Hormesis depends upon the life-stage and duration of exposure: Examples for a pesticide and a nanomaterial

Tests to assess toxic effects on the reproduction of adult C. elegans after 72h exposure for two chemicals, (3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)), also known as diuron, and silver nanoparticles (Ag NPs) indicated potential, although not significant hormesis. Follow up toxicity tests comparing the potential hormesis concentrations with controls at high replication confirmed that the stimulatory effect was repeatable and also statistically significant within the test. To understand the relevance of the hormesis effects for overall population fitness, full life-cycle toxicity tests were conducted for each chemical. When nematodes were exposed to DCMU over the full life-span, the hormesis effect for reproduction seen in short-term tests was no longer evident. Further at the putative hormesis concentrations, a negative effect of DCMU on time to maturation was also seen. For the Ag NPs, the EC50 for effects on reproduction in the life-cycle exposure was substantially lower than in the short-term test, the EC50s estimated by a three parameter log logistic model being 2.9mg/L and 0.75mg/L, respectively. This suggests that the level of toxicity for Ag NPs for C. elegans reproduction is dependant on the life stage exposed and possibly the duration of the exposure. Further, in the longer duration exposures, hormesis effects on reproduction seen in the short-term exposures were no longer apparent. Instead, all concentrations reduced both overall brood size and life-span. These results for both chemicals suggest that the hormesis observed for a single endpoint in short-term exposure may be the result of a temporary reallocation of resources between traits that are not sustained over the full life-time. Such reallocation is consistent with energy budget theories for organisms subject to toxic stress.

Uptake routes and toxicokinetics of silver nanoparticles and silver ions in the earthworm Lumbricus rubellus

Current bioavailability models, such as the free ion activity model and biotic ligand model, explicitly consider that metal exposure will be mainly to the dissolved metal in ionic form. With the rise of nanotechnology products and the increasing release of metal-based nanoparticles (NPs) to the environment, such models may increasingly be applied to support risk assessment. It is not immediately clear, however, whether the assumption of metal ion exposure will be relevant for NPs. Using an established approach of oral gluing, a toxicokinetics study was conducted to investigate the routes of silver nanoparticles (AgNPs) and Ag(+) ion uptake in the soil-dwelling earthworm Lumbricus rubellus. The results indicated that a significant part of the Ag uptake in the earthworms is through oral/gut uptake for both Ag(+) ions and NPs. Thus, sealing the mouth reduced Ag uptake by between 40% and 75%. An X-ray analysis of the internal distribution of Ag in transverse sections confirmed the presence of increased Ag concentrations in exposed earthworm tissues. For the AgNPs but not the Ag(+) ions, high concentrations were associated with the gut wall, liver-like chloragogenous tissue, and nephridia, which suggest a pathway for AgNP uptake, detoxification, and excretion via these organs. Overall, the results indicate that Ag in the ionic and NP forms is assimilated and internally distributed in earthworms and that this uptake occurs predominantly via the gut epithelium and less so via the body wall. The importance of oral exposure questions the application of current metal bioavailability models, which implicitly consider that the dominant route of exposure is via the soil solution, for bioavailability assessment and modeling of metal-based NPs.

Combined Effects from γ Radiation and Fluoranthene Exposure on Carbon Transfer from Phytoplankton to Zooplankton

Risk assessment does not usually take into account mixtures of contaminants, thus potentially under- or overestimating environmental effects. We investigated how the transfer of carbon between a primary producer, Pseudokirchneriella subcapitata, and a consumer, Daphnia magna, is affected by acute exposure of γ radiation (GR) in combination with the polycyclic aromatic hydrocarbon fluoranthene (FA). We exposed D. magna to five concentrations of FA and five acute doses of GR as single contaminants and in nine binary combinations. We compared the observed data for three end points (incorporation of carbon by D. magna, D. magna ingestion rates, and growth) to the predicted joint effects of the mixed stressors based on the independent action (IA) concept. There were deviations from the IA predictions, especially for ingestion rates and carbon incorporation by D. magna, where antagonistic effects were observed at the lower doses, while synergism was seen at the highest doses. Our results highlight the importance of investigating the effects of exposure to GR in a multistressor context. In mixtures of GR and FA, the IA-predicted effects seem to be conservative as antagonism between the two stressors was the dominant pattern, possibly due to stimulation of cellular antioxidative stress mechanisms.

