However, it is often impractical, or impossible, to collect empirical data concerning all the interactions at any scale because the type of interaction differs across species and levels of biological organization. We applied an agent-based model to simulate the effects of a hypothetical chemical stressor and inter-specific competition (both alone and together) on greenback cutthroat trout (GCT), a listed species under the US Endangered Species Act, in two temperature scenarios. The trout life cycle is modeled using the Dynamic Energy Budget theory. The chemical stressor is represented by a reduction in ingestion efficiency, and competition is implemented by introducing a population of brown trout. Results show that chemical exposure is the major stressor in the colder temperature scenario, whereas competition mostly affected the GCT population in the warmer environment. Moreover, the effects of the stressors at the individual level were not predictive of the type of interactions between stressors (additive, antagonistic, synergistic) at the population level, which differed between the two-temperature scenarios. We conclude that mechanistic models can help to identify generalities about interactions among environmental and stressor properties, create in-silico experiments to provide different scenarios for conservation purposes, and explore multiple-exposure consequences at higher levels of biological organization. In this way they can provide useful tools for improving ecological risk assessment and informing management decisions.

Stoichiometric Ecotoxicology for a Multisubstance World

Nutritional and contaminant stressors influence organismal physiology, trophic interactions, community structure, and ecosystem-level processes; however, the interactions between toxicity and elemental imbalance in food resources have been examined in only a few ecotoxicity studies. Integrating well-developed ecological theories that cross all levels of biological organization can enhance our understanding of ecotoxicology. In the present article, we underline the opportunity to couple concepts and approaches used in the theory of ecological stoichiometry (ES) to ask ecotoxicological questions and introduce stoichiometric ecotoxicology, a subfield in ecology that examines how contaminant stress, nutrient supply, and elemental constraints interact throughout all levels of biological organization. This conceptual framework unifying ecotoxicology with ES offers potential for both empirical and theoretical studies to deepen our mechanistic understanding of the adverse outcomes of chemicals across ecological scales and improve the predictive powers of ecotoxicology.

Sublethal effect modelling for environmental risk assessment of chemicals: Problem definition, model variants, application and challenges

Bioenergetic models, and specifically dynamic energy budget (DEB) theory, are gathering a great deal of interest as a tool to predict the effects of realistically variable exposure to toxicants over time on an individual animal. Here we use aquatic ecological risk assessment (ERA) as the context for a review of the different model variants within DEB and the closely related DEBkiss theory (incl. reserves, ageing, size & maturity, starvation). We propose a coherent and unifying naming scheme for all current major DEB variants, explore the implications of each model's underlying assumptions in terms of its capability and complexity and analyse differences between the models (endpoints, mathematical differences, physiological modes of action). The results imply a hierarchy of model complexity which could be used to guide the implementation of simplified model variants. We provide a decision tree to support matching the simplest suitable model to a given research or regulatory question. We detail which new insights can be gained by using DEB in toxicokinetic-toxicodynamic modelling, both generally and for the specific example of ERA, and highlight open questions. Specifically, we outline a moving time window approach to assess time-variable exposure concentrations and discuss how to account for cross-generational exposure. Where possible, we suggest valuable topics for experimental and theoretical research.

Effects of polystyrene diet on Tenebrio molitor larval growth, development and survival: Dynamic Energy Budget (DEB) model analysis

The presence of polystyrene (PS) waste increases constantly. Styrofoam, the most popular form of PS, is one of the major plastic pollutants in the environment. An efficient and environmentally friendly method of PS recycling is still needed. The biodegradation of PS by insects has been presented by researchers as a promising alternative to chemical, mechanical and thermal methods. The main aim of this study was to assess the survival, growth, and development of yellow mealworms (the larvae of Tenebrio molitor) fed with PS to determine if the insects are able to use PS as a source of mass and energy. The Dynamic Energy Budget (DEB) model was used to analyze the effects of food type on the growth trajectory and metabolism of tested organisms. We investigated five possible modes of influence of PS diet on DEB model parameters including a decrease of food availability, an increase in somatic maintenance power, an increase in costs for structure, allocation of energy, and a decrease in somatic maintenance power. Our results show that changes in the development of larvae fed with PS are mainly caused by a decrease in reserves density and reaction of the organism to the insufficient food supply. The inability or difficulty in completing the life cycle of T. molitor larvae fed with PS raises doubts about the use of mealworms as an effective technology for utilizing polystyrene.

