Acute Toxicity of Major Geochemical Ions to Fathead Minnows (Pimephales promelas): Part B-Modeling Ion Toxicity.

Mathematical models are presented for the acute median lethal concentrations of major geochemical ions (Na+ , K+ , Ca2+ , Mg2+ , Cl- , SO4 2- , HCO3 - /CO3 2- ) to fathead minnows (Pimephales promelas), based on an extensive series of experiments presented in a companion article. Toxicity relationships across different dilution waters, individual salts, and salt mixtures suggest six independent mechanisms of toxicity to consider in modeling efforts, including Mg/Ca-specific toxicity, osmolarity-related toxicity, SO4 -specific toxicity, K-specific toxicity, effects of high pH/alkalinity, and a multiple ion-related toxicity at low Ca distinct from the other mechanisms. Models are evaluated using chemical activity-based exposure metrics pertinent to each mechanism, but concentration-based alternative models that are simpler to apply are also addressed. These models are compared to those previously provided for Ceriodaphnia dubia, and various issues regarding their application to risk assessments are discussed. Environ Toxicol Chem 2022;41:2095-2106. © 2022 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Acute Toxicity of Major Geochemical Ions to Fathead Minnows (Pimephales Promelas): Part A-Observed Relationships for Individual Salts and Salt Mixtures.

The results of a series of experiments on the acute toxicity of major geochemical ions (Na+ , K+ , Ca2+ , Mg2+ , Cl- , SO4 2- , HCO3 - /CO3 2- ) to fathead minnows (Pimephales promelas) are reported. Tests of individual major ion salts in various dilution waters demonstrated that the toxicities of Na, Mg, and K salts decrease as the overall ion content of the dilution water increases. For Na and Mg salts, this is attributable to Ca content as previously reported for Ceriodaphnia dubia. For K salts, the cause is unclear, but it is not due to Na as reported for C. dubia. In an unregulated test at high pH (9.3), NaHCO3 was also found to be twice as toxic compared to when the pH was reduced to 8.4. Experiments with binary salt mixtures indicated the existence of multiple independent mechanisms of action. These include K-specific toxicity and Ca/Mg-specific toxicity previously reported for C. dubia, but also apparent toxicities related to SO4 and to high pH/alkalinity in CO3 /HCO3 -dominated exposures. Previous work with C. dubia also suggested a general ion toxicity involving all ions that was correlated with osmolarity. For fathead minnow, similar correlations were observed, but multiple mechanisms were indicated. At higher Ca, this general toxicity could be attributable to osmotic effects, but at lower Ca, osmolarity may be more a covariate than a cause, with this toxicity being related to a combined effect of ions other than via osmolarity. Environ Toxicol Chem 2022;41:2078-2094. © 2022 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

The effects of arsenic speciation on accumulation and toxicity of dietborne arsenic exposures to rainbow trout.

The effects on juvenile rainbow trout survival, growth, food consumption, and food conversion efficiency from dietborne exposures to inorganic arsenic (arsenite, arsenate) and to the organoarsenicals monomethylarsonate (MMA), dimethylarsinate (DMA), and arsenobetaine (AsB) were investigated in two experiments: (1) a 28-d exposure using live diets of oligochaete worms separately exposed via water to these five arsenic compounds and (2) a 56-d exposure using pellet diets prepared from commercial fish food to which arsenite, MMA, or DMA were added. In the live diet experiment, the degree to which worms could be contaminated with the organoarsenicals was limited by toxicity to the worms and other experimental constraints, so that their toxicity relative to inorganic arsenic could not be fully established, but AsB was concluded to have low toxicity, consistent with published results for mammals. For the pellet diet experiment, MMA and DMA were found to be at least an order of magnitude less toxic than inorganic As on the basis of concentration in the diet, as well as much less toxic on the basis of accumulation in the fish. The need to consider speciation in aquatic risk assessments for arsenic was further demonstrated by tissue analyses of three macroinvertebrate species from a mining-impacted stream, which showed large variations in both total arsenic and the relative amounts of inorganic and organic arsenic. Additionally, although effects of arsenic on trout appear to be well correlated with inorganic arsenic, worms were found to be more sensitive to waterborne DMA than to inorganic arsenic, showing that low toxicity of organoarsenicals cannot be assumed for all aquatic organisms. Various difficulties in evaluating and applying studies on dietborne exposures and fish growth are also discussed.

