Effects of physiochemical properties of test media on nanoparticle toxicity to Daphnia magna Straus

Development of a Quasi-Quantitative Structure-Activity Relationship Model for Prediction of the Immobilization Response of Daphnia magna Exposed to Metal-Based Nanomaterials

The conventional Hill equation model is suitable to fit dose-response data obtained from performing (eco)toxicity assays. Models based on quasi-quantitative structure-activity relationships (QSARs) to estimate the Hill coefficient ( n H ) ${n}_{{\rm{H}}})$ were developed with the aim of predicting the response of the invertebrate species Daphnia magna to exposure to metal-based nanomaterials. Descriptors representing the pristine properties of nanoparticles and media conditions were coded to a quasi-simplified molecular input line entry system and correlated to experimentally derived values of n H ${n}_{{\rm{H}}}$ . Monte Carlo optimization was used to model the set of n H ${n}_{{\rm{H}}}$  values, and the model was trained on the basis of reported dose-response relationships of 60 data sets (n = 367 individual response observations) of 11 metal-based nanomaterials as obtained from 20 literature reports. The model simulates the training data well, with only 2.3% deviation between experimental and modeled response data. The technique was employed to predict the dose-response relationships of 15 additional data sets (n = 72 individual observations) not included in model development of seven metal-based nanomaterials from 10 literature reports, with an average error of 3.5%. Combining the model output with either the median effective concentration value or any other known effect level as obtained from experimental data allows the prediction of full dose-response curves of D. magna immobilization. This model is an accurate screening tool that allows the determination of the shape and slope of dose-response curves, thereby greatly reducing experimental effort in case of novel advanced metal-based nanomaterials or the prediction of responses in altered exposure media. This screening model is compliant with the 3Rs (replacement, reduction, and refinement) principle, which is embraced by the scientific and regulatory communities dealing with nano-safety. Environ Toxicol Chem 2022;41:1439-1450. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Transcriptomic analysis and transgenerational effects of ZnO nanoparticles on Daphnia magna: Endocrine-disrupting potential and energy metabolism

The widespread application of zinc oxide nanoparticles (ZnO NPs) has raised concerns over the adverse effects on aquatic species. In this study, transcriptomic analysis was applied to evaluate the chronic toxicity of ZnO NPs on the freshwater invertebrate Daphnia magna and the intergenerational effects were then further investigated. Parent daphnia (F0) were exposed to ZnO NPs at 3, 60, and 300 μg L^-1 for 21 days. ZnO NPs significantly inhibited the reproduction (first pregnancy and spawning time, total number of offspring) and growth (molting frequency and body length) of F0. Here, differentially expressed genes (DEGs) involved in lysosomal and phagosome, energy metabolism and endocrine disruption pathways were significantly downregulated. Furthermore, disruption on the transport and catabolic processes probably resulted in the particle accumulation. The inhibited pathways related to energy metabolism may partially account for the body length, molting and reproductive restriction. The suppression of growth and reproduction may attribute to the down-regulation of insulin secretion and ovarian steroidogenesis pathways, respectively. Partial recovery of growth and reproductive inhibition in F1 - F3 descended from the F0 generation exposure did not support constant transgenerational effects. This study unravels the molecular mechanisms and transgenerational consequences of the toxicity of nanoparticles on Daphnia.

Ordered weighted average based grouping of nanomaterials with Arsinh and dose response similarity models

In the context of the EU GRACIOUS project, we propose a novel procedure for similarity assessment and grouping of nanomaterials. This methodology is based on the (1) Arsinh transformation function for scalar properties, (2) full curve shape comparison by application of a modified Kolmogorov-Smirnov metric for bivariate properties, (3) Ordered Weighted Average (OWA) aggregation-based grouping distance, and (4) hierarchical clustering. The approach allows for grouping of nanomaterials that is not affected by the dataset, so that group membership will not change when new candidates are included in the set of assessed materials. To facilitate the application of the proposed methodology, a software script was developed by using the R programming language which is currently under migration to a web tool. The presented approach was tested against a dataset, derived from literature review, related to immobilization of Daphnia magna and reporting information on several nanomaterials and properties.

