Toxicokinetic and toxicodynamic modeling explains carry-over toxicity from exposure to diazinon by slow organism recovery

Distribution, risk evaluation, and source allocation of cesium and strontium in surface soil in a mining city

Radioactive nuclides cesium (Cs) and strontium (Sr) possess long half-lives, with 135Cs at approximately 2.3 million years and 87Sr at about 49 billion years. Their persistent accumulation can result in long-lasting radioactive contamination of soil ecosystems. This study employed geo-accumulation index (Igeo), pollution load index (PLI), potential ecological risk index (PEPI), health risk assessment model (HRA), and Monte Carlo simulation to evaluate the pollution and health risks of Cs and Sr in the surface soil of different functional areas in a typical mining city in China. Positive matrix factorization (PMF) model was used to elucidate the potential sources of Cs and Sr and the respective contribution rates of natural and anthropogenic sources. The findings indicate that soils in the mining area exhibited significantly higher levels of Cs and Sr pollution compared to smelting factory area, agricultural area, and urban residential area. Strontium did not pose a potential ecological risk in any studied functional area. The non-carcinogenic health risk of Sr to the human body in the study area was relatively low. Because of the lack of parameters for Cs, the potential ecological and human health risks of Cs was not calculated. The primary source of Cs in the soil was identified as the parent material from which the soil developed, while Sr mainly originated from associated contamination caused by mining activities. This research provides data for the control of Cs and Sr pollution in the surface soil of mining city.

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.

Behavioral toxicity, histopathological alterations and oxidative stress in Tubifex tubifex exposed to aromatic carboxylic acids- acetic acid and benzoic acid: A comparative time-dependent toxicity assessment

This study evaluated Acetic acid (AA) and Benzoic acid's (BA) acute and sublethal toxicity by observing mortality, behavioral responses, and changes in the levels of oxidative stress enzymes in Tubifex tubifex. Exposure-induced changes in antioxidant activity (Catalase, Superoxide dismutase), oxidative stress (Malondialdehyde concentrations), and histopathological alterations in the tubificid worms were also noted across exposure intervals. The 96 h LC₅₀ values of AA and BA to T. tubifex were 74.99 and 37.15 mg/l, respectively. Severity in behavioral alterations (including increased mucus production, wrinkling, and reduction in clumping) and autotomy showed concentration-dependent trends for both toxicants. Although histopathological effects also showed marked degeneration in the alimentary and integumentary systems in highest exposure groups (worms exposed to 14.99 mg/l for AA and 7.42 mg/l for BA) for both toxicants. Antioxidant enzymes (catalase and superoxide dismutase) also showed a marked increase of up to 8-fold and 10-fold for the highest exposure group of AA and BA respectively. While species sensitivity distribution analysis revealed T. tubifex as most sensitive to AA and BA compared to other freshwater vertebrates and invertebrates, General Unified Threshold model of Survival (GUTS) predicted individual tolerance effects (GUTS-IT), with slower potential for toxicodynamic recovery, as a more likely pathway for population mortality. Study findings demonstrate BA with greater potential for ecological effects compared to AA within 24 h of exposure. Furthermore, ecological risks to critical detritus feeders like T. tubifex may have severe implications for ecosystem services and nutrient availability within freshwater habitats.

Modeling integrated biomarker response (IBR) index for the mussel Mytilus galloprovincialis (Lamark 1819) exposed to heavy metal mixture using the CCF design

Marine pollutants such as heavy metals (HMs) are considered among the most copious oxidative stress (OS) inducers in marine organisms which leads to reactive oxygen species (ROS) formation. Complementary to our previous bioassays studies, the present research focuses on Catalase (CAT), Glutathione S-transferase (GST) and Malondialdehyde (MDA) as oxidative stress biomarkers and the integrated biomarker response (IBR) indexes (IBR₁ and IBR₂) as an ecotoxicological assessment tool in Mytilus galloprovincialis using central composite face centered (CCF) design. The oxidative stress biomarkers were measured in adult mussels (45-55 mm) on 3 days-exposed under different sub-lethal concentrations of cadmium (Cd), zinc (Zn), and copper (Cu). Using multiple regressions, ANOVA analysis revealed that experimental data fitted to second-order (quadratic) polynomial equations. The results showed that types, concentrations and metals combinations has a direct effect in CAT and GST activities, MDA level and IBR indexes. Additionally, metal-metal interactions were found synergistic (supra-additive), antagonistic (infra-additive) or zero interaction in the toxicological effect. As necessary, the optimization of the experimental results was done in order to determine the optimal conditions for the oxidative stress responses and IBR indexes. It was demonstrated that the CCF design combined with the multi-biomarker approach and IBR index can be used as an appropriate tool in ecotoxicological modulation and prediction of oxidative stress and antioxidant status by heavy metals in the mussels Mytilus galloprovincialis.

Assessment toxic effects of exposure to 3-indoleacetic acid via hemato-biochemical, hormonal, and histopathological screening in rats

The current study purposed to investigate the 3-indoleacetic acid (IAA) possible adverse impacts on hematological parameters, hepatorenal function, cardiac, and skeletal muscles as well as testes of rats and histopathological alterations of respective organs and to determine the extent of reversing any adverse impacts occurred in animals after IAA withdrawal. Rats were exposed orally to 500 mg/kg BW by gastric intubation once daily for 14 days, after which one-half was sacrificed and the remaining half left for a further 14 days without IAA exposure. The exposure of rats to IAA produced anemia, leukopenia, neutrophilia, lymphopenia, and a significant increase in activities of serum transaminase, gamma-glutamyl transferase, creatine kinase-myocardial band, creatine kinase-muscle type, and levels of serum creatinine, sodium, chloride, and potassium. Furthermore, serum levels of testosterone, gonadotropins, and leptin significantly declined. The changes in most of measured parameters continued after IAA withdrawal. Histopathological alterations in different tissues supported these changes. In conclusion, subacute exposure to IAA at a high concentration could exert hematotoxicity and toxic effects on many soft organs and its withdrawal led to incomplete recovery of animals. Thus, IAA should be used cautiously as extensive use of it at high concentrations can cause harmful effects on the environment, animals and human beings.

The Capacity of Freshwater Ecosystems to Recover from Exceedences of Aquatic Life Criteria

In the United States, national chemical water quality criteria for the protection of aquatic life assume that aquatic ecosystems have sufficient resiliency to recover from criteria exceedences occurring up to once every 3 years. This resiliency assumption was critically reviewed through two approaches: (1) synthesis of case studies, and (2) population modeling. The population modeling examined differences in recovery of species with widely different life histories. One invertebrate (Hyalella azteca) and four fish species were modeled (fathead minnow, brook trout, lake trout, and shortnose sturgeon) with various disturbance magnitudes and intervals. The synthesis of ecosystem case studies showed generally faster recoveries for insect communities rather than fish, and recoveries from pulse (acute) disturbances were often faster than recoveries from press (chronic) disturbances. When the recovery dataset excluded severe disturbances that seemed unrepresentative of common facility discharge upsets that might cause criteria exceedences, the median recovery time was 1 year, 81% of the cases were considered recovered within 3 years, and 95% were considered recovered within 10 years. The modeling projected that short-lived fish species with high recovery times could thrive despite enduring 50% mortality disturbances every other year. However, long-lived fish species had longer recovery times and declined under the one disturbance every 3 years scenario. Overall, the analyses did not refute the long-standing judgements that 3 years is generally sufficient for recovery from nonrepetitive, moderate intensity disturbances of a magnitude up to 2× the chronic criteria in waters without other pollution sources or stresses. However, these constraints may not always be met and if long-lived fish species are a concern, longer return intervals such as 5-10 years could be indicated. Environ Toxicol Chem 2022;41:2887-2910. Published 2022. This article is a U.S. Government work and is in the public domain in the USA.