Short-term soil bioassays may not reveal the full toxicity potential for nanomaterials; bioavailability and toxicity of silver ions (AgNO₃) and silver nanoparticles to earthworm Eisenia fetida in long-term aged soils

This study investigated if standard risk assessment hazard tests are long enough to adequately provide the worst case exposure for nanomaterials. This study therefore determined the comparative effects of the aging on the bioavailability and toxicity to earthworms of soils dosed with silver ions and silver nanoparticles (Ag NP) for 1, 9, 30 & 52 weeks, and related this to the total Ag in the soil, Ag in soil pore water and earthworm tissue Ag concentrations. For ionic Ag, a classical pattern of reduced bioavailability and toxicity with time aged in the soil was observed. For the Ag NP, toxicity increased with time apparently driven by Ag ion dissolution from the added Ag NPs. Internal Ag in the earthworms did not always explain toxicity and suggested the presence of an internalised, low-toxicity Ag fraction (as intact or transformed NPs) after shorter aging times. Our results indicate that short-term exposures, without long-term soil aging, are not able to properly assess the environmental risk of Ag NPs and that ultimately, with aging time, Ag ion and Ag NP effect will merge to a common value.

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

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

Identifying biochemical phenotypic differences between cryptic species

Molecular genetic methods can distinguish divergent evolutionary lineages in what previously appeared to be single species, but it is not always clear what functional differences exist between such cryptic species. We used a metabolomic approach to profile biochemical phenotype (metabotype) differences between two putative cryptic species of the earthworm Lumbricus rubellus. There were no straightforward metabolite biomarkers of lineage, i.e. no metabolites that were always at higher concentration in one lineage. Multivariate methods, however, identified a small number of metabolites that together helped distinguish the lineages, including uncommon metabolites such as Nε-trimethyllysine, which is not usually found at high concentrations. This approach could be useful for characterizing functional trait differences, especially as it is applicable to essentially any species group, irrespective of its genome sequencing status.

CeO2 nanoparticles induce no changes in phenanthrene toxicity to the soil organisms Porcellionides pruinosus and Folsomia candida

Cerium oxide nanoparticles (CeO2 NPs) are used as diesel fuel additives to catalyze oxidation. Phenanthrene is a major component of diesel exhaust particles and one of the most common pollutants in the environment. This study aimed at determining the effect of CeO2 NPs on the toxicity of phenanthrene in Lufa 2.2 standard soil for the isopod Porcellionides pruinosus and the springtail Folsomia candida. Toxicity tests were performed in the presence of CeO2 concentrations of 10, 100 or 1000mg Ce/kg dry soil and compared with results in the absence of CeO2 NPs. CeO2 NPs had no adverse effects on isopod survival and growth or springtail survival and reproduction. For the isopods, LC50s for the effect of phenanthrene ranged from 110 to 143mg/kg dry soil, and EC50s from 17.6 to 31.6mg/kg dry soil. For the springtails, LC50s ranged between 61.5 and 88.3mg/kg dry soil and EC50s from 52.2 to 76.7mg/kg dry soil. From this study it may be concluded that CeO2 NPs have a low toxicity and do not affect toxicity of phenanthrene to isopods and springtails.

Analytical approaches to support current understanding of exposure, uptake and distributions of engineered nanoparticles by aquatic and terrestrial organisms

Initiatives to support the sustainable development of the nanotechnology sector have led to rapid growth in research on the environmental fate, hazards and risk of engineered nanoparticles (ENP). As the field has matured over the last 10 years, a detailed picture of the best methods to track potential forms of exposure, their uptake routes and best methods to identify and track internal fate and distributions following assimilation into organisms has begun to emerge. Here we summarise the current state of the field, focussing particularly on metal and metal oxide ENPs. Studies to date have shown that ENPs undergo a range of physical and chemical transformations in the environment to the extent that exposures to pristine well dispersed materials will occur only rarely in nature. Methods to track assimilation and internal distributions must, therefore, be capable of detecting these modified forms. The uptake mechanisms involved in ENP assimilation may include a range of trans-cellular trafficking and distribution pathways, which can be followed by passage to intracellular compartments. To trace toxicokinetics and distributions, analytical and imaging approaches are available to determine rates, states and forms. When used hierarchically, these tools can map ENP distributions to specific target organs, cell types and organelles, such as endosomes, caveolae and lysosomes and assess speciation states. The first decade of ENP ecotoxicology research, thus, points to an emerging paradigm where exposure is to transformed materials transported into tissues and cells via passive and active pathways within which they can be assimilated and therein identified using a tiered analytical and imaging approach.