An energy-based model to analyze growth data of earthworms exposed to two fungicides

The pesticide risk assessment for earthworms is currently performed using standardized tests, the model species Eisenia fetida, and the analyses of the data obtained are performed with ad hoc statistical tools. We assessed the impact of two fungicides on the entire growth pattern of the earthworm species Aporrectodea caliginosa, which is highly representative of agricultural fields. Individuals of three different ages (from hatching to 56 days old) were exposed to Cuprafor micro® (copper oxychloride) and Swing® Gold (dimoxystrobin and epoxiconazole). Data were analyzed with an energy-based toxicodynamic model coupled with a toxicokinetic model. The copper fungicide caused a drastic growth inhibition once the no effect concentration (NEC), estimated at 65 mg kg^-1 of copper, was exceeded. The Swing® Gold negatively affected the growth with NEC values estimated at 0.387 mg kg^-1 and 0.128 mg kg^-1 for the dimoxystrobin and the epoxiconazole in this fungicide formulation, respectively. The time-profile of the effects on A. caliginosa individuals was fully accounted for by the model, whatever their age of exposure. Furthermore, toxicity data analyses, supported by measurements of fungicide concentrations in earthworm at the end of the experiment, allowed bettering understanding of the mechanisms of action of the fungicides towards earthworm growth.

Caenorhabditis elegans As a Promising Alternative Model for Environmental Chemical Mixture Effect Assessment-A Comparative Study

A key challenge of mixture toxicity testing is that a multitude of substances with even more combinations need to be tested in a broad dose range. Consequently testing in rodent bioassays, the current gold standard of toxicity testing, is hardly feasible. High-throughput compatible cell culture systems, however, suffer from limitations with respect to toxicokinetics, tissue interactions, and compensatory mechanisms. Therefore, simple organisms like the nematode Caenorhabditis elegans, combining relevant advantages of complex in vivo and fast in vitro assays might prove highly valuable within a testing strategy for mixtures. To investigate the comparability between results obtained with C. elegans and traditional rodent assays, we used five azole fungicides as well investigated model substances. Our findings suggest that azoles act additively in C. elegans which is in line with previous results in rats. Additionally, we show that toxicokinetics are one important factor for the differences in the relative toxicity of the azoles in both species. Importantly, we also demonstrate that in contrast to most rodent in vivo studies, C. elegans assays provide well-defined concentration-response relationships which are a very good basis for the prediction of mixture effects. We conclude that C. elegans may be an appropriate model for mixture toxicity testing at least within a first step to identify and prioritize relevant mixtures for further testing.

The use of gene expression to unravel the single and mixture toxicity of abamectin and difenoconazole on survival and reproduction of the springtail Folsomia candida

Pesticides risk assessments have traditionally focused on the effects on standard parameters, such as mortality, reproduction and development. However, one of the first signs of adverse effects that occur in organisms exposed to stress conditions is an alteration in their genomic expression, which is specific to the type of stress, sensitive to very low contaminant concentrations and responsive in a few hours. The aim of the present study was to evaluate the single and binary mixture toxicity of commercial products of abamectin (Kraft^® 36 EC) and difenoconazole (Score^® 250 EC) to Folsomia candida. Laboratory toxicity tests were conducted to access the effects of these pesticides on springtail survival, reproduction and gene expression. The reproduction assays gave EC₅₀ and EC₁₀ values, respectively, of 6.3 and 1.4 mg a.s./kg dry soil for abamectin; 1.0 and 0.12 mg a.s./kg dry soil for Kraft^® 36 EC; and 54 and 23 mg a.s./kg dry soil for Score^® 250 EC. Technical difenoconazole did not have any effect at the concentrations tested. No significant differences in gene expression were found between the abamectin concentrations tested (EC₁₀ and EC₅₀) and the solvent control. Exposure to Kraft^® 36 EC, however, significantly induced Cyp6 expression at the EC₅₀ level, while VgR was significantly downregulated at both the EC₁₀ and EC₅₀. Exposure to the simple pesticide mixture of Kraft^® 36 EC + Score^® 250 EC caused significant up regulation of ABC transporter, and significant down regulation of VgR relative to the controls. GABA receptor also showed significant down-regulation between the EC₁₀ and EC₅₀ mixture treatments. Results of the present study demonstrate that pesticide-induced gene expression effects precede and occur at lower concentrations than organism-level responses. Integrating "omic" endpoints in traditional bioassays may thus be a promising way forward in pesticide toxicity evaluations.