Mechanistic insights to lactic and formic acid toxicity on benthic oligochaete worm Tubifex tubifex

Lactic and formic acid are two commonly found monocarboxylic organic acids. Lactic acid is discharged into the water bodies as acidic industrial effluent from the food, cosmetic, chemical, and pharmaceutical industries, whereas formic acid is discharged from various paper, leather tanning, and textile processing industries. The present study investigated the toxicity of both organic acids upon the benthic oligochaete worm Tubifex tubifex. The 96-h median lethal concentration (LC₅₀) values for lactic and formic acid are determined as 143.81 mg/l and 57.99 mg/l respectively. The effects of two sublethal concentrations (10% and 30% of 96 h LC₅₀) of these acids on differential expression of oxidative stress enzymes are investigated. The comparative analysis of acute toxicity demonstrates that formic acid exposure is more detrimental to T. tubifex than lactic acid. The in silico structural analysis predicts that formic acid can interact with cytochrome c oxidase of the electron transport system and thereby inhibits its functionality and induces reactive oxygen species production. Integrated biomarker response (IBR) analysis illustrates that overall oxidative stress of formic acid to T. tubifex is significantly higher than that of lactic acid, which supports the structural analysis. It is concluded from this study that toxicokinetic-toxicodynamic and species sensitivity distributions studies are helpful for ecological risk management of environmental toxicants.

Sensitive and selective magnetic dispersive microextraction of diazinon from urine samples by molecularly imprinted polymer based on core-shell metal-organic frameworks

A core-shell magnetic metal-organic framework (Fe₃O₄ SiO₂/ PAEDTC@ MIL- 101 (Fe)) was synthesized as the substrate and then covered with a surface molecularly imprinted polymer (MIP) layer. Next, Fe₃O₄ SiO₂/ PAEDTC@ MIL- 101 (Fe) @ MIP was characterized by XRD, FT-IR, BET, VSM, TEM, and FE-SEM techniques and applied for selective, fast, and sensitive magnetic dispersive solid-phase microextraction (M-DµSPE) of diazinon from urine samples by the GC- FID detection method. The key experimental variables affecting M-DµSPE were studied and optimized by central composite design (CCD). Under optimum conditions (5 mL; sample at pH: 7.0, the mass of solid sorbent; 6 mg, extraction time; 4 min, acetonitrile as an eluent solvent; 1.5 mL, and desorption time; 3 min, and then reconstituted with 100 µL of methanol), the proposed method exhibits high sensitivity with limits of detection and quantification of 0.005 and 0.017 ng mL^-1, respectively. Excellent extraction recovery (98.5 %), wide linearity range (0.02-200000 ng mL^-1, R² > 0.992), high enrichment factors (47-53), and satisfactory precision (<6.3 % RSD) were achieved. The MIP- MOF@ M-DµSPE -GC-FID method can be used with high precision and wide linearity to extract and analyze trace levels of diazinon in real urine samples.

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.

Modelling chronic toxicokinetics and toxicodynamics of copper in mussels considering ionoregulatory homeostasis and oxidative stress

Chronic toxicity of copper (Cu) at sublethal levels is associated with ionoregulatory disturbance and oxidative stress. These factors were considered in a toxicokinetic-toxicodynamic model in the present study. The ionoregulatory disturbance was evaluated by the activity of the Na⁺/K⁺-ATPase enzyme (NKA), while oxidative stress was presented by lipid peroxidation (LPO) and glutathione-S-transferase (GST) activity. NKA activity was related to the binding of Cu²⁺ and Na ⁺ to NKA. LPO and GST activity were linked with the simulated concentration of unbound Cu. The model was calibrated using previously reported data and empirical data generated when zebra mussels were exposed to Cu. The model clearly demonstrated that Cu might inhibit NKA activity by reducing the number of functional pump sites and the limited Cu-bound NKA turnover rate. An ordinary differential equation was used to describe the relationship between the simulated concentration of unbound Cu and LPO/GST activity. Although this method could not explain the fluctuations in these biomarkers during the experiment, the measurements were within the confidence interval of estimations. Model simulation consistently shows non-significant differences in LPO and GST activity at two exposure levels, similar to the empirical observation.

Complex delayed and transgenerational effects driven by the interaction of heat and insecticide in the maternal generation of the wheat aphid, Sitobion avenae

BACKGROUND

Experience of an earlier environment plays an important role in the induction of delayed and even intergenerational phenotypes of an organism. Evidence suggests that rapid adaptation to an environmental stressor can change the performance of organisms, and even enable them to deal with other stressors. The goal of this study was to determine the effects of adult imidacloprid exposure on life-history traits within and between generations of the cereal aphid, Sitobion avenae, under three developmental conditions: constant temperature, 22°C; a low-intensity thermal condition, 22 + 34°C for 2 h per day; and a high-intensity thermal condition, 22 + 38°C for 2 h per day.

RESULTS

Early thermal experience not only changed the tolerance of S. avenae to the insecticide, imidacloprid, but also caused adults to incur fitness costs: the higher the heat intensity, the higher the costs. Negative transgenerational impacts of combined heat and insecticide stressors were limited to the developmental stage, whereas positive stimulation of heat intensity was observed during the adult stage. Overall, nymphal thermal experience exacerbated the detrimental effects of adult insecticidal exposure on the intrinsic rate of population increase in the maternal generation, but stimulated a net reproductive rate in the succeeding offspring generation.

CONCLUSION

These findings underpin the importance of considering the experience of the early developmental environment, but also enhance our understanding of the transgenerational effects of combined thermal and insecticide stressors on the population fate of S. avenae. They also help to assess the efficacy of chemical control in a warming world. © 2021 Society of Chemical Industry.

Robust Likelihood-Based Approach for Automated Optimization and Uncertainty Analysis of Toxicokinetic-Toxicodynamic Models

Toxicokinetic-toxicodynamic (TKTD) models offer a mechanistic understanding of individual-level toxicity over time and allow for meaningful extrapolations from laboratory tests to exposure conditions in the field. Thereby, they hold great potential for ecotoxicological studies, both in a regulatory context as well as for basic research. In contrast to mechanistic effect models at higher levels of biological organization, TKTD models can be, and generally are, parameterized by fitting them to data (results from toxicity tests). Fitting models comes with a range of statistical and numerical challenges, which may hamper the application of TKTD models in a practical setting. Especially in the context of environmental risk assessment, there is a need for robust and user-friendly software tools to automatically extract the best-fitting model parameters and quantify their uncertainty from any data set. The study presents a general outline for TKTD model analysis, rooted in likelihood-based ("frequentist") inference. The general outline is followed by a presentation of the specific algorithm that has been implemented into software for the robust and automated analysis of toxicity data for survival. However, the presented approach is more broadly applicable to low-dimensional problems. Integr Environ Assess Manag 2021;17:388-397. © 2020 SETAC.

High Tolerance and Delayed Responses of Daphnia magna to Neonicotinoid Insecticide Imidacloprid: Toxicokinetic and Toxicodynamic Modeling

Species sensitivity to neonicotinoids has been shown to be highly variable among aquatic invertebrates. Toxicokinetic and toxicodynamic (TKTD) models were constructed to mechanistically elucidate the susceptibility of Daphnia magna to imidacloprid. D. magna was highly tolerant to single short-term exposure to imidacloprid (96-h LC50 of 8.47 μg/mL), but delayed and carry-over toxicity occurred under repeated pulse exposures. Kinetic distribution of imidacloprid between exoskeleton and soft tissues of D. magna was evaluated using a newly developed method. Approximately 84% imidacloprid was distributed to soft tissues but was rapidly depurated from the tissue (t_1/2 of 1.2 h), resulting in low bioaccumulation and high tolerance. TKTD modeling also successfully simulated the survival of D. magna after pulsed exposures. The calculated recovery time was 45 d, indicating significant delayed and carry-over toxicity of the insecticide. While complete elimination of imidacloprid only took about 5 h (TK), slow damage recovery (45 d) caused slow organism recovery (TD). Consequently, although D. magna was tolerant to imidacloprid due to fast depuration from soft tissue, long damage recovery time significantly enhanced the toxicity under repeated pulse exposures. Our study highlights the necessity of integrating delayed and carry-over toxicity quantification in assessing the risk of neonicotinoids to aquatic invertebrates.