Effect of soil organic matter content and pH on the toxicity of ZnO nanoparticles to Folsomia candida

Organic matter (OM) and pH may influence nanoparticle fate and effects in soil. This study investigated the influence of soil organic matter content and pH on the toxicity of ZnO-NP and ZnCl2 to Folsomia candida in four natural soils, having between 2.37% and 14.7% OM and [Formula: see text] levels between 5.0 and 6.8. Porewater Zn concentrations were much lower in ZnO-NP than in ZnCl2 spiked soils, resulting in higher Freundlich sorption constants for ZnO-NP. For ZnCl2 the porewater Zn concentrations were significantly higher in less organic soils, while for ZnO-NP the highest soluble Zn level (23mgZn/l) was measured in the most organic soil, which had the lowest pH. Free Zn(2+) ion concentrations were higher for ZnCl2 than for ZnO-NP and were greatly dependent on pH (pHpw) and dissolved organic carbon content of the pore water. The 28-d EC50 values for the effect of ZnCl2 on the reproduction of F. candida increased with increasing OM content from 356 to 1592mgZn/kg d.w. For ZnO-NP no correlation between EC50 values and OM content was found and EC50 values ranged from 1695 in the most organic soil to 4446mgZn/kg d.w. in the higher pH soil. When based on porewater and free Zn(2+) concentrations, EC50 values were higher for ZnCl2 than for ZnO-NP, and consistently decreased with increasing pHpw. This study shows that ZnO-NP toxicity is dependent on soil properties, but is mainly driven by soil pH.

The importance of experimental time when assessing the effect of temperature on toxicity in poikilotherms

Temperature is an important factor affecting toxicity, determining chemical toxicokinetics and toxicodynamics in poikilothermic organisms. Because metabolic rates are also affected by temperature, interactions between the emergence of toxic effects and time are very likely. The aim of the present study was to investigate how temperature affects the toxicity of copper toward the nematode Caenorhabditis elegans when measured during short, fixed time frames compared with full life cycles. Copper toxicity was tested in 2 experiments at 4 or 6 temperatures in the range of 11 °C to 24 °C, with Cu concentrations spanning from 1 mg Cu/L agar to 40 mg Cu/L agar, respectively. Reproduction and mortality were determined across the entire life cycle, and the time to production of first egg and the population growth rate were calculated. The results showed that the 50% effect concentrations (EC50s) of Cu increased 1.5-fold to 2.5-fold with increasing temperature within the tested range, depending on endpoint. When calculating EC50 on reproduction after 24 h or 96 h, the typical setup for temperature-chemical interaction studies, results ranged from no temperature effect to effects much larger than those for the full life cycle. Studies of temperature effects on toxicity must therefore be carefully designed in relation to the research question being investigated.

Metabolomic analysis of soil communities can be used for pollution assessment

Metabolic profiling can be used to assess the changes in biochemical profiles of soil communities living in contaminated sites. The term "community metabolomics" is proposed for the application of metabolomics techniques to the study of the entire community of a soil sample. The authors anticipate the present study to be a starting point for the use of this technique to assess how communities respond to factors such as pollution and climate change.

Earthworms produce phytochelatins in response to arsenic

Phytochelatins are small cysteine-rich non-ribosomal peptides that chelate soft metal and metalloid ions, such as cadmium and arsenic. They are widely produced by plants and microbes; phytochelatin synthase genes are also present in animal species from several different phyla, but there is still little known about whether these genes are functional in animals, and if so, whether they are metal-responsive. We analysed phytochelatin production by direct chemical analysis in Lumbricus rubellus earthworms exposed to arsenic for a 28 day period, and found that arsenic clearly induced phytochelatin production in a dose-dependent manner. It was necessary to measure the phytochelatin metabolite concentrations directly, as there was no upregulation of phytochelatin synthase gene expression after 28 days: phytochelatin synthesis appears not to be transcriptionally regulated in animals. A further untargetted metabolomic analysis also found changes in metabolites associated with the transsulfuration pathway, which channels sulfur flux from methionine for phytochelatin synthesis. There was no evidence of biological transformation of arsenic (e.g. into methylated species) as a result of laboratory arsenic exposure. Finally, we compared wild populations of earthworms sampled from the field, and found that both arsenic-contaminated and cadmium-contaminated mine site worms had elevated phytochelatin concentrations.