Lethal and sublethal toxicity of abamectin and difenoconazole (individually and in mixture) to early life stages of zebrafish

In recent years, the need for the development of alternative test methods for the conventional acute fish toxicity test (AFT) with adult fish has often been discussed. In addition, concerns have been raised on the potential risks related with environmentally realistic pesticide mixtures since risk evaluations have traditionally been based on individual pesticides. The insecticide/acaricide abamectin and the fungicide difenoconazole are the main pesticides that are intensively used in Brazilian strawberry crop and are hence likely to occur simultaneously in edge-of-field waterbodies. The aim of the present study was therefore to evaluate the lethal and sublethal toxicity of single and mixture exposures of these pesticides to zebrafish early life stages (embryos and juveniles). By comparing the derived toxicity data of the individual compounds with that previously determined for zebrafish adults, the order of life stage sensitivity was juvenile > adult > embryo. The pesticide mixture revealed a dose-level dependent deviation of the independent action model, with antagonism at low dose levels and synergism at high dose levels. Sublethal parameters (especially those related with locomotion) were considerably more sensitive than lethality. Subsequently, the inclusion of sublethal parameters may greatly improve the sensitivity of FET tests and hence its suitability as a substitution of adult fish testing in risk assessment evaluations.

Sensitivities of three tropical indigenous freshwater invertebrates to single and mixture exposures of diuron and carbofuran and their commercial formulations

As compared to their temperate counterparts, few toxicity tests have been conducted so far into the evaluation of the sensitivity of indigenous tropical species to pesticides. Especially mixture toxicity assessments appear to be scarce. To contribute to increase our knowledge in this arena, we evaluated the acute toxicity of diuron and carbofuran and their mixtures to the neotropical oligochaetes Allonais inaequalis and Dero furcatus, and the ostracod Strandesia trispinosa. Tests were performed with both the pure active ingredients, as well as their formulated products. The toxicity of the latter to the three test organisms was generally greater than that of the pure active ingredients, although absolute differences were rather small. The sensitivity of the indigenous species was slightly greater than temperate test species from the same taxonomic groups. The concentration addition conceptual model best described the results of the mixture toxicity data. Derived deviations of this model appeared to be dependent on the test organism and as to whether the pesticides were applied as active ingredients or their commercial products. Reported field concentrations of the two pesticides indicate risks to freshwater biota, especially if they are both present. The test species used in the present study are concluded to be suitable candidates as surrogate test organisms in local pesticide risk evaluations.

Dynamic energy budget models in ecological risk assessment: From principles to applications

In ecological risk assessment of chemicals, hazard identification and hazard characterisation are most often based on ecotoxicological tests and expressed as summary statistics such as No Observed Effect Concentrations or Lethal Concentration values and No Effect Concentrations. Considerable research is currently ongoing to further improve methodologies to take into account toxico kinetic aspects in toxicological assessments, extrapolations of toxic effects observed on individuals to population effects and combined effects of multiple chemicals effects. In this context, the principles of the Dynamic Energy Budget (DEB), namely the conserved allocation of energy to different life-supporting processes in a wide variety of different species, have been applied successfully to the development of a number of DEB models. DEB models allow the incorporation of effects on growth, reproduction and survival within one consistent framework. This review aims to discuss the principles of the DEB theory together with available DEB models, databases available and applications in ecological risk assessment of chemicals for a wide range of species and taxa. Future perspectives are also discussed with particular emphasis on ongoing research efforts to develop DEB models as open source tools to further support the research and regulatory community to integrate quantitative biology in ecotoxicological risk assessment.