Quantifying the effect of nano-TiO2 on the toxicity of lead on C. dubia using a two-compartment modeling approach

Nanoparticles (NPs) can significantly influence toxicity imposed by toxic metals. However, this impact has not been quantified. In this research, we investigated the effect of nano-TiO₂ on lead (Pb) accumulation and the resultant toxicity using water flea Ceriodaphnia dubia (C. dubia) as the testing organism. We used a two-compartment modeling approach, which included a two-compartment accumulation model and a toxicodynamic model, on the basis of Pb body tissue accumulation, to quantify the impact of nano-TiO₂ on Pb toxicity. The effect of algae on the combined toxicity of Pb and nano-TiO₂ was also quantified. The two-compartment accumulation model could well quantify Pb accumulation kinetics in two-compartments of C. dubia, the gut and the rest of the body tissue in the presence of nano-TiO₂. Modeling results suggested that the gut quickly accumulates Pb through active uptake from the mouth, but the rest of the body tissue slowly accumulates Pb from the gut. The predicted Pb distribution within C. dubia was verified by depuration modeling results from an independent depuration test. The survivorship of C. dubia as a function of Pb accumulated in the body tissue and exposure time can be well described using a toxicodynamic model. The effects of algae on Pb accumulation in different compartments of C. dubia and the toxicity in the presence of nano-TiO₂ were also well described using the two-compartment modeling approach. Therefore, the novel two-compartment modeling approach provides a useful tool for assessing the effect of NPs on aquatic ecosystems where toxic metals are present.

Making the Biotic Ligand Model kinetic, easier to develop, and more flexible for deriving water quality criteria

In aquatic environments, the ecological risks posed by metals are greatly affected by water chemistry, thereby creating challenges for water quality management. Biotic ligand models (BLMs) have become the most widely used tools to interpret and predict water chemistry effects. Traditional BLM development methods require a large number of toxicity tests and organisms, and model predictions are limited to certain toxicity statistics (e.g., 48-h median effective concentration, 48-h EC₅₀), to which the models were calibrated. To address these limitations, we propose a new method to develop BLMs by integrating them into the toxicokinetic-toxicodynamic (TK-TD) framework. Metal bioaccumulation was predicted from metal exposure and water chemistry using the BLM-type toxicokinetics, whilst metal toxicity was predicted from metal bioaccumulation using the toxicodynamics. Using the new method, we developed a kinetic BLM of cadmium for Daphnia magna with only six toxicity tests and 1540 daphnids; this represents a 60-80% reduction compared to the traditional methods. The model was validated in the presence of commercial dissolved organic matter (DOM) and in natural waters sampled from 12 lakes. The kinetic BLM was able to accurately simulate the protective effects of the commercial DOM by employing the Stockholm humic model, whilst the complexation capabilities of some natural DOM were overestimated. We further used the model to predict Cd EC₅₀ and no-effect concentrations for different waters, generating predictions close to the effect concentrations reported in the literature. Overall, our method requires fewer resources and presents an easier approach to develop BLMs; the kinetic BLM is more flexible and can serve as a useful tool for developing water quality criteria.

Effect Modeling Quantifies the Difference Between the Toxicity of Average Pesticide Concentrations and Time-Variable Exposures from Water Quality Monitoring

Synthetic chemicals are frequently detected in water bodies, and their concentrations vary over time. Water monitoring programs typically employ either a sequence of grab samples or continuous sampling, followed by chemical analysis. Continuous time-proportional sampling yields the time-weighted average concentration, which is taken as proxy for the real, time-variable exposure. However, we do not know how much the toxicity of the average concentration differs from the toxicity of the corresponding fluctuating exposure profile. We used toxicokinetic-toxicodynamic models (invertebrates, fish) and population growth models (algae, duckweed) to calculate the margin of safety in moving time windows across measured aquatic concentration time series (7 pesticides) in 5 streams. A longer sampling period (14 d) for time-proportional sampling leads to more deviations from the real chemical stress than shorter sampling durations (3 d). The associated error is a factor of 4 or less in the margin of safety value toward underestimating and an error of factor 9 toward overestimating chemical stress in the most toxic time windows. Under- and overestimations occur with approximate equal frequency and are very small compared with the overall variation, which ranged from 0.027 to 2.4 × 10¹⁰ (margin of safety values). We conclude that continuous, time-proportional sampling for a period of 3 and 14 d for acute and chronic assessment, respectively, yields sufficiently accurate average concentrations to assess ecotoxicological effects. Environ Toxicol Chem 2020;39:2158-2168. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Critical knowledge gaps and relevant variables requiring consideration when performing aquatic ecotoxicity assays

The increasing diversity and complexity of contaminants released in the environment continuously lead to new challenges when applying ecotoxicity assays. This paper comprises a review concerning exposure assessment and highlights important variables that should be taken into account when investigating aquatic media toxicity under both laboratory or field conditions. Thus, to reflect as much as possible what occurs in nature, ecotoxicity assays must carefully consider these variables in their experimental design. This includes contaminant properties, the selected bioindicators and biomarkers, the dose mode/regime, concentration vs. load, exposure to single vs. multiple contaminants and exposure of single vs. multiple species. Many of these, however, are not usually taken into account, leading to critical knowledge gaps in this area, discussed in detail herein.

Improving Risk Assessment by Predicting the Survival of Field Gammarids Exposed to Dynamic Pesticide Mixtures

Exposure assessment of pesticides has substantially improved over time, with methods that now include a combination of advanced analytical techniques and fate/transport models to evaluate their spatiotemporal distribution. However, the current regulatory environmental risk assessment considers thresholds from laboratory studies completed under standardized conditions that do not reflect environmental dynamics. Using the General Unified Threshold model for Survival (GUTS) model framework, we predicted the impact of time-varying pesticide exposures on the survival of gammarids in a small agricultural stream. The LP₅₀ values were used as an additional metric for assessing risks (defined in GUTS as a multiplication factor applied to the concentration time series to induce 50% mortality by the end of exposure). Although real-case exposures to individual pesticides were predicted to produce little to no impact on survival, the LP₅₀ values indicate acute (LP₅₀ ≤ 100) and/or chronic (LP₅₀ ≤ 10) toxicities for azoxystrobin, chlorpyrifos, diazinon, and imidacloprid, while risk to propiconazole exposure was considered very low (LP₅₀ ≫ 100). Finally, the model was extended to reflect mixture toxicity via concentration addition. It predicted risks under acute and chronic exposures to organophosphates and neonicotinoids. Given that gammarids are simultaneously exposed to multiple chemicals and other stressors throughout their lifetime, a decline in survival probabilities due to chemical stress can likely influence their overall fitness. We recognize that some assumptions require validation, but our work included a level of realism that can assist risk managers when evaluating the cumulative consequences of chemical exposure.

Mosquito larvae that survive a heat spike are less sensitive to subsequent exposure to the pesticide chlorpyrifos

While extreme high temperatures are an important aspect of global warming, their effects on organisms are relatively understudied, especially in ecotoxicology. Sequential exposure to heat spikes and pesticides is a realistic scenario as both are typically transient stressors and are expected to further increase in frequency under global warming. We tested the effects of exposure to a lethal heat spike and subsequently to an ecologically relevant lethal pulse exposure of the pesticide chlorpyrifos in the larvae of mosquito Culex pipiens. The heat spike caused direct and delayed mortality, and resulted in a higher heat tolerance and activity of acetylcholinesterase, and a lower fat content in the survivors. The chlorpyrifos exposure caused mortality, accelerated growth rate, and decreased the heat tolerance and the activity of acetylcholinesterase. The preceding heat spike did not change how chlorpyrifos reduced the heat tolerance. Notably, the preceding heat spike did lower the lethal effect of the pesticide, which makes an important novel finding at the interface of ecotoxicology and global change biology, and adds a new dimension to the "climate-induced toxicant sensitivity" (CITS) concept. This may be due to both survival selection and cross-tolerance, and therefore likely a widespread phenomenon. Our results emphasize the importance of including extreme high temperatures as an important transient global change stressor in ecotoxicology.

The Exposure Order Strongly Modifies How a Heat Spike Increases Pesticide Toxicity

The exposure order may strongly affect the impact of stressors, yet is largely ignored for the frequently occurring combinations of toxicants with natural stressors. We tested how exposure order shaped the interactive effects of serial exposure to the pesticide chlorpyrifos and to a heat spike in the larvae of the mosquito Culex pipiens. Notably, the chlorpyrifos-induced mortality was much more magnified by the heat spike and a synergism was already detected at the low concentration when exposure to chlorpyrifos followed the heat spike. This suggests that the preceding heat spike weakened the larvae as reflected in their lower net energy budget, moreover the chlorpyrifos-induced inhibition of its target enzyme (acetylcholinesterase) was only magnified by the heat spike when it was the first stressor. Also the chlorpyrifos-induced reduction in heat tolerance was stronger when the pesticide pulse followed the heat spike, and was buffered by the heat spike when this was the second stressor. Our results provide the first evidence that the exposure order can strongly change the magnifying effect of an important climate change factor on the toxicity of a pesticide. This highlights the importance of exposure order in ecological risk assessment of toxicants under realistic combinations with natural stressors.