Low temperatures enhance the toxicity of copper and cadmium to Enchytraeus crypticus through different mechanisms

Knowledge about how toxicity changes with temperature is important for determining the extent of safety factors required when extrapolating from standard laboratory conditions to variable field scenarios. In the present study, the authors evaluated the toxicity of Cu and Cd to the potworm Enchytraeus crypticus at 6 temperatures in the range of 11 °C to 25 °C. For both metals, reproductive toxicity decreased approximately 2.5-fold with increasing temperature. This is contrary to what most other studies have found. Measurements of the bioavailable fraction of the metals in the soils and the internal metal concentrations in the worms over time showed that the major cause of change in toxicity with temperature for Cu was the worms' ability to regulate internal concentration at high temperatures. Uptake of Cd increased with time at all temperatures and with higher rates at high temperatures. Hence, the lower toxicity of Cd at high temperatures is proposed to be due to the E. crypticus being more efficient at immobilizing Cd and/or repairing damages at high compared to low temperatures. The present study concludes that no consistent relationship between metal toxicity and temperature across species can be made. The metabolic dependence of the species in terms of regulating metal uptake, excretion, immobilization, damage, and repair processes, will be crucial factors in determining species susceptibility to metals at varying temperatures.

A new medium for Caenorhabditis elegans toxicology and nanotoxicology studies designed to better reflect natural soil solution conditions

A new toxicity test medium for Caenorhabditis elegans is presented. The test solution is designed to provide a better representation of natural soil pore water conditions than currently available test media. The medium has a composition that can readily be modified to allow for studies of the influences of a range of environmentally relevant parameters on nematode biology and toxicology. Tests conducted in the new medium confirmed that nematodes' reproduction was possible at a range of solution pH levels, offering the potential to conduct toxicity studies under a variety of conditions. A test to establish silver nanoparticle and dissolved silver nitrate toxicity, a study type not feasible in M9 or agar media due to precipitation and nanoparticle agglomeration, indicated lower silver nanoparticle (median effective concentration [EC50] of 6.5 mg Ag/L) than silver nitrate (EC50 0.28 mg Ag/L) toxicity. Characterization identified stable nanoparticle behavior in the new test medium.

Modelling the effects of copper on soil organisms and processes using the free ion approach: towards a multi-species toxicity model

The free ion approach has been previously used to calculate critical limit concentrations for soil metals based on point estimates of toxicity. Here, the approach was applied to dose-response data for copper effects on seven biological endpoints in each of 19 European soils. The approach was applied using the concept of an effective dose, comprising a function of the concentrations of free copper and 'protective' major cations, including H(+). A significant influence of H(+) on the toxicity of Cu(2+) was found, while the effects of other cations were inconsistent. The model could be generalised by forcing the effect of H(+) and the slope of the dose-response relationship to be equal for all endpoints. This suggests the possibility of a general bioavailability model for copper effects on organisms. Furthermore, the possibility of such a model could be explored for other cationic metals such as nickel, zinc, cadmium and lead.

Metal-based nanoparticles in soil: fate, behavior, and effects on soil invertebrates

Metal-based nanoparticles (NPs) (e.g., silver, zinc oxide, titanium dioxide, iron oxide) are being widely used in the nanotechnology industry. Because of the release of particles from NP-containing products, it is likely that NPs will enter the soil compartment, especially through land application of sewage sludge derived from wastewater treatment. This review presents an overview of the literature dealing with the fate and effects of metal-based NPs in soil. In the environment, the characteristics of NPs (e.g., size, shape, surface charge) and soil (e.g., pH, ionic strength, organic matter, and clay content) will affect physical and chemical processes, resulting in NP dissolution, agglomeration, and aggregation. The behavior of NPs in soil will control their mobility and their bioavailability to soil organisms. Consequently, exposure characterization in ecotoxicological studies should obtain as much information as possible about dissolution, agglomeration, and aggregation processes. Comparing existing studies is a challenging task, because no standards exist for toxicity tests with NPs. In many cases, the reporting of associated characterization data is sparse, or missing, making it impossible to interpret and explain observed differences in results among studies.