Physiological modes of action across species and toxicants: the key to predictive ecotoxicology

As ecotoxicologists we strive for a better understanding of how chemicals affect our environment. Humanity needs tools to identify those combinations of man-made chemicals and organisms most likely to cause problems. In other words: which of the millions of species are at risk from pollution? And which of the tens of thousands of chemicals contribute most to the risk? We identified our poor knowledge on physiological modes of action (how a chemical affects the energy allocation in an organism), and how they vary across species and toxicants, as a major knowledge gap. We also find that the key to predictive ecotoxicology is the systematic, rigorous characterization of physiological modes of action because that will enable more powerful in vitro to in vivo toxicity extrapolation and in silico ecotoxicology. In the near future, we expect a step change in our ability to study physiological modes of action by improved, and partially automated, experimental methods. Once we have populated the matrix of species and toxicants with sufficient physiological mode of action data we can look for patterns, and from those patterns infer general rules, theory and models.

Single and mixture toxicity of abamectin and difenoconazole to adult zebrafish (Danio rerio)

Concerns have been raised in recent years on the potential risks related with pesticide mixtures that are likely to be present in agricultural edge-of-field waterbodies. Despite the high use of pesticides in tropical countries like Brazil, studies evaluating pesticide mixtures are especially scarce in the tropics. The insecticide abamectin and the fungicide difenoconazole are the main pesticides intensively used in Brazilian strawberry crop and are hence likely to occur simultaneously. The aim of the present study was therefore to evaluate the toxicity of abamectin, difenoconazole and their mixture to the tropical fish Danio rerio. Laboratory toxicity tests with the individual pesticides indicated 48 h-LC₅₀ values of 59 μg L^-1 for abamectin and 1.4 mg L^-1 for difenoconazole. Mixtures of the two pesticides revealed a synergistic deviation of the independent action model. Implications of study findings for the aquatic risk assessment of pesticide mixtures, especially in tropical countries and indications for future research are discussed.

Toxicity-based toxicokinetic/toxicodynamic assessment of bioaccumulation and nanotoxicity of zerovalent iron nanoparticles in Caenorhabditis elegans

Elucidating the relationships between the toxicity-based-toxicokinetic (TBTK)/toxicodynamic (TD) properties of engineered nanomaterials and their nanotoxicity is crucial for human health-risk analysis. Zerovalent iron (Fe⁰) nanoparticles (NPs) are one of the most prominent NPs applied in remediating contaminated soils and groundwater. However, there are concerns that Fe⁰NP application contributes to long-term environmental and human health impacts. The nematode Caenorhabditis elegans is a surrogate in vivo model that has been successfully applied to assess the potential nanotoxicity of these nanomaterials. Here we present a TBTK/TD approach to appraise bioaccumulation and nanotoxicity of Fe⁰NPs in C. elegans. Built on a present C. elegans bioassay with estimated TBTK/TD parameters, we found that average bioconcentration factors in C. elegans exposed to waterborne and food-borne Fe⁰NPs were ~50 and ~5×10^-3, respectively, whereas 10% inhibition concentrations for fertility, locomotion, and development, were 1.26 (95% CI 0.19-5.2), 3.84 (0.38-42), and 6.78 (2.58-21) μg·g^-1, respectively, implicating that fertility is the most sensitive endpoint in C. elegans. Our results also showed that biomagnification effects were not observed in waterborne or food-borne Fe⁰NP-exposed worms. We suggest that the TBTK/TD assessment for predicting NP-induced toxicity at different concentrations and conditions in C. elegans could enable rapid selection of nanomaterials that are more likely to be nontoxic in larger animals. We conclude that the use of the TBTK/TD scheme manipulating C. elegans could be used for rapid evaluation of in vivo toxicity of NPs or for drug screening in the field of nanomedicine.