Mechanisms underlying silicon-dependent metal tolerance in the marine diatom Phaeodactylum tricornutum

Anthropogenic activities have significantly changed the stoichiometry and concentrations of nutrients in coastal waters. Silicon (Si) has become a potential limiting nutrient due to disproportionate nitrogen, phosphorus, and silicate inputs into these areas. The disrupted nutrient ratios can cause changes to metal sensitivity and accumulation in marine diatoms, an important group of eukaryotic phytoplankton that requires silicon for growth. In this study, we examined the effects of Si availability on the metal sensitivity in the diatom P. tricornutum. We found that Si starvation dramatically compromised its cadmium, copper, and lead tolerances. Interestingly, multiple lines of evidence indicated that Si-enriched cells had higher metal adsorption and influx rates than Si-starved cells. Yet Si-enriched cells also had a greater ability to respond to and counteract metal toxicity via elevated expression of membrane and vacuolar metal transporters and greater antioxidant activities which scavenge reactive oxygen species created by metal stress.

The roles of silicon in combating cadmium challenge in the Marine diatom Phaeodactylum tricornutum

Marine phytoplankton possess a sophisticated homeostatic network to counteract metal toxicity. Changes in environmental conditions such as ambient nutrient concentrations can significantly impact their intrinsic metal sensitivity. In this study, we evaluated the role of silicon (Si) in counteracting cadmium (Cd) toxicity in the marine diatom Phaeodactylum tricornutum. We first demonstrated that Si enrichment dramatically enhanced Cd tolerance and changed the Cd accumulation in the diatom. Our modeling suggested that Si-enriched cells adsorbed more Cd but had a higher Cd elimination rate than the Si-starved cells. Examinations by atomic force microscopy and X-ray photoelectron spectroscopy revealed that the Si-enriched cells had better silification and more SiO^- in the cell walls, which markedly lowered the surface potential of the diatom cells and allowed them to attract more Cd. Although the Si-enriched cells tended to have a high Cd burden when facing Cd stress, they suppressed the increase of intracellular Cd by both down-regulating the influx transporter ZIP and up-regulating the efflux transporter ATPase5-1B. Our study shows the significant roles Si plays in maintaining metal homeostasis and combating Cd challenge in marine diatoms.

Linking Morphology, Toxicokinetic, and Toxicodynamic Traits of Aquatic Invertebrates to Pyrethroid Sensitivity

Pyrethroid insecticides are known to be highly toxic to most aquatic nontarget organisms, but little is known about the mechanisms causing some species to be highly sensitive while others are hardly affected by the pyrethroids. The aim of the present study was to measure the sensitivity (EC₅₀-values) of 10 aquatic invertebrates toward a 24 h pulse of the pyrethroid cypermethrin and subsequently test if the difference in sensitivity could be explained by measured morphological and physiological traits and modeled toxicokinetic (TK) and toxicodynamic (TD) parameters. Large differences were observed for the measured uptake and elimination kinetics, with bioconcentration factors (BCFs) ranging from 53 to 2337 at the end of the exposure. Similarly, large differences were observed for the TDs, and EC₅₀-values after 168 h varied 120-fold. Modeling the whole organism cypermethrin concentrations indicated compartmentation into a sorbed fraction and two internal fractions: a bioavailable and non-bioavailable internal fraction. Strong correlations between surface/volume area and the TK parameters (sorption and uptake rate constants and the resulting BCF) were found, but none of the TK parameters correlated with sensitivity. The only parameter consistently correlating with sensitivity across all species was the killing rate constant of the GUTS-RED-SD model (the reduced general unified threshold models of survival assuming stochastic death), indicating that sensitivity toward cypermethrin is more related to the TD parameters than to TK parameters.

In vitro biotransformation of pharmaceuticals and pesticides by trout liver S9 in the presence and absence of carbamazepine

The aim of the present study was to develop (i) a technique for identifying metabolites of organic contaminants by using an in vitro system of trout S9 and liquid chromatography-high-resolution mass spectrometry-based identification method and (ii) to apply this technique to identify the interactive potential of carbamazepine on the formation rate of other metabolites. The pharmaceuticals carbamazepine and propranolol and the pesticides azoxystrobin, diazinon, and fipronil were selected as test contaminants. As a result, a total of ten metabolites were identified for the five parent substances, six of which were confirmed using reference standards. Metabolic reactions included hydroxylation, epoxidation, S-oxidation, and dealkylation. The metabolic transformation rate ranged from 0.2 to 3.5 pmol/mg protein/min/μmol substrate. In the binary exposure experiment with increasing carbamazepine concentration, the formation rates of diazinon and fipronil metabolites (MDI2 and MFP2, respectively) increased, while formation of metabolites of propranolol and azoxystrobin (MPR1, MPR2, MPR3, and MAZ1) slowed down. Meanwhile, S9 pre-exposed to carbamazepine produced diazoxon, a toxic metabolite of diazinon, and pyrimidinol, a less toxic metabolite, more rapidly. These results suggest that carbamazepine, a perennial environmental pollutant, might modulate the toxicity of other substances such as diazinon but further in vivo studies are needed.

A Bayesian framework for estimating parameters of a generic toxicokinetic model for the bioaccumulation of organic chemicals by benthic invertebrates: Proof of concept with PCB153 and two freshwater species

Toxicokinetic (TK) models are relevant and widely used to predict chemical concentrations in biological organisms. The importance of dietary uptake for aquatic invertebrates has been increasingly assessed in recent years. However, the model parameters are estimated on limited specific laboratory data sets that are bounded by several uncertainties. The aim of this study was to implement a Bayesian framework for simultaneously estimating the parameters of a generic TK model for benthic invertebrate species from all data collected. We illustrate our approach on the bioaccumulation of PCB153 by two species with different life traits and therefore exposure routes: Chironomus riparius larvae exposed to spiked sediment for 7 days and Gammarus fossarum exposed to spiked sediment and/or leaves for 7 days and then transferred to a clean media for 7 more days. The TK models assuming first-order kinetics were fitted to the data using Bayesian inference. The median model predictions and their 95% credibility intervals showed that the model fit the data well. From a methodological point of view, this paper illustrates that simultaneously estimating all model parameters from all available data by Bayesian inference, while considering the correlation between parameters and different types of data, is a real added value for TK modeling. Moreover, we demonstrated the ability of a generic TK model considering uptake and elimination routes as modules to add according to the availability of the data measured. From an ecotoxicological point of view, we show differences in PCB153 bioaccumulation between chironomids and gammarids, explained by the different life traits of these two organisms.

Gill damage and delayed mortality of Northern shrimp (Pandalus borealis) after short time exposure to anti-parasitic veterinary medicine containing hydrogen peroxide

Hydrogen peroxide (H₂O₂) is used as anti-parasitic veterinary medicine in salmon farms worldwide. In the period from 2009 to 2018 a total of 135 million kg of H₂O₂ was used in Norway, the world's largest producer of Atlantic salmon. Since the treatment water is discharged to the sea, concerns have been raised about effects of H₂O₂ on the coastal ecosystem. In the present study, Northern shrimp (Pandalus borealis) have been exposed to short pulses of H₂O₂ in the PARAMOVE^® formulation, followed by a recovery period in clean seawater. The exposure concentrations represented 100, 1000 and 10 000 times dilutions of the prescribed treatment concentration for salmon; 15 mg/L, 1.5 mg/L and 0.15 mg/L H₂O₂. Significantly increased mortality was observed after 2 h exposure to 15 mg/L H₂O₂ (50%) and after 2 h exposure to 1.5 mg/L H₂O₂ on 3 consecutive days (33%), but no mortality was observed after 2 h exposure to 0.15 mg/L. The mortality occurred 2-4 days after the first pulse of exposure. The patterns of acute effects (immobility and death) could be captured with a toxicokinetic-toxicodynamic model (GUTS), which allows extrapolations to LC50s for constant exposure, or thresholds for effects given untested exposure profiles. Effects of H₂O₂ were also detected in shrimp that survived until the end of the recovery period. The feeding rate was 66% lower than in the control after 12 days of recovery for the three-pulse 1.5 mg/L exposure. Furthermore, dose dependent tissue damage was detected in the gills and evidence of lipid peroxidation in the hepatopancreas in shrimp exposed for 1 h to 1.5 mg/L and 15 mg/L and kept in recovery for 8 days. Fluorescence intensity in the hepatopancreas of treated shrimp increased 47% and 157% at 1.5 mg/L and 15 mg/L, respectively, compared to the control. Local hydrodynamic conditions will determine how fast the concentration of H₂O₂ will be diluted and how far it will be transported horizontally and vertically. Results from dispersion modelling (literature data) together with the current experiments indicate that treatment water with toxic concentrations of H₂O₂ (1.5 mg/L) could reach P. borealis living more than 1 km from a treated salmon farm.