The impacts of metal-based engineered nanomaterial mixtures on microbial systems: A review

The last decade has witnessed tremendous growth in the commercial use of metal-based engineered nanomaterials (ENMs) for a wide range of products and processes. Consequently, direct and indirect release into environmental systems may no longer be considered negligible or insignificant. Yet, there is an active debate as to whether there are real risks to human or ecological health with environmental exposure to ENMs. Previous research has focused primarily on the acute effects of individual ENMs using pure cultures under controlled laboratory environments, which may not accurately reveal the ecological impacts of ENMs under real environmental conditions. The goal of this review is to assess our current understanding of ENM effects as we move from exposure of single to multiple ENMs or microbial species. For instance, are ENMs' impacts on microbial communities predicted by their intrinsic physical or chemical characteristics or their effects on single microbial populations; how do chronic ENM interactions compare to acute toxicity; does behavior under simplified laboratory conditions reflect that in environmental media; finally, is biological stress modified by interactions in ENM mixtures relative to that of individual ENM? This review summarizes key findings and our evolving understanding of the ecological effects of ENMs under complex environmental conditions on microbial systems, identifies the gaps in our current knowledge, and indicates the direction of future research.

The Toxicity of Nanoparticles to Organisms in Freshwater

Nanotechnology is a rapidly growing industry yielding many benefits to society. However, aquatic environments are at risk as increasing amounts of nanoparticles (NPs) are contaminating waterbodies causing adverse effects on aquatic organisms. In this review, the impacts of environmental exposure to NPs, the influence of the physicochemical characteristics of NPs and the surrounding environment on toxicity and mechanisms of toxicity together with NP bioaccumulation and trophic transfer are assessed with a focus on their impacts on bacteria, algae and daphnids. We identify several gaps which need urgent attention in order to make sound decisions to protect the environment. These include uncertainty in both estimated and measured environmental concentrations of NPs for reliable risk assessment and for regulating the NP industry. In addition toxicity tests and risk assessment methodologies specific to NPs are still at the research and development stage. Also conflicting and inconsistent results on physicochemical characteristics and the fate and transport of NPs in the environment suggest the need for further research. Finally, improved understanding of the mechanisms of NP toxicity is crucial in risk assessment of NPs, since conventional toxicity tests may not reflect the risks associated with NPs. Behavioural effects may be more sensitive and would be efficient in certain situations compared with conventional toxicity tests due to low NP concentrations in field conditions. However, the development of such tests is still lacking, and further research is recommended.

The Toxicity of Nonaged and Aged Coated Silver Nanoparticles to Freshwater Alga Raphidocelis subcapitata

The transformation of coated silver nanoparticles (AgNPs) and their impacts on aquatic organisms require further study. The present study investigated the role of aging on the transformation of differently coated AgNPs and their sublethal effects on the freshwater alga Raphidocelis subcapitata. The stability of AgNPs was evaluated over 32 d, and the results indicated that transformation of AgNPs occurred during the incubation; however, coating-specific effects were observed. Fresh AgNPs increased reactive oxygen species (ROS) formation, whereas aged AgNPs induced excessive ROS generation compared with their fresh counterparts. Increased ROS levels caused increased lipid peroxidation (LPO) in treatment groups exposed to both fresh and aged NPs, although LPO was comparatively higher in algae exposed to aged AgNPs. The observed increase in catalase (CAT) activity of algal cells was attributed to early stress responses induced by excessive intracellular ROS generation, and CAT levels were higher in the aged NP treatment groups. In conclusion, AgNPs increased ROS levels and LPO in algae and caused the activation of antioxidant enzymes such as CAT. Overall, the results suggest that aging and coating of AgNPs have major impacts on AgNP transformation in media and their effects on algae. Environ Toxicol Chem 2019;38:2371-2382. © 2019 SETAC.