Estimating n-octanol-water partition coefficients for neutral highly hydrophobic chemicals using measured n-butanol-water partition coefficients.

Direct measurement of the n-octanol partition coefficients (KOW) for highly hydrophobic organic chemicals is extremely difficult because of the extremely low concentrations present in the water phase. n-Butanol/water partition coefficients (KBW) are generally much lower than KOW due to the increased solubility of solute in the alcohol saturated aqueous phase, and therefore become easier to measure. We measured the KBW for 25 neutral organic chemicals having measured log KOWs ranging from 2 to 9 and 4 additional highly hydrophobic chemicals, with unmeasured KOWs, having estimated log KOWs ranging from 6 to 18. The measured log KBW and log KOW values were linearly related, r2 = 0.978, and using the regression developed from the data, KOWs were predicted for the 4 highly hydrophobic chemicals with unmeasured KOWs. The resulting predictions were orders of magnitude lower than those predicted by a variety of computational models and suggests the estimates of KOW in the literature for highly hydrophobic chemicals (i.e., log KOW greater than 10) are likely incorrect by several orders of magnitude.

Chronic toxicity of major ion salts and their mixtures to Ceriodaphnia dubia.

In previous work we intensively studied the acute responses of the cladoceran Ceriodaphnia dubia to major geochemical ions (Na+ , K+ , Ca2+ , Mg2+ , Cl- , SO42- , and HCO3- /CO32- ), culminating in the development of models to predict acute toxicity from ionic composition. To quantitatively evaluate whether the toxicological behavior of major ions observed for C. dubia extends to chronic toxicity, we conducted 58 chronic toxicity tests with individual major salts and binary mixtures thereof. Chronic responses paralleled those demonstrated previously for acute exposure, specifically 1) similar relative toxicity of individual salts; 2) different Na salts showing similar potency when exposure is expressed as osmolarity; 3) toxicity of Mg, Ca, and K salts related to cation activity; 4) decreased toxicity of Na and Mg salts when Ca activity is increased at less than toxic concentrations; 5) additive behavior for salt mixtures sharing a common cation; and 6) independent behavior for salt mixtures with dissimilar cations, except Mg/Ca mixtures which appeared additive. Acute-to-chronic ratios were fairly consistent among salts, with values of approximately 1.8 for acute 50% lethal concentration (LC50) to chronic 50% effect concentration (EC50) and 2.8 for LC50/EC20 when expressed on an activity basis. Adjusting the previous acute toxicity model for acute-to-chronic ratios yielded chronic models that predict chronic toxicity within the range of intertest variability. Because these models are informed by a wide range of ion mixtures, they should provide robust assessment tools for natural waters enriched with major ions. Environ Toxicol Chem 2019;38:769-783. © 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.

The acute toxicity of major ion salts to Ceriodaphnia dubia. III. Mathematical models for mixture toxicity.

Based on previous research on the acute toxicity of major ions (Na+ , K+ , Ca2+ , Mg2+ , Cl- , SO42- , and HCO3- /CO32- ) to Ceriodaphnia dubia, a mathematical model was developed for predicting the median lethal concentration (LC50) for any ion mixture, excepting those dominated by K-specific toxicity. One component of the model describes a mechanism of general ion toxicity to which all ions contribute and predicts LC50s as a function of osmolarity and Ca activity. The other component describes Mg/Ca-specific toxicity to apply when such toxicity exceeds the general ion toxicity and predicts LC50s as a function of Mg and Ca activities. This model not only tracks well the observed LC50s from past research used for model development but also successfully predicts LC50s from new toxicity tests on synthetic mixtures of ions emulating chemistries of various ion-enriched effluents and receiving waters. It also performs better than a previously published model for major ion toxicity. Because of the complexities of estimating chemical activities and osmolarity, a simplified model based directly on ion concentrations was also developed and found to provide useful predictions. Environ Toxicol Chem 2018;37:247-259. Published 2017 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.