Applications of dynamic models in predicting the bioaccumulation, transport and toxicity of trace metals in aquatic organisms

This review evaluates the three dynamic models (biokinetic model: BK, physiologically based pharmacokinetic model: PBPK, and toxicokinetic-toxicodynamic model: TKTD) in our understanding of the key questions in metal ecotoxicology in aquatic systems, i.e., bioaccumulation, transport and toxicity. All the models rely on the first-order kinetics principle of metal uptake and elimination. The BK model basically treats organisms as a single compartment, and is both physiologically and geochemically based. With a good understanding of each kinetic parameter, bioaccumulation of metals in any aquatic organisms can be studied holistically and mechanistically. Modeling efforts are not merely restrained from the prediction of metal accumulation in the tissues, but instead provide the direction of the key processes that need to be addressed. PBPK is more physiologically based since it mainly addresses the transportation, transformation and distribution of metals in the organisms. It can be treated conceptually as a multi-compartmental kinetic model, whereas the physiology is driving the development of any good PBPK model which is no generic for aquatic animals and contaminants. There are now increasingly applications of the PBPK modeling specifically in metal studies, which reveal many important processes that are impossible to be teased out by direct experimental measurements without adequate modeling. TKTD models further focus on metal toxicity in addition to metal bioaccumulation. The TK part links exposure and bioaccumulation, while the TD part links bioaccumulation and toxic effects. The separation of TK and TD makes it possible to model processes, e.g., toxicity modification by environmental factors, interaction between different metals, at both the toxicokinetic and toxicodynamic levels. TKTD models provide a framework for making full use of metal toxicity data, and thus provide more information for environmental risk assessments. Overall, the three models reviewed here will continue to provide guiding principles in our further studies of metal bioaccumulation and toxicity in aquatic organisms.

Toxicokinetic-toxicodynamic modeling of cadmium and lead toxicity to larvae and adult zebrafish

Toxicity of hazard materials to organism is different between larvae and adult zebrafish. However, this different effect was seldom considered in toxicological modeling. Here, we measured Cd and Pb toxicity for larvae and adult zebrafish (Danio rerio) and assessed whether metal toxicity can be better simulated by the one-compartment or two-compartment toxicokinetic (TK) and toxicodynamic (TD) models with assumption of stochastic death (SD) and individual tolerance (IT), respectively. Results showed that, for larvae, the one-compartment model generally fitted the observed accumulation and survival better than two-compartment model. In contrast, for adult, the two-compartment model simulation satisfied the observed accumulation and survival better than one-compartment model. In addition, both the SD and the IT models generally described the Cd or Pb toxicity well, although the IT model predictions were slightly better than the SD model in adult fish, the opposite phenomenon was observed in larvae. Our results suggested that variations in both TK and TD parameters might be needed to quantify the toxicity sensitivity in larvae and adult zebrafish, and accounting these variations in mechanistic toxicological effect models (e.g. TK-TD) will allow more accurate predictions of hazard materials effects to organisms.

Modeling the Sensitivity of Aquatic Macroinvertebrates to Chemicals Using Traits

In this study, a trait-based macroinvertebrate sensitivity modeling tool is presented that provides two main outcomes: (1) it constructs a macroinvertebrate sensitivity ranking and, subsequently, a predictive trait model for each one of a diverse set of predefined Modes of Action (MOAs) and (2) it reveals data gaps and restrictions, helping with the direction of future research. Besides revealing taxonomic patterns of species sensitivity, we find that there was not one genus, family, or class which was most sensitive to all MOAs and that common test taxa were often not the most sensitive at all. Traits like life cycle duration and feeding mode were identified as important in explaining species sensitivity. For 71% of the species, no or incomplete trait data were available, making the lack of trait data the main obstacle in model construction. Research focus should therefore be on completing trait databases and enhancing them with finer morphological traits, focusing on the toxicodynamics of the chemical (e.g., target site distribution). Further improved sensitivity models can help with the creation of ecological scenarios by predicting the sensitivity of untested species. Through this development, our approach can help reduce animal testing and contribute toward a new predictive ecotoxicology framework.

Predicting the survival of zebrafish larvae exposed to fluctuating pulses of lead and cadmium

Aquatic organisms are often exposed to time-varied concentrations of contaminants due to pulsed inputs in natural water. Traditional toxicology experiments are usually carried out in a constant exposure pattern, which is inconsistent with the actual environment. In this study, a refined toxicokinetic-toxicodynamic (TK-TD) model was used to study the toxic effects of Pb and Cd on zebrafish larvae under three pulse exposures with 2, 4, and 6 h, respectively. The parameter sensitivity analysis showed that J_{M, max} had the greatest impact on the output of the model. Cd or Pb pulse exposure resulted in less death than constant exposure at the same time-weighted average (TWA) concentrations. Survival fraction in larvae under 6 h interval between two pulses of Pb or Cd was larger than that under 2 h and 4 h interval. Toxicity under constant exposure of Cd or Pb was greater than that under 2, 4, and 6 h interval pulse exposure because the cumulative Cd or Pb concentration in the body under constant exposure was greater than that under pulse exposure. The results also showed that the stochastic death (SD) model was more suitable than the individual tolerance (IT) model for predicting the survival fraction of larvae under pulse exposure to Pb and Cd, which was indicated by higher R² (0.670-0.940) in SD model than that (0.588-0.861) in IT model. Our model provides approaches for laboratory toxicity testing and modeling approaches for addressing the toxicity of heavy metal pulsed exposure.

Predicting the toxic effects of Cu and Cd on Chlamydomonas reinhardtii with a DEBtox model

Predicting the sublethal effects of pollutants to aquatic organism is essential in realistic chemical risk assessment. However, only a few dynamics models for sublethal endpoints are available. Here, we investigated the toxic effects of the essential metal Cu and the nonessential metal Cd on Chlamydomonas reinhardtiiunder both single and combined exposure, compared the effectiveness of different effect endpoints as toxic effect factors, and developed a Dynamic Energy Budget toxicology (DEBtox) model to predict the sublethal effects of Cu and Cd on C. reinhardtii. The results showed that the chlorophyll fluorescence parameter is a better toxic effect indicator than others for short-term exposure (<24 h), while algal cell growth is preferred for long-term exposure (2-6 days). The developed DEBtox model could successfully predict single metal toxicity to C. reinhardtii, while the combined metal DEBtox model slightly overestimates the joint toxicity of Cu-Cd due to the antagonistic effect of Cu-Cd on C. reinhardtii. This study is helpful to understanding and better predictions of metal sublethal toxic effects on aquatic organisms.

A Spatio-Temporal Exposure-Hazard Model for Assessing Biological Risk and Impact

We developed a simulation model for quantifying the spatio-temporal distribution of contaminants (e.g., xenobiotics) and assessing the risk of exposed populations at the landscape level. The model is a spatio-temporal exposure-hazard model based on (i) tools of stochastic geometry (marked polygon and point processes) for structuring the landscape and describing the exposed individuals, (ii) a dispersal kernel describing the dissemination of contaminants from polygon sources, and (iii) an (eco)toxicological equation describing the toxicokinetics and dynamics of contaminants in affected individuals. The model was implemented in the briskaR package (biological risk assessment with R) of the R software. This article presents the model background, the use of the package in an illustrative example, namely, the effect of genetically modified maize pollen on nontarget Lepidoptera, and typical comparisons of landscape configurations that can be carried out with our model (different configurations lead to different mortality rates in the treated example). In real case studies, parameters and parametric functions encountered in the model will have to be precisely specified to obtain realistic measures of risk and impact and accurate comparisons of landscape configurations. Our modeling framework could be applied to study other risks related to agriculture, for instance, pathogen spread in crops or livestock, and could be adapted to cope with other hazards such as toxic emissions from industrial areas having health effects on surrounding populations. Moreover, the R package has the potential to help risk managers in running quantitative risk assessments and testing management strategies.