Comparative dissolution, uptake, and toxicity of zinc oxide particles in individual aquatic species and mixed populations

Potential differences in species susceptibility to nanoparticle (NP) contaminants make the use of multispecies community toxicity testing strategies beneficial in understanding NP risk to aquatic environments. Because of the limited knowledge of zinc oxide (ZnO) NP fate and toxicity, we conducted multispecies exposures and compared the responses of individual species to the same species in a community comprised of algae (Chlamydomonas reinhardtii), bacteria (Escherichia coli), crustaceans (Daphnia magna), and zebrafish (Danio rerio). Different-sized ZnO particles and ionic Zn were compared to investigate the contribution of particulate and dissolved Zn to aquatic organism toxicity. Each organism and community was exposed to Zn sources at 0.08, 0.8, and 8 mg Zn/L. The present results indicate that all 3 types of Zn elicited differential toxicity among test organisms, with stronger adverse outcomes observed in single species than within a community. The community assay (nanocosm) we developed increased resilience to all Zn exposures by 5 to 10% compared to individual exposures at equivalent concentrations. In addition, the uptake and toxicity of ZnO particles to aquatic communities appear to be driven by rapid dissolution and the concomitant impacts of zinc ion toxicity, and the size of the ZnO particles had little impact on uptake or toxicity. The nanocosm assay could be a useful screening tool for rapidly assessing the potential impacts of nanomaterials to aquatic species. Environ Toxicol Chem 2019;38:591-602. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

The toxicity of coated silver nanoparticles to Daphnia carinata and trophic transfer from alga Raphidocelis subcapitata

Nanoparticles (NPs) are causing threats to the environment. Silver NPs (AgNPs) are increasingly used in commercial products and may end up in freshwater ecosystems. The freshwater organisms are vulnerable due to water-borne and dietary exposure to AgNPs. Surface properties play an important role in the fate and behavior of AgNPs in the aquatic environment and their effects on organisms. However, effects of surface properties of AgNPs on organisms are poorly understood. In this study, we explored the effects of AgNPs coated with three different ligands; Tyrosine (T-AgNP), Epigallocatechin gallate (E-AgNP) and Curcumin (C-AgNP) in relation to the toxicity to a key aquatic organism; Daphnia carinata. The study focused on how coatings determine fate of NPs in the medium, mortality, feeding behaviour, bioaccumulation and trophic transfer from the freshwater alga, Raphidocelis subcapitata to daphnids. NP stability tests indicated that T-AgNPs were least stable in the ASTM daphnia medium while C-AgNPs were most stable. 48 h EC50 values of AgNPs to D. carinata were in the order of E-AgNP (19.37 μg L-1) > C-AgNP (21.37 μg L-1) > T-AgNP (49.74 μg L-1) while the 48 h EC50 value of Ag+ ions was 1.21 μg L-1. AgNP contaminated algae significantly decreased the feeding rates of daphnids. However, no significant differences were observed in feeding rates between algae contaminated with differently coated AgNPs. Trophic transfer studies showed that AgNPs were transferred from algae to daphnids. The bioacumulation of AgNPs in algae and the diet-borne bioaccumulation of AgNPs in daphnids varied for differently coated AgNPs. Bioaccumulation of C-AgNPs in algae was 1.5 time higher than T-AgNPs. However, the accumulation of T-AgNPs in daphnids via trophic transfer was 2.6 times higher than T-AgNPs. The knowledge generated from this study enhances the understanding of surface property dependent toxicity, bioaccumulation and trophic transfer of AgNPs in aquatic environments.