The time-dependent synergism of the six-component mixtures of substituted phenols, pesticides and ionic liquids to Caenorhabditis elegans

Traditional environmental risk assessment rarely focused on exposures to multi-component mixtures which may cause toxicological interactions and usually ignored that toxicity is a process in time, which may underestimate the environment risk of mixtures. In this paper, six chemicals belonging to three categories, two substituted phenols, two pesticides and two Ionic liquids, were picked to construct a six-component mixture system. To systematically examine the effects of various concentration compositions, the uniform design ray method was employed to design nine mixture rays with nine mixture ratios and for every mixture ray 12 concentration levels were specified by the fixed ratio ray design. The improved combination index was used to evaluate the combined toxicities of the mixtures to Caenorhabditis elegans (C. elegans) in the exposure times of 6, 12 and 24h. It was shown that the mixture rays display time-dependent synergism, i.e. the range of synergistic effect narrows and the strength of synergism runs down with exposure time, which illustrates that the mixture toxicity of some chemicals is not a sum of individual toxicities at some exposure times and it is necessary to consider the toxicological interaction in mixtures.

Toxicokinetic models and related tools in environmental risk assessment of chemicals

Environmental risk assessment of chemicals for the protection of ecosystems integrity is a key regulatory and scientific research field which is undergoing constant development in modelling approaches and harmonisation with human risk assessment. This review focuses on state-of-the-art toxicokinetic tools and models that have been applied to terrestrial and aquatic species relevant to environmental risk assessment of chemicals. Both empirical and mechanistic toxicokinetic models are discussed using the results of extensive literature searches together with tools and software for their calibration and an overview of applications in environmental risk assessment. These include simple tools such as one-compartment models, multi-compartment models to physiologically-based toxicokinetic (PBTK) models, mostly available for aquatic species such as fish species and a number of chemical classes including plant protection products, metals, persistent organic pollutants, nanoparticles. Data gaps and further research needs are highlighted.

Extending standard testing period in honeybees to predict lifespan impacts of pesticides and heavy metals using dynamic energy budget modelling

Concern over reported honeybee (Apis mellifera spp.) losses has highlighted chemical exposure as a risk. Current laboratory oral toxicity tests in A. mellifera spp. use short-term, maximum 96 hour, exposures which may not necessarily account for chronic and cumulative toxicity. Here, we use extended 240 hour (10 day) exposures to examine seven agrochemicals and trace environmental pollutant toxicities for adult honeybees. Data were used to parameterise a dynamic energy budget model (DEBtox) to further examine potential survival effects up to 30 day and 90 day summer and winter worker lifespans. Honeybees were most sensitive to insecticides (clothianidin > dimethoate ≫ tau-fluvalinate), then trace metals/metalloids (cadmium, arsenic), followed by the fungicide propiconazole and herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). LC₅₀s calculated from DEBtox parameters indicated a 27 fold change comparing exposure from 48 to 720 hours (summer worker lifespan) for cadmium, as the most time-dependent chemical as driven by slow toxicokinetics. Clothianidin and dimethoate exhibited more rapid toxicokinetics with 48 to 720 hour LC₅₀s changes of <4 fold. As effects from long-term exposure may exceed those measured in short-term tests, future regulatory tests should extend to 96 hours as standard, with extension to 240 hour exposures further improving realism.

Integrated presentation of ecological risk from multiple stressors

Current environmental risk assessments (ERA) do not account explicitly for ecological factors (e.g. species composition, temperature or food availability) and multiple stressors. Assessing mixtures of chemical and ecological stressors is needed as well as accounting for variability in environmental conditions and uncertainty of data and models. Here we propose a novel probabilistic ERA framework to overcome these limitations, which focusses on visualising assessment outcomes by construct-ing and interpreting prevalence plots as a quantitative prediction of risk. Key components include environmental scenarios that integrate exposure and ecology, and ecological modelling of relevant endpoints to assess the effect of a combination of stressors. Our illustrative results demonstrate the importance of regional differences in environmental conditions and the confounding interactions of stressors. Using this framework and prevalence plots provides a risk-based approach that combines risk assessment and risk management in a meaningful way and presents a truly mechanistic alternative to the threshold approach. Even whilst research continues to improve the underlying models and data, regulators and decision makers can already use the framework and prevalence plots. The integration of multiple stressors, environmental conditions and variability makes ERA more relevant and realistic.