The acute toxicity of major ion salts to Ceriodaphnia dubia. II. Empirical relationships in binary salt mixtures.

Many human activities increase concentrations of major geochemical ions (Na+1 , K+1 , Ca+2 , Mg+2 , Cl-1 , SO4-2 , and HCO3-1 /CO3-2 ) in freshwater systems, and can thereby adversely affect aquatic life. Such effects involve several toxicants, multiple toxicity mechanisms, various ion interactions, and widely varying ion compositions across different water bodies. Previous studies of individual salt toxicities have defined some useful relationships; however, adding single salts to waters results in atypical compositions and does not fully address mixture toxicity. To better understand mechanisms and interactions for major ion toxicity, 29 binary mixture experiments, each consisting of 7 to 8 toxicity tests, were conducted on the acute toxicity of major ion salts and mannitol to Ceriodaphnia dubia. These tests showed multiple mechanisms of toxicity, including: 1) nonspecific ion toxicity, correlated with osmolarity and to which all ions contribute; and 2) cation-dependent toxicities for potassium (K), magnesium (Mg), and calcium (Ca) best related to their chemical activities. These mechanisms primarily operate independently, except for additive toxicity of Mg-dependent and Ca-dependent toxicities. These mixture studies confirmed ameliorative effects of Ca on sodium (Na) and Mg salt toxicities and of Na on K salt toxicity, and further indicated lesser ameliorative effects of Ca on K salt toxicity and Mg on Na salt toxicity. These results provide a stronger basis for assessing risks from the complex mixtures of ions found in surface waters. Environ Toxicol Chem 2017;36:1525-1537. Published 2016 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.

The acute toxicity of major ion salts to Ceriodaphnia dubia: I. influence of background water chemistry.

The ions Na+ , K+ , Ca2+ , Mg2+ , Cl- , SO42- , and HCO3- /CO32- (referred to in the present study as "major ions") are present in all freshwaters and physiologically required by aquatic organisms but can increase to harmful levels from a variety of anthropogenic activities. It is also known that the toxicities of major ion salts can vary depending on the concentrations of other ions, and understanding these relationships is key to establishing appropriate environmental limits. The authors present a series of experiments with Ceriodaphnia dubia to evaluate the acute toxicity of 12 major ion salts and to determine how toxicity of these salts varies as a function of background water chemistry. All salts except CaSO4 and CaCO3 were acutely toxic below saturation, with the lowest median lethal concentrations found for K salts. All 10 salts that showed toxicity also showed some degree of reduced toxicity as the ionic content of the background water increased. Experiments that independently varied Ca:Mg ratio, Na:K ratio, Cl:SO4 ratio, and alkalinity/pH demonstrated that Ca concentration was the primary factor influencing the toxicities of Na and Mg salts, whereas the toxicities of K salts were primarily influenced by the concentration of Na. These experiments also indicated multiple mechanisms of toxicity and suggested important aspects of dosimetry; the toxicities of K, Mg, and Ca salts were best related to the chemical activity of the cation, whereas the toxicities of Na salts also reflected an influence of the anions and were well correlated with osmolarity. Understanding these relationships between major ion toxicity and background water chemistry should aid in the development of sensible risk-assessments and regulatory standards. Environ Toxicol Chem 2016;35:3039-3057. Published 2016 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.

Impact of natural organic matter on particle behavior and phototoxicity of titanium dioxide nanoparticles.