Combined Effects of Test Media and Dietary Algae on the Toxicity of CuO and ZnO Nanoparticles to Freshwater Microcrustaceans Daphnia magna and Heterocypris incongruens: Food for Thought

The chemical composition of the test medium as well as the presence of algae (microcrustaceans' food) affects the bioavailability and thus the toxicity of metal nanoparticles (NP) to freshwater microcrustaceans. This study evaluated the effect of the addition of algae (Rapidocelis subcapitata at 7.5 × 10⁶ cells/mL) on the toxicity of CuO (primary size 22⁻25 nm) and ZnO NP (10⁻15 nm) to planktic Daphnia magna and benthic Heterocypris incongruens in artificial (mineral) and natural freshwater (lake water). The toxicity of ionic controls, CuSO₄ and ZnSO₄, was evaluated in parallel. When algae were added and the toxicity was tested in mineral medium, 48 h EC₅₀ of CuO and ZnO NP to D. magna was ~2 mg metal/L and 6-day LC₅₀ of H. incongruens was 1.1 mg metal/L for CuO and 0.36 mg metal/L for ZnO. The addition of algae to D. magna test medium mitigated the toxicity of CuO and ZnO NP 4⁻11-fold when the test was conducted in natural water but not in the artificial freshwater. The addition of algae mitigated the toxicity of CuSO₄ (but not ZnSO₄) to D. magna at least 3-fold, whatever the test medium. In the 6-day H. incongruens tests (all exposures included algae), only up to 2-fold differences in metal NP and salt toxicity between mineral and natural test media were observed. To add environmental relevance to NP hazard assessment for the freshwater ecosystem, toxicity tests could be conducted in natural water and organisms could be fed during the exposure.

Dietary transfer of zinc oxide particles from algae (Scenedesmus obliquus) to daphnia (Ceriodaphnia dubia)

The rapid increase in production and usage of ZnO particles in recent years has instigated the concerns regarding their plausible effects on the environment. Current study explores the trophic transfer potential of ZnO particles of different sizes (50, 100 nm and bulk particles) from algae (Scenedesmus obliquus) to daphnia (Ceriodaphnia dubia) and the contribution of ZnO_(ions) (effect of dissolved Zn ions that remain in test medium after separation NPs) to the overall toxicity of ZnO_(total) (impact of both particle and dissolved Zn ions). Toxicity and uptake of ZnO_(total) and ZnO_(ions) in algae were found to be dependent on the concentration and particle size. Feeding of Zn accumulated algae (517 ± 28, 354.7 ± 61 and 291 ± 20 µg/g dry wt.) post-exposure to 61 µM of ZnO_(total) of 50, 100 nm and bulk ZnO particles caused a significant decrease in the survival (15-20%) of daphnia. A significant amount of Zn accumulation was observed in daphnia even after the 48 h depuration period. Biomagnification factor was found to be nearly 1 for all the sizes of ZnO particles tested. For 50 nm ZnO, the BMF was higher when compared to other two sizes, reaching the mean value of 1.06 ± 0.01 at 61 µM. Further analysis revealed that the dietary uptake of different sizes of ZnO particles caused ultra-structural damages and degradation of internal organs in daphnia.

Comparative assessment of toxicity of ZnO and amine-functionalized ZnO nanorods toward Daphnia magna in acute and chronic multigenerational tests

Zinc oxide nanomaterials (ZnO NM) have been used in a large number of applications due to their interesting physicochemical properties. However, the increasing use of ZnO NM has led to concerns regarding their environmental impacts. In this study, the acute and chronic toxicity of ZnO nanorods (NR) bare (ZnONR) and amine-functionalized (ZnONR@AF) toward the freshwater microcrustacean Daphnia magna was evaluated. The ZnO NR were characterized by transmission electron microscopy (TEM), X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and the zeta potential and hydrodynamic diameter (HD). The acute EC50_(48h) values for D. magna revealed that the ZnONR@AF were more toxic than the ZnONR. The generation of reactive oxygen species (ROS) was observed in both NM. Regarding the chronic toxicity, the ZnONR@AF were again found to be more toxic than the ZnONR toward D. magna. An effect on longevity was observed for ZnONR, while ZnONR@AF affected the reproduction, growth and longevity. In the multigenerational recovery test, we observed that maternal exposure can affect the offspring even when these organisms are not directly exposed to the ZnO NR.