Coupling toxicokinetic-toxicodynamic and population models for assessing aquatic ecological risks to time-varying pesticide exposures

Population modeling evaluations of pesticide exposure time series were compared with aspects of a currently used risk assessment process. The US Environmental Protection Agency's Office of Pesticide Programs models daily aquatic 30-yr pesticide exposure distributions in its risk assessments, but does not routinely make full use of the information in such time series. We used mysid shrimp Americamysis bahia toxicity and demographic data to demonstrate the value of a toxicokinetic-toxicodynamic model coupled with a series of matrix population models in risk assessment refinements. This species is a small epibenthic marine crustacean routinely used in regulatory toxicity tests. We demonstrate how the model coupling can refine current risk assessments using only existing standard regulatory toxicity test results. Several exposure scenarios (each with the same initial risk characterization as determined by a more traditional organism-based approach) were created within which population modeling documented risks different from those of assessments based on the traditional approach. We also present different acute and chronic toxicity data scenarios by which toxicokinetic-toxicodynamic coupled with population modeling can distinguish responses that traditional risk evaluations are not designed to detect. Our results reinforce the benefits of this type of modeling in risk evaluations, especially related to time-varying exposure concentrations. Environ Toxicol Chem 2018;37:2633-2644. Published 2018 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Modelling the effects of PSII inhibitor pulse exposure on two algae in co-culture

A weakness of standard testing procedures is that they do not consider interactions between organisms, and they focus only on single species. Furthermore, these procedures do not take into account pulse exposure. However, pulse exposure is of particular importance because in streams, after crop application and during and after precipitation, herbicide concentrations fluctuate widely and can exceed the Annual Average Environmental Quality Standards (AA-EQS), which aim to protect the aquatic environment. The sensitivity of the algae Scenedesmus vacuolatus and Pseudokirchneriella subcapitata in a co-culture exposed to pulses is thus analysed in this study. As a first step, the growths of the algae in co-culture are investigated. For initial cell densities fixed, respectively, to 100,000 and 50,000 cells/mL, the growth of each alga is exponential over at least 48 h. S. vacuolatus seems to influence the growth of P. subcapitata negatively. Allelopathy is a possible explanation for this growth inhibition. The toxicity of the herbicide isoproturon is later tested on the algae S. vacuolatus and P. subcapitata cultured alone and in the co-culture. Despite the supplementary stress on the algae in the co-culture competing for nutrients, the toxicity of the herbicide is lower for the two algae when they are in the co-culture than when they are in separated culture. A model is adapted and used to predict the cell-density inhibition on the alga S. vacuolatus in the co-culture with the alga P. subcapitata exposed to a pulse concentration of isoproturon. Four laboratory experiments are performed to validate the model. The comparison between the laboratory and the modelled effects shows good agreement. The differences can be considered minor most of time. For future studies, it is important to ensure that the cell count is precise, as it is used to determine the parameters of the model. The differences can be also induced by the fact that the cell number of the alga P. subcapitata re-suspended in a new OECD medium after the centrifugation process cannot be fixed.

Modeling the Toxicokinetics of Multiple Metals in the Oyster Crassostrea hongkongensis in a Dynamic Estuarine Environment

Metal contamination is a major problem in many estuaries. Toxicokinetic models are useful tools for predicting metal accumulation in estuarine organisms and managing the associated ecological risks. However, obtaining toxicokinetic parameter values with sufficient predictive power is challenging for dynamic estuarine waters. In this study, we determined the toxicokinetics of multiple metals in the oyster Crassostrea hongkongensis in a dynamic estuary polluted by metals using a 48 day transplant experiment. During the experiment, metal concentrations in oysters, water, and suspended particles were intensively monitored at 3 day intervals. The toxicokinetic parameters were then estimated using the Markov chain Monte Carlo (MCMC) method. The calibrated model was capable of successfully simulating the time-course of metal bioaccumulation in oysters and was further validated by predicting the bioaccumulation at another site in the estuary. Furthermore, the model was used to assess the relative importance of different pathways in metal bioaccumulation. With the MCMC method, distributions instead of single values were assigned to model parameters. This method makes the model predictions probabilistic with clearly defined uncertainties, and they are thus particularly useful for the risk assessment of metals in aquatic systems.

An ecotoxicological view on neurotoxicity assessment

The numbers of potential neurotoxicants in the environment are raising and pose a great risk for humans and the environment. Currently neurotoxicity assessment is mostly performed to predict and prevent harm to human populations. Despite all the efforts invested in the last years in developing novel in vitro or in silico test systems, in vivo tests with rodents are still the only accepted test for neurotoxicity risk assessment in Europe. Despite an increasing number of reports of species showing altered behaviour, neurotoxicity assessment for species in the environment is not required and therefore mostly not performed. Considering the increasing numbers of environmental contaminants with potential neurotoxic potential, eco-neurotoxicity should be also considered in risk assessment. In order to do so novel test systems are needed that can cope with species differences within ecosystems. In the field, online-biomonitoring systems using behavioural information could be used to detect neurotoxic effects and effect-directed analyses could be applied to identify the neurotoxicants causing the effect. Additionally, toxic pressure calculations in combination with mixture modelling could use environmental chemical monitoring data to predict adverse effects and prioritize pollutants for laboratory testing. Cheminformatics based on computational toxicological data from in vitro and in vivo studies could help to identify potential neurotoxicants. An array of in vitro assays covering different modes of action could be applied to screen compounds for neurotoxicity. The selection of in vitro assays could be guided by AOPs relevant for eco-neurotoxicity. In order to be able to perform risk assessment for eco-neurotoxicity, methods need to focus on the most sensitive species in an ecosystem. A test battery using species from different trophic levels might be the best approach. To implement eco-neurotoxicity assessment into European risk assessment, cheminformatics and in vitro screening tests could be used as first approach to identify eco-neurotoxic pollutants. In a second step, a small species test battery could be applied to assess the risks of ecosystems.

Testing the time-scale dependence of delayed interactions: A heat wave during the egg stage shapes how a pesticide interacts with a successive heat wave in the larval stage

Under global change organisms are exposed to multiple, potentially interacting stressors. Especially interactions between successive stressors are poorly understood and recently suggested to depend on their timing of exposure. We particularly need studies assessing the impact of exposure to relevant stressors at various life stages and how these interact. We investigated the single and combined impacts of a heat wave (mild [25 °C] and extreme [30 °C]) during the egg stage, followed by successive exposure to esfenvalerate (ESF) and a heat wave during the larval stage in damselflies. Each stressor caused mortality. The egg heat wave and larval ESF exposure had delayed effects on survival, growth and lipid peroxidation (MDA). This resulted in deviations from the prediction that stressors separated by a long time interval would not interact: the egg heat wave modulated the interaction between the stressors in the larval stage. Firstly, ESF caused delayed mortality only in larvae that had been exposed to the extreme egg heat wave and this strongly depended upon the larval heat wave treatment. Secondly, ESF only increased MDA in larvae not exposed to the egg heat wave. We found little support for the prediction that when there is limited time between stressors, synergistic interactions should occur. The intermediate ESF concentration only caused delayed mortality when combined with the larval heat wave, and the lowest ESF concentrations only increased oxidative damage when followed by the mild larval heat wave. Survival selection mitigated the interaction patterns between successive stressors that are individually lethal, and therefore should be included in a predictive framework for the time-scale dependence of the outcome of multistressor studies with pollutants. The egg heat wave shaping the interaction pattern between successive pesticide exposure and a larval heat wave highlights the connectivity between the concepts of 'heat-induced pesticide sensitivity' and 'pesticide-induced heat sensitivity'.