Insights into zinc and cadmium biology in the nematode Caenorhabditis elegans

Zinc is an essential metal that is involved in a wide range of biological processes, and aberrant zinc homeostasis is implicated in multiple human diseases. Cadmium is chemically similar to zinc, but it is a nonessential environmental pollutant. Because zinc deficiency and excess are deleterious, animals require homeostatic mechanisms to maintain zinc levels in response to dietary fluctuations. The nematode Caenorhabditis elegans is emerging as a powerful model system to investigate zinc trafficking and homeostasis as well as cadmium toxicity. Here we review genetic and molecular studies that have combined to generate a picture of zinc homeostasis based on the transcriptional control of zinc transporters in intestinal cells. Furthermore, we summarize studies of cadmium toxicity that reveal intriguing parallels with zinc biology.

Feedbacks and tipping points in organismal response to oxidative stress

Biological feedbacks play a crucial role in determining effects of toxicants, radiation, and other environmental stressors on organisms. Focusing on reactive oxygen species (ROS) that are increasingly recognized as a crucial mediator of many stressor effects, we investigate how feedback strength affects the ability of organisms to control negative effects of exposure. We do this by developing a general theoretical framework for describing effects of a wide range of stressors and species. The framework accounts for positive and negative feedbacks representing cellular processes: (i) production of ROS due to metabolism and the stressor, (ii) ROS reactions with cellular compounds that cause damage, and (iii) cellular control of both ROS and damage. We suggest functional forms that capture generic properties of cellular control mechanisms constituting the feedbacks, assess stability of equilibrium states in the resulting models, and investigate tipping points at which cellular control breaks down causing unregulated increase of ROS and damage. Depending on the chosen functional forms, the models can have zero, one, or two positive steady states; except in one singular case, the steady state with lowest values of ROS and damage is locally stable. We found two types of tipping points: those induced by changes in the parameters (including the stressor intensity), and those induced by the history of exposure, i.e. ROS and damage levels. The relatively simple models effectively describe several patterns of cellular responses to stress, such as the covariation of ROS and damage, the break-down of cellular control leading to explosion of ROS and/or damage, increase in damage even when ROS is (near)-constant, and the effects of exposure history on the ability of the cell to handle additional stress. The models quantify dynamics of cellular control, and could therefore be used to estimate the metabolic costs of stress and help integrate them into models that use energetic considerations to model organism's response to the environment. Although developed with unicellular organisms in mind, our models can be applied to all multicellular organisms that utilize similar feedbacks when dealing with stress.

Dynamic energy-based modeling of uranium and cadmium joint toxicity to Caenorhabditis elegans

Toxicokinetic - toxicodynamic energy-based models offer new alternatives to the commonly used approaches for the analysis of mixture toxicity data. Based on the Dynamic Energy Budget theory, DEBtox models enable the description of several endpoints over time simultaneously under the same framework. However, such model still has to be faced with experimental data in a multi-contamination context. In this study, the predictive capacities of a DEBtox model to describe the uranium and cadmium joint toxicity over the entire growth and reproduction period of the soil nematode Caenorhabditis elegans was examined. The two reference additivity approaches, Concentration Addition and Response addition, implemented in the DEBtox model were tested. Assuming no interaction between the two toxicants through Response addition, the DEBtox model allowed a rather accurate fit of the U and Cd joint effects on the growth and reproduction of C. elegans: an interaction between the two metals at the toxicokinetic or toxicodynamic level seems thus unlikely or has only minor consequences. Interestingly, this study underlines that even if the compounds of a mixture share the same DEBtox physiological mode of action (in this case a decrease in assimilation), the Response addition approach may provide a better fit of joint toxicity data than the Concentration addition approach. Moreover, the present work highlighted limitations in the model predictions which are related to the simplifications of the DEBtox framework and its adaptations to the physiology of C. elegans and which lead to an overestimation of the U and Cd joint toxicity in some cases.