Time-Dependent Toxicity Responses in Daphnia magna Exposed to CuO and ZnO Nanoparticles

Aggregation and dissolution of CuO and ZnO nanoparticles (NPs) increased with increasing exposure time (24, 48, and 72 h). Acute toxicity of CuO NPs to Daphnia magna also increased significantly with increasing exposure time (p < 0.05), whereas exposure time did not significantly affect acute toxicity of ZnO NPs. The dissolved Cu concentration of CuO NPs was much lower than the median effective concentration (EC₅₀) value (44 μg L^-1 at 72 h), implying that the increase in acute toxicity was caused by particles rather than by dissolved ions. However, the dissolved Zn concentration of ZnO NPs was higher than the EC₅₀ value (600 μg L^-1 at 72 h), suggesting this acute toxicity may be caused by dissolved ions. Moreover, CuO NPs induced greater lipid peroxidation than Cu ions did at an exposure time of 72 h, whereas converse results were observed for ZnO NPs.

Toxicity of CuO Nanoparticles to Structure and Metabolic Activity of Allium cepa Root Tips

Roots of Allium cepa were exposed to six CuO NPs suspensions (0, 5, 10, 20, 40, 80 mg L^-1) in this study. Results revealed that with the increase of CuO NPs concentration, the Cu content in roots increased significantly. Compared to control, onion roots treated with CuO NPs (except 5 mg L^-1 suspension) grew slowly after 24 h. The surface of the root cap and meristematic zone were obviously damaged. The apical meristem of roots treated by 10 mg L^-1 and above concentrations stopped division. The nucleus of meristematic cells deformed, and nucleoli number increased. The plasmolysis occurred, and the cell membrane and nuclear membrane fractured. With the increase of CuO NPs concentration, the MDA content increased, and the root activity decreased. When dealt with 80 mg L^-1 CuO NPs for 72 h, onion roots appeared to be corroded.

Influence of pH and media composition on suspension stability of silver, zinc oxide, and titanium dioxide nanoparticles and immobilization of Daphnia magna under guideline testing conditions

In aquatic toxicity testing of engineered nanoparticles (ENPs) the process of agglomeration is very important as it may alter bioavailability and toxicity. In the present study, we aimed to identify test conditions that are favorable for maintaining stable ENP suspensions. We evaluated the influence of key environmental parameters: pH (2-12) and ionic strength using M7, Soft EPA (S EPA) medium, and Very Soft EPA (VS EPA) medium; and observed the influence of these parameters on zeta potential, zeta average, and acute immobilization of Daphnia magna for three different ENPs. Despite being sterically stabilized, test suspensions of silver (Ag) ENPs formed large agglomerates in both VS EPA and M7 media; and toxicity was found to be higher in VS EPA medium due to increased dissolution. Low-agglomerate suspensions for zinc oxide (ZnO) could be obtained at pH 7 in VS EPA medium, but the increase in dissolution caused higher toxicity than in M7 medium. Titanium dioxide (TiO2) ENPs had a point of zero charge in the range of pH 7-8. At pH 7 in VS EPA, agglomerates with smaller hydrodynamic diameters (~200nm) were present compared to the high ionic strength M7 medium where hydrodynamic diameters reached micrometer range. The stable suspensions of TiO2 ENPs caused immobilization of D. magna, 48-h EC50 value of 13.7mgL(-1) (95% CI, 2.4mg-79.1mgL(-1)); whereas no toxicity was seen in the unstable, highly agglomerated M7 medium suspensions, 48-h EC50 >100mgL(-1). The current study provides a preliminary approach for methodology in testing and assessing stability and toxicity of ENPs in aquatic toxicity tests of regulatory relevance.