History Matters: Pre-Exposure to Wastewater Enhances Pesticide Toxicity in Invertebrates

Disturbance regimes determine communities' structure and functioning. Nonetheless, little effort has been undertaken to understand interactions of press and pulse disturbances. In this context, leaf-shredding macroinvertebrates can be chronically exposed to wastewater treatment plant effluents (i.e., press disturbance) before experiencing pesticide exposure following agricultural runoff (i.e., pulse disturbance). It is assumed that wastewater pre-exposure alters animals' sensitivity to pesticides. To test this hypothesis, we exposed model-populations of the shredder Gammarus fossarum to wastewater at three field-relevant dilution levels (i.e., 0%, 50%, and 100%). After 2, 4, and 6 weeks, survival, leaf consumption, dry weight, and energy reserves were monitored. Additionally, animals were assessed for their sensitivity toward the neonicotinoid insecticide thiacloprid using their feeding rate as response variable. Both wastewater treatments reduced gammarids' survival, leaf consumption, dry weight, and energy reserves. Moreover, both wastewater pre-exposure scenarios increased animals' sensitivity toward thiacloprid by up to 2.5 times compared to the control. Our results thus demonstrate that press disturbance as posed by wastewater pre-exposure can enhance susceptibility of key players in ecosystem functioning to further (pulse) disturbances. Therefore, applying mitigation measures such as advanced treatment technologies seems sensible to support functional integrity in the multiple-stress situation.

Toxicodynamic modeling of zebrafish larvae to metals using stochastic death and individual tolerance models: comparisons of model assumptions, parameter sensitivity and predictive performance

Process-based toxicodynamic (TD) models are playing an increasing role in predicting chemical toxicity to aquatic organism. Stochastic death (SD) and individual tolerance distribution (IT) are two often used assumptions in TD models which could lead to different consequences for risk assessment of chemicals. Here, using the toxicity data of single (Cu, Zn, Cd, and Pb) and their binary metal mixtures on survival of zebrafish larvae, we assessed the parameter sensitivity and evaluated the predictive performance of SD and IT models. The sensitivity analysis indicated the parameters related to toxicodynamics such as k _k and threshold, had a great influence on the SD model's output and α had a great influence on the IT model's output. The predicted survival probability was highly sensitive to the assumptions of SD or IT models, and the SD model explained toxicity of single metal and binary metal mixtures better than IT model. Our results suggested that SD model is more suitable in assessing the metal toxicity to zebrafish larvae. Moreover, different combinations of laboratory metal-specific and species-specific experiments with SD and IT models need further study for better understanding and predicting toxic effects for different metals and organisms.

Toxic Mixtures in Time-The Sequence Makes the Poison

"The dose makes the poison". This principle assumes that once a chemical is cleared out of the organism (toxicokinetic recovery), it no longer has any effect. However, it overlooks the other process of re-establishing homeostasis, toxicodynamic recovery, which can be fast or slow depending on the chemical. Therefore, when organisms are exposed to two toxicants in sequence, the toxicity can differ if their order is reversed. We test this hypothesis with the freshwater crustacean Gammarus pulex and four toxicants that act on different targets (diazinon, propiconazole, 4,6-dinitro-o-cresol, 4-nitrobenzyl chloride). We found clearly different toxicity when the exposure order of two toxicants was reversed, while maintaining the same dose. Slow toxicodynamic recovery caused carry-over toxicity in subsequent exposures, thereby resulting in a sequence effect-but only when toxicodynamic recovery was slow relative to the interval between exposures. This suggests that carry-over toxicity is a useful proxy for organism fitness and that risk assessment methods should be revised as they currently could underestimate risk. We provide the first evidence that carry-over toxicity occurs among chemicals acting on different targets and when exposure is several days apart. It is therefore not only the dose that makes the poison but also the exposure sequence.

Time-Dependent Effects in Algae for Chemicals with Different Adverse Outcome Pathways: A Novel Approach

Chemicals affect unicellular algae as a result of toxicokinetic and toxicodynamic processes. The internal concentration of chemicals in algae cells typically reaches equilibrium within minutes, while damage cumulatively increases over hours. The time gap between the steady state of internal exposure and damage development is thus suspected to span up to hours, mainly due to toxicodynamic processes. The quantification of rate-limited toxicodynamic processes, aggregated as a progressive effect from an initiating molecular event through biological key events toward the adverse outcome on algae growth inhibition, might discriminate between different adverse outcome pathways (AOPs). To support our hypothesis, we selected six chemicals according to different physicochemical properties and three distinctly dissimilar AOPs. The time courses of internal concentrations were linked to the observed affected Scenedesmus vacuolatus growth using toxicokinetic-toxicodynamic modeling. Effects on cell growth were explained by effect progression and not by the time to reach internal equilibrium concentration. Effect progression rates ranged over 6 orders of magnitude for all chemicals but varied by less than 1 order of magnitude within similar AOP (photosystem II inhibitors > reactive chemicals > lipid biosynthesis inhibitors), meaning that inhibitors of photosystem II advance an effect toward algae growth fastest compared to reactive chemicals and inhibitors of lipid biosynthesis.

Computationally Efficient Implementation of a Novel Algorithm for the General Unified Threshold Model of Survival (GUTS)

The General Unified Threshold model of Survival (GUTS) provides a consistent mathematical framework for survival analysis. However, the calibration of GUTS models is computationally challenging. We present a novel algorithm and its fast implementation in our R package, GUTS, that help to overcome these challenges. We show a step-by-step application example consisting of model calibration and uncertainty estimation as well as making probabilistic predictions and validating the model with new data. Using self-defined wrapper functions, we show how to produce informative text printouts and plots without effort, for the inexperienced as well as the advanced user. The complete ready-to-run script is available as supplemental material. We expect that our software facilitates novel re-analysis of existing survival data as well as asking new research questions in a wide range of sciences. In particular the ability to quickly quantify stressor thresholds in conjunction with dynamic compensating processes, and their uncertainty, is an improvement that complements current survival analysis methods.

What causes the difference in synergistic potentials of propiconazole and prochloraz toward pyrethroids in Daphnia magna?

Azole fungicides (imidazoles and triazoles) are known to function synergistically with several compounds, especially with pyrethroid insecticides, most likely by inhibiting cytochrome P450. Different azole fungicides have been shown to differ in their synergistic potentials usually with the imidazoles being stronger synergists than the triazoles. This study investigated whether the toxicokinetic and toxicodynamic (TKTD) properties of the imidazole prochloraz and triazole propiconazole can explain their different synergistic potential toward the freshwater macroinvertebrate Daphnia magna. Pulse exposure to external concentrations of propiconazole (1.4μM) and prochloraz (1.7μM) for 18h resulted in internal concentrations of 22.7 and 53.5μmolkg(-1)w.w. for propiconazole and prochloraz, respectively. This 2-fold difference in bioaccumulation corresponded very well with the observed 2.7-fold lower external EC50-estimate (7 days) for prochloraz compared to propiconazole. The estimated IC50 for the in vivo inhibition of cytochrome P450 (ECOD) activity, however, measured as transformation of 7-ethoxycoumarin into 7-hydroxycoumarin, was almost 500-fold higher for prochloraz (IC50: 0.011±0.002μM) compared to propiconazole (IC50: 4.9±0.06μM). When indirectly measuring the binding strength of the two azoles, daphnids exposed to propiconazole recovered roughly 80% of their ECOD activity compared to the control shortly after being moved to azole-free medium, indicating that propiconazole causes reversible inhibition of cytochrome P450. In contrast, the ECOD-activity remained inhibited in the prochloraz-exposed daphnids for 12h following transfer to azole-free medium, which correlated with elimination of the measured internal prochloraz concentration (DT95≈13h). These results indicate that lethal toxicity of the azole fungicides is mainly driven by toxicokinetics through their hydrophobicities resulting in different internal concentrations. Their synergistic potential toward pyrethroid toxicity, on the other hand, is mainly governed by their toxicodynamic effects measured as the differences in IC50-values toward in vivo cytochrome P450 (ECOD) activity together with the proposed binding strength measured indirectly through the recovery of ECOD activity as a function of internal azole concentrations.