Variable Temperature Stress in the Nematode Caenorhabditis elegans (Maupas) and Its Implications for Sensitivity to an Additional Chemical Stressor

A wealth of studies has investigated how chemical sensitivity is affected by temperature, however, almost always under different constant rather than more realistic fluctuating regimes. Here we compared how the nematode Caenorhabditis elegans responds to copper at constant temperatures (8-24°C) and under fluctuation conditions of low (±4°C) and high (±8°C) amplitude (averages of 12, 16, 20°C and 16°C respectively). The DEBkiss model was used to interpret effects on energy budgets. Increasing constant temperature from 12-24°C reduced time to first egg, life-span and population growth rates consistent with temperature driven metabolic rate change. Responses at 8°C did not, however, accord with this pattern (including a deviation from the Temperature Size Rule), identifying a cold stress effect. High amplitude variation and low amplitude variation around a mean temperature of 12°C impacted reproduction and body size compared to nematodes kept at the matching average constant temperatures. Copper exposure affected reproduction, body size and life-span and consequently population growth. Sensitivity to copper (EC50 values), was similar at intermediate temperatures (12, 16, 20°C) and higher at 24°C and especially the innately stressful 8°C condition. Temperature variation did not increase copper sensitivity. Indeed under variable conditions including time at the stressful 8°C condition, sensitivity was reduced. DEBkiss identified increased maintenance costs and increased assimilation as possible mechanisms for cold and higher copper concentration effects. Model analysis of combined variable temperature effects, however, demonstrated no additional joint stressor response. Hence, concerns that exposure to temperature fluctuations may sensitise species to co-stressor effects seem unfounded in this case.

Death Dilemma and Organism Recovery in Ecotoxicology

Why do some individuals survive after exposure to chemicals while others die? Either, the tolerance threshold is distributed among the individuals in a population, and its exceedance leads to certain death, or all individuals share the same threshold above which death occurs stochastically. The previously published General Unified Threshold model of Survival (GUTS) established a mathematical relationship between the two assumptions. According to this model stochastic death would result in systematically faster compensation and damage repair mechanisms than individual tolerance. Thus, we face a circular conclusion dilemma because inference about the death mechanism is inherently linked to the speed of damage recovery. We provide empirical evidence that the stochastic death model consistently infers much faster toxicodynamic recovery than the individual tolerance model. Survival data can be explained by either, slower damage recovery and a wider individual tolerance distribution, or faster damage recovery paired with a narrow tolerance distribution. The toxicodynamic model parameters exhibited meaningful patterns in chemical space, which is why we suggest toxicodynamic model parameters as novel phenotypic anchors for in vitro to in vivo toxicity extrapolation. GUTS appears to be a promising refinement of traditional survival curve analysis and dose response models.

Nested interactions in the combined toxicity of uranium and cadmium to the nematode Caenorhabditis elegans

Uranium is a natural, ubiquitous radioactive element for which elevated concentrations can be found in the vicinity of some nuclear fuel cycle facilities or intensive farming areas, and most often in mixtures with other contaminants such as cadmium, due to co-occurrence in geological ores (e.g. U- or P-ore). The study of their combined effects on ecosystems is of interest to better characterize such multi-metallic polluted sites. In the present study, the toxicity of binary mixture of U and Cd on physiological parameters of the soil nematode Caenorhabditis elegans was assessed over time. Descriptive modeling using concentration and response addition reference models was applied to compare observed and expected combined effects and identify possible synergistic or antagonistic interactions. A strong antagonism between U and Cd was identified for length increase and brood size endpoints. The study revealed that the combined effects might be explained by two nested antagonistic interactions. We demonstrate that the first interaction occurred in the exposure medium. We also identified a significant second antagonistic interaction which occurred either during the toxicokinetic or toxicodynamic steps. These findings underline the complexity of interactions that may take place between chemicals and thus, highlight the importance of studying mixtures at various levels to fully understand underlying mechanisms.