A metabolomics based test of independent action and concentration addition using the earthworm Lumbricus rubellus

A major challenge in ecotoxicology is to understand the effects of multiple toxicants on organisms. Here we assess the effects on survival, weight change, cocoon production and metabolism caused by exposure to two similarly acting (imidacloprid/thiacloprid) and two dissimilarly acting (chlorpyrifos/Nickel) chemicals on the earthworm Lumbricus rubellus. We assessed the standard models of concentration addition (CA) and independent action (IA), in conjunction with a metabolomics based approach to elucidate mechanisms of effect. For imidacloprid and thiacloprid the reproductive effects indicated probable additivity. Although this suggests joint effects through a similar mechanism, metabolite changes for each pesticide actually indicated distinct effects. Further, earthworms exposed to a 0.5 toxic unit equitoxic mixture demonstrated metabolic effects intermediate between those for each pesticide, indicating a non-interactive, independent joint effect. For higher effect level mixtures (1 and 1.5 toxic units), metabolite changes associated with thiacloprid exposure began to dominate. The metabolomic effects of the two dissimilarly acting chemicals were distinct, confirming separate modes of action and both proved more toxic than anticipated from previous studies. In the mixtures, phenotypic effects were in accordance with IA estimates, while metabolite changes were dominated by Ni effects, even though chlorpyrifos contributed most to reproductive toxicity. This could be attributed to the greater systematic effect of Ni when compared to the more specifically acting chlorpyrifos.

Metabolic profiling detects early effects of environmental and lifestyle exposure to cadmium in a human population

BACKGROUND

The 'exposome' represents the accumulation of all environmental exposures across a lifetime. Top-down strategies are required to assess something this comprehensive, and could transform our understanding of how environmental factors affect human health. Metabolic profiling (metabonomics/metabolomics) defines an individual's metabolic phenotype, which is influenced by genotype, diet, lifestyle, health and xenobiotic exposure, and could also reveal intermediate biomarkers for disease risk that reflect adaptive response to exposure. We investigated changes in metabolism in volunteers living near a point source of environmental pollution: a closed zinc smelter with associated elevated levels of environmental cadmium.

METHODS

High-resolution ¹H NMR spectroscopy (metabonomics) was used to acquire urinary metabolic profiles from 178 human volunteers. The spectral data were subjected to multivariate and univariate analysis to identify metabolites that were correlated with lifestyle or biological factors. Urinary levels of 8-oxo-deoxyguanosine were also measured, using mass spectrometry, as a marker of systemic oxidative stress.

RESULTS

Six urinary metabolites, either associated with mitochondrial metabolism (citrate, 3-hydroxyisovalerate, 4-deoxy-erythronic acid) or one-carbon metabolism (dimethylglycine, creatinine, creatine), were associated with cadmium exposure. In particular, citrate levels retained a significant correlation to urinary cadmium and smoking status after controlling for age and sex. Oxidative stress (as determined by urinary 8-oxo-deoxyguanosine levels) was elevated in individuals with high cadmium exposure, supporting the hypothesis that heavy metal accumulation was causing mitochondrial dysfunction.

CONCLUSIONS

This study shows evidence that an NMR-based metabolic profiling study in an uncontrolled human population is capable of identifying intermediate biomarkers of response to toxicants at true environmental concentrations, paving the way for exposome research.

Can the joint effect of ternary mixtures be predicted from binary mixture toxicity results?

The joint effect of the majority of chemical mixtures can be predicted using the reference model of Concentration Addition (CA). It becomes a challenge, however, when the mixtures include chemicals that synergise or antagonise the effect of each other. In this study we examine if the deviation from CA of seven ternary mixtures of interacting chemicals can be predicted from knowledge of the binary mixture responses involved. We hypothesise that the strongest interactions will take place in the binary mixtures and that the size of the ternary mixture response can be predicted from knowledge of the binary interactions. The hypotheses were tested using a stepwise modelling approach of incorporating the information held in binary mixtures into a ternary mixture model, and comparing the model predictions with observed ternary mixture toxicity data derived from studies of interacting chemical mixtures on the floating plant Lemna minor and the bacteria Vibrio fischeri. The results showed that for both the antagonistic and the synergistic ternary mixtures the ternary model predictions were superior to the conventional CA reference model and provided robust estimations of the size of the experimentally derived ternary mixture toxicity effects.