Due to their inherent phototoxicity and inevitable environmental release, titanium dioxide nanoparticles (nano-TiO2) are increasingly studied in the field of aquatic toxicology. One of the particular interests is the interactions between nano-TiO2 and natural organic matter (NOM). In this study, a series of experiments was conducted to study the impacts of Suwannee River natural organic matter (SRNOM) on phototoxicity and particle behaviors of nano-TiO2. For Daphnia magna, after the addition of 5mg/L SRNOM, LC50 value decreased significantly from 1.03 (0.89-1.20) mg/L to 0.26 (0.22-0.31) mg/L. For zebrafish larvae, phototoxic LC50 values were 39.9 (95% CI, 25.9-61.2) mg/L and 26.3 (95% CI, 18.3-37.8) mg/L, with or without the presence of 5mg/L SRNOM, respectively. There was no statistically significant change of these LC50 values. The impact of SRNOM on phototoxicity of nano-TiO2 was highly dependent on test species, with D. magna being the more sensitive species. The impact on particle behavior was both qualitatively and quantitatively examined. A global predictive model for particle behavior was developed with a three-way interaction of SRNOM, TiO2 concentration, and time and an additive effect of ionic strength. Based on power analyses, 96-h exposure in bioassays was recommended for nanoparticle-NOM interaction studies. The importance of reactive oxygen species (ROS) quenching of SRNOM was also systematically studied using a novel exposure system that isolates the effects of environmental factors. These experiments were conducted with minimal impacts of other important interaction mechanisms (NOM particle stabilization, NOM UV attenuation, and NOM photosensitization). This study highlighted both the particle stabilization and ROS quenching effects of NOM on nano-TiO2 in an aquatic system. There is an urgent need for representative test materials, together with key environmental factors, for future risk assessment and regulations of nanomaterials.

Modeling TiO2 nanoparticle phototoxicity: The importance of chemical concentration, ultraviolet radiation intensity, and time.

As a semiconductor with wide band gap energy, TiO2 nanoparticles (nano-TiO2) are highly photoactive, and recent efforts have demonstrated phototoxicity of nano-TiO2 to aquatic organisms. However, a dosimetry model for the phototoxicity of nanomaterials that incorporates both direct UV and photo-activated chemical toxicity has not yet been developed. In this study, a set of Hyalella azteca acute toxicity bioassays at multiple light intensities and nano-TiO2 concentrations, and with multiple diel light cycles, was conducted to assess how existing phototoxicity models should be adapted to nano-TiO2. These efforts demonstrated (a) adherence to the Bunsen-Roscoe law for the reciprocity of light intensity and time, (b) no evidence of damage repair during dark periods, (c) a lack of proportionality of effects to environmental nano-TiO2 concentrations, and (d) a need to consider the joint effects of nano-TiO2 phototoxicity and direct UV toxicity.

The effects of chronological age and size on toxicity of zinc to juvenile brown trout.

A series of toxicity tests were conducted to investigate the role of chronological age on zinc tolerance in juvenile brown trout (Salmo trutta). Four different incubation temperatures were used to control the maturation of the juveniles before zinc exposures. These 96-h exposures used flow-through conditions and four chronological ages of fish with weights ranging from 0.148 to 1.432 g. Time-to-death (TTD) data were collected throughout the exposure along with the final mortality. The results indicate that chronological age does not play a predictable role in zinc tolerance for juvenile brown trout. However, a relationship between zinc tolerance and fish size was observed in all chronological age populations, which prompted us to conduct additional exploratory data analysis to quantify how much of an effect size had during this stage of development. The smallest fish (0.148-0.423 g) were shown to be less sensitive than the largest fish (0.639-1.432 g) with LC50 values of 868 and 354 µg Zn/L, respectively. The Kaplan-Meier product estimation method was used to determine survival functions from the TTD data and supports the LC50 results with a greater median TTD for smaller fish than larger juvenile fish. These results indicate that fish size or a related characteristic may be a significant determinant of susceptibility and should be considered in acute zinc toxicity tests with specific attention paid to the expected exposure scenario in the field.

Chronic TiO2 nanoparticle exposure to a benthic organism, Hyalella azteca: impact of solar UV radiation and material surface coatings on toxicity.