EU Regulation of Nanobiocides: Challenges in Implementing the Biocidal Product Regulation (BPR)

The Biocidal Products Regulation (BPR) contains several provisions for nanomaterials (NMs) and is the first regulation in the European Union to require specific testing and risk assessment for the NM form of a biocidal substance as a part of the information requirements. Ecotoxicological data are one of the pillars of the information requirements in the BPR, but there are currently no standard test guidelines for the ecotoxicity testing of NMs. The overall objective of this work was to investigate the implications of the introduction of nano-specific testing requirements in the BPR and to explore how these might be fulfilled in the case of copper oxide nanoparticles. While there is information and data available in the open literature that could be used to fulfill the BPR information requirements, most of the studies do not take the Organisation for Economic Co-operation and Development's nanospecific test guidelines into consideration. This makes it difficult for companies as well as regulators to fulfill the BPR information requirements for nanomaterials. In order to enable a nanospecific risk assessment, best practices need to be developed regarding stock suspension preparation and characterization, exposure suspensions preparation, and for conducting ecotoxicological test.

Assessment of silver nanoparticle toxicity for common carp (Cyprinus carpio) fish embryos using a novel method controlling the agglomeration in the aquatic media

Formation of agglomerates and their rapid sedimentation during aquatic ecotoxicity testing of nanoparticles is a major issue with a crucial influence on the risk assessment of nanomaterials. The present work is aimed at developing and testing a new approach based on the periodic replacement of liquid media during an ecotoxicological experiment which enabled the efficient monitoring of exposure conditions. A verified mathematical model predicted the frequencies of media exchanges which checked for formation of agglomerates from silver nanoparticles AgNP with 50 nm average size of the original colloid. In the model experiments, embryos of common carp Cyprinus carpio were exposed repeatedly for 6 h to AgNPs (5-50 μm Ag L(-1)) either under semistatic conditions (exchange of media after 6 h) or in variants with frequent media exchanges (varying from 20 to 300 min depending on the AgNP colloid concentration and the desired maximum agglomerate size of 200 or 400 nm). In contrast to other studies, where dissolved free metals are usually responsible for toxic effects, our 144-h experiments demonstrated the importance of AgNP agglomerates in the adverse effects of nanosilver. Direct adsorption of agglomerates on fish embryos locally increased Ag concentrations which resulted in pronounced toxicity particularly in variants with larger 400 nm agglomerates. The present study demonstrates the suitability of the novel methodology in controlling the conditions during aquatic nanomaterial toxicity testing. It further provided insights into the mechanisms underlying the effects of AgNP, which rank on a global scale among the most widely used nanomaterials.

Exposure medium: key in identifying free Ag+ as the exclusive species of silver nanoparticles with acute toxicity to Daphnia magna

It is still not very clear what roles the various Ag species play in the toxicity of silver nanoparticles (AgNPs). In this study, we found that traditional exposure media result in uncontrollable but consistent physicochemical transformation of AgNPs, causing artifacts in determination of median lethal concentration (LC50) and hindering the identification of Ag species responsible for the acute toxicity of AgNPs to Daphnia magna. This obstacle was overcome by using 8 h exposure in 0.1 mmol L(-1) NaNO3 medium, in which we measured the 8-h LC50 of seven AgNPs with different sizes and coatings, and determined the concentrations of various Ag species. The LC50 as free Ag(+) of the seven AgNPs (0.37-0.44 μg L(-1)) agreed very well with that of AgNO3 (0.40 μg L(-1)), and showed the lowest value compared to that as total Ag, total Ag(+), and dissolved Ag, demonstrating free Ag(+) is exclusively responsible for the acute toxicity of AgNPs to D. magna, while other Ag species in AgNPs have no contribution to the acute toxicity. Our results demonstrated the great importance of developing appropriate exposure media for evaluating risk of nanomaterials.