Using toxicokinetic-toxicodynamic modeling as an acute risk assessment refinement approach in vertebrate ecological risk assessment

Recent guidance identified toxicokinetic-toxicodynamic (TK-TD) modeling as a relevant approach for risk assessment refinement. Yet, its added value compared to other refinement options is not detailed, and how to conduct the modeling appropriately is not explained. This case study addresses these issues through 2 examples of individual-level risk assessment for 2 hypothetical plant protection products: 1) evaluating the risk for small granivorous birds and small omnivorous mammals of a single application, as a seed treatment in winter cereals, and 2) evaluating the risk for fish after a pulsed treatment in the edge-of-field zone. Using acute test data, we conducted the first tier risk assessment as defined in the European Food Safety Authority (EFSA) guidance. When first tier risk assessment highlighted a concern, refinement options were discussed. Cases where the use of models should be preferred over other existing refinement approaches were highlighted. We then practically conducted the risk assessment refinement by using 2 different models as examples. In example 1, a TK model accounting for toxicokinetics and relevant feeding patterns in the skylark and in the wood mouse was used to predict internal doses of the hypothetical active ingredient in individuals, based on relevant feeding patterns in an in-crop situation, and identify the residue levels leading to mortality. In example 2, a TK-TD model accounting for toxicokinetics, toxicodynamics, and relevant exposure patterns in the fathead minnow was used to predict the time-course of fish survival for relevant FOCUS SW exposure scenarios and identify which scenarios might lead to mortality. Models were calibrated using available standard data and implemented to simulate the time-course of internal dose of active ingredient or survival for different exposure scenarios. Simulation results were discussed and used to derive the risk assessment refinement endpoints used for decision. Finally, we compared the "classical" risk assessment approach with the model-based approach. These comparisons showed that TK and TK-TD models can bring more realism to the risk assessment through the possibility to study realistic exposure scenarios and to simulate relevant mechanisms of effects (including delayed toxicity and recovery). Noticeably, using TK-TD models is currently the most relevant way to directly connect realistic exposure patterns to effects. We conclude with recommendations on how to properly use TK and TK-TD model in acute risk assessment for vertebrates.

Prediction of acute toxicity of cadmium and lead to zebrafish larvae by using a refined toxicokinetic-toxicodynamic model

The biotic ligand model (BLM) and the toxicokinetic-toxicodynamic (TK-TD) model are essential in predicting the acute toxicity of metals in various species and exposure conditions; however, these models are usually separately utilized. In this study, a mechanistic TK-TD model was developed to predict the acute toxicity of 10(-6)M Cd and 10(-6)M Pb to zebrafish (Danio rerio) larvae. The novel approach links the BLM with relevant TK processes to simulate the bioaccumulation processes of Cd or Pb as a function of the maximum uptake rate of each metal, the affinity constants, and the concentrations of free metal ions and H(+) in test solutions. Results showed that the refined TK-TD model can accurately predict the accumulation and acute toxicity of Cd and Pb to zebrafish larvae at pH 5.5, 6.5, and 7.0.

Constructing Time-Resolved Species Sensitivity Distributions Using a Hierarchical Toxico-Dynamic Model

Classical species sensitivity distribution (SSD) is used to assess the threat to ecological communities posed by a contaminant and derive a safe concentration. It suffers from several well-documented weaknesses regarding its ecological realism and statistical soundness. Criticism includes that SSD does not take time-dependence of the data into account, that safe concentrations obtained from SSD might not be entirely protective of the target communities, and that there are issues of statistical representativity and of uncertainty propagation from the experimental data. We present a hierarchical toxico-dynamic (TD) model to simultaneously address these weaknesses: TD models incorporate time-dependence and allow improvement of the ecological relevance of safe concentrations, while the hierarchical approach affords appropriate propagation of uncertainty from the original data. We develop this model on a published data set containing the salinity tolerance over 72 h of 217 macroinvertebrate taxa, obtained through rapid toxicity testing (RTT). The shrinkage properties of the hierarchical model prove particularly adequate for modeling inhomogeneous RTT data. Taking into account the large variability in the species response, the model fits the whole data set well. Moreover, the model predicts a time-independent safe concentration below that obtained with classical SSD at 72 h, demonstrating under-protectiveness of the classical approach.

Modelling the effects of pulse exposure of several PSII inhibitors on two algae

Subsequent to crop application and during precipitation events, herbicides can reach surface waters in pulses of high concentrations. These pulses can exceed the Annual Average Environmental Quality Standards (AA-EQS), defined in the EU Water Framework Directive, which aims to protect the aquatic environment. A model was developed in a previous study to evaluate the effects of pulse exposure for the herbicide isoproturon on the alga Scenedesmus vacuolatus. In this study, the model was extended to other substances acting as photosystem II inhibitors and to other algae. The measured and predicted effects were equivalent when pulse exposure of atrazine and diuron were tested on S. vacuolatus. The results were consistent for isoproturon on the alga Pseudokirchneriella subcapitata. The model is thus suitable for the effect prediction of phenylureas and triazines and for the algae used: S. vacuolatus and P. subcapitata. The toxicity classification obtained from the dose-response curves (diuron>atrazine>isoproturon) was conserved for the pulse exposure scenarios modelled for S. vacuolatus. Toxicity was identical for isoproturon on the two algae when the dose-response curves were compared and also for the pulse exposure scenarios. Modelling the effects of any pulse scenario of photosystem II inhibitors on algae is therefore feasible and only requires the determination of the dose-response curves of the substance and growth rate of unexposed algae. It is crucial to detect the longest pulses when measurements of herbicide concentrations are performed in streams because the model showed that they principally affect the cell density inhibition of algae.

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.

Hydra viridissima (green Hydra) rapidly recovers from multiple magnesium pulse exposures

The time taken for organisms to recover from a pulsed toxicant exposure is an important consideration when water quality guidelines are applied to intermittent events in the environment. Organisms may appear to have recovered by standard toxicity testing methods but could carry residual toxicant or damage that may make them more sensitive to subsequent pulses. Such cumulative effects may render guidelines underprotective. The present study evaluated recovery of the freshwater cnidarian Hydra viridissima following multiple pulse exposure to magnesium (Mg). The H. viridissima were exposed to 4-h pulses of 790 mg/L and 1100 mg/L separated by 2-h, 10-h, 18-h, 24-h, 48-h, and 72-h recovery periods. Twenty-four-hour pulses of 570 mg/L, 910 mg/L, and 940 mg/L were separated by 24-h, 96-h, and 168-h recovery periods. All treatments showed similar or reduced sensitivity to the second pulse when compared with the single pulse, indicating that full recovery occurred prior to a second pulse-exposure. Five variations of equivalent time-weighted average concentrations were used to compare sensitivity of Hydra with various pulse scenarios. The sensitivity of the organisms to the multiple pulses was significantly lower than the time-weighted average continuous exposure response in 3 of the 4 scenarios tested, indicating that the Hydra benefited from interpulse recovery periods. The findings will be utilized alongside those from other species to inform the use of a site-specific, duration-based water quality guideline for Mg, and they provide an example of the use of empirical data in the regulation of toxicant pulses in the environment.

Analyzing the effectiveness of using branchial NKA activity as a biomarker for assessing waterborne copper toxicity in tilapia (Oreochromis mossambicus): A damage-based modeling approach

Branchial Na(＋)-K(＋)-ATPase (NKA) activity has been suggested as a promising biomarker for assessing metal stress in aquatic organisms. However, studies that systematically show the effectiveness of using NKA activity to detect metal exposure and toxicity at the individual level are limited. In this study, we aimed to determine whether branchial NKA activity mechanistically responds to the accumulation of waterborne copper (Cu) and accounts for observed toxicity over time under environmentally-relevant and aquafarming Cu exposure levels (0.2, 1 and 2 mg L(-1)). Temporal trends in Cu accumulation and the corresponding responses of branchial NKA activity resulting from Cu exposure were investigated in laboratory experiments conducted on juvenile tilapia (Oreochromis mossambicus), a freshwater teleost that shows potential as a bioindicator of real-time and historical metal pollution. We used the process-based damage assessment model (DAM) to inspect the time course of Cu toxicity by integrating the compensation process between Cu-induced inhibition and repair of branchial NKA activity. NKA activity acted as a sensitive biomarker for Cu exposure and accumulation in tilapia, which showed induced impairment of osmoregulation and lethality when they were exposed to environmentally relevant levels (0.2 mg L(-1)), but not to higher exposure levels (1 and 2 mg L(-1)) in aquaculture farms or contaminated aquatic ecosystems. This study highlights the benefits and limitations of using branchial NKA activity as a sensitive biomarker to assess the health status of a fish population and its ecosystem.