There is limited information on the chronic effects of nanomaterials to benthic organisms, as well as environmental mitigating factors that might influence this toxicity. The present study aimed to fill these data gaps by examining various growth endpoints (weight gain, instantaneous growth rate, and total protein content) for up to a 21 d sediment exposure of TiO2 nanoparticles (nano-TiO2) to a representative benthic species, Hyalella azteca. An uncoated standard, P25, and an Al(OH)3 coated nano-TiO2 used in commercial products were added to sediment at 20 mg/L or 100 mg/L Under test conditions, UV exposure alone was shown to be a greater cause of toxicity than even these high levels of nano-TiO2 exposure, indicating that different hazards need to be addressed in toxicity testing scenarios. In addition, this study showed the effectiveness of a surface coating on the decreased photoactivity of the material, as the addition of an Al(OH)3 coating showed a dramatic decrease in reactive oxygen species (ROS) production. However, this reduced photoactivity was found to be partially restored when the coating had been degraded, leading to the need for future toxicity tests which examine the implications of weathering events on particle surface coatings.

Species sensitivity and dependence on exposure conditions impacting the phototoxicity of TiO2 nanoparticles to benthic organisms.

Toxicity of titanium dioxide nanoparticles (nano-TiO2 ) to aquatic organisms can be greatly increased after exposure to ultraviolet (UV) radiation. This phenomenon has received some attention for water column species; however, investigations of nano-TiO2 phototoxicity for benthic organisms are still limited. In the present study, bioassays of 3 representative benthic organisms (Hyalella azteca, Lumbriculus variegatus, and Chironomus dilutus) were conducted to evaluate nano-TiO2 phototoxicity. When exposed to 20 mg/L of nano-TiO2 and various light intensities (0-30 W/m(2)), H. azteca was the most sensitive, with a median lethal dose of 40.7 (95% confidence interval, 36.3-44.7) Wh/m(2), and hence is a potential model organism in future toxicological guidelines for photoactive nanomaterials to freshwater benthos. Without the presence of nano-TiO2 , no mortality was observed in L. variegatus and C. dilutus exposed to UV intensity ranging from 0 W/m(2) to 41 W/m(2). However, a sharp drop of H. azteca survival was observed when UV intensity was higher than 9.4 W/m(2), demonstrating the importance of UV-only effects on the ultimate phototoxicity of nanomaterials. Furthermore, both bioavailability and surface attachment of nano-TiO2 onto organisms were affected by the exposure scenario, supported by the exposure scenario-dependent phototoxicity seen in H. azteca and C. dilutus. Overall, the present study demonstrates the importance of species sensitivity and exposure scenarios in future test guidelines of nano-phototoxicity.

Adverse outcome pathways: a conceptual framework to support ecotoxicology research and risk assessment.

Ecological risk assessors face increasing demands to assess more chemicals, with greater speed and accuracy, and to do so using fewer resources and experimental animals. New approaches in biological and computational sciences may be able to generate mechanistic information that could help in meeting these challenges. However, to use mechanistic data to support chemical assessments, there is a need for effective translation of this information into endpoints meaningful to ecological risk-effects on survival, development, and reproduction in individual organisms and, by extension, impacts on populations. Here we discuss a framework designed for this purpose, the adverse outcome pathway (AOP). An AOP is a conceptual construct that portrays existing knowledge concerning the linkage between a direct molecular initiating event and an adverse outcome at a biological level of organization relevant to risk assessment. The practical utility of AOPs for ecological risk assessment of chemicals is illustrated using five case examples. The examples demonstrate how the AOP concept can focus toxicity testing in terms of species and endpoint selection, enhance across-chemical extrapolation, and support prediction of mixture effects. The examples also show how AOPs facilitate use of molecular or biochemical endpoints (sometimes referred to as biomarkers) for forecasting chemical impacts on individuals and populations. In the concluding sections of the paper, we discuss how AOPs can help to guide research that supports chemical risk assessments and advocate for the incorporation of this approach into a broader systems biology framework.