Phytotoxicity of individual and binary mixtures of rare earth elements (Y, La, and Ce) in relation to bioavailability

A systematic review and meta-analysis on the root effects and toxic mechanisms of rare earth elements

Rare earth elements (REEs) have attracted much attention because of their unique physical and chemical properties. The root system is the plant organ most directly in contact with REEs, and it is critical to understand the mechanisms of interaction between the two. This paper investigates the effects of REEs on plant enrichment and fractionation, as well as on various developmental and toxicity indices of the root system. REEs are more likely to be deposited on the root surface under the influence of root secretion. The complexation between the two affects the uptake and fractionation of REEs and the altered pattern of root secretion. The toxicity mechanisms of REEs on plant root cells were lied in: (1) REEs generate reactive oxygen species after entering the plant, leading to oxidative stress and damage to plant cells; (2) REEs with higher charge-to-volume ratios compete for organic ligands with or displace Ca2+, further disrupting the normal function of plant root cells. It was shown that the sensitivity of inter-root microorganisms to REEs varied depending on the content and physicochemical properties of REEs. The paper also concluded with a meta-analysis of phytotoxicity induced by REEs, which showed that REEs affect plant physiological parameters. REEs, as a source of oxidative stress, triggered lipid peroxidation damage in plants and enhanced the activity of antioxidant enzymes, thus revealing the significant toxicity of REEs to plants. The phytotoxic effects of REEs increased with time and concentration. These results help to elucidate the ecotoxicology of rare earth-induced phytotoxicity.

Biostimulant Response of Foliar Application of Rare Earth Elements on Physiology, Growth, and Yield of Rice

Rare earth elements (REEs) have been intentionally used in Chinese agriculture since the 1980s to improve crop yields. Around the world, REEs are also involuntarily applied to soils through phosphate fertilizers. These elements are known to alleviate damage in plants under abiotic stresses, yet there is no information on how these elements act in the physiology of plants. The REE mode of action falls within the scope of the hormesis effect, with low-dose stimulation and high-dose adverse reactions. This study aimed to verify how REEs affect rice plants' physiology to test the threshold dose at which REEs could act as biostimulants in these plants. In experiment 1, 0.411 kg ha-1 (foliar application) of a mixture of REE (containing 41.38% Ce, 23.95% La, 13.58% Pr, and 4.32% Nd) was applied, as well as two products containing 41.38% Ce and 23.95% La separately. The characteristics of chlorophyll a fluorescence, gas exchanges, SPAD index, and biomass (pot conditions) were evaluated. For experiment 2, increasing rates of the REE mix (0, 0.1, 0.225, 0.5, and 1 kg ha-1) (field conditions) were used to study their effect on rice grain yield and nutrient concentration of rice leaves. Adding REEs to plants increased biomass production (23% with Ce, 31% with La, and 63% with REE Mix application) due to improved photosynthetic rate (8% with Ce, 15% with La, and 27% with REE mix), favored by the higher electronic flow (photosynthetic electron transport chain) (increase of 17%) and by the higher Fv/Fm (increase of 14%) and quantum yield of photosystem II (increase of 20% with Ce and La, and 29% with REE Mix), as well as by increased stomatal conductance (increase of 36%) and SPAD index (increase of 10% with Ce, 12% with La, and 15% with REE mix). Moreover, adding REEs potentiated the photosynthetic process by increasing rice leaves' N, Mg, K, and Mn concentrations (24-46%). The dose for the higher rice grain yield (an increase of 113%) was estimated for the REE mix at 0.72 kg ha-1.

A 50-year systemic review of bioavailability application in Soil environmental criteria and risk assessment

Bioavailability has become a critical factor in improving ecological risk assessment and environmental remediation efficiency in contaminated soil research. However, the soil environmental quality standards and risk assessment procedures used in most countries are still based on the total amount of pollutants for lacking sufficient understanding of the exposure pathways and action mechanisms of pollutants. we collected relevant literature from the Web of Science database, spanning the period from 1950 to 2021 by using Citespace to analyze the scientific development of bioavailability. As of January 09, 2022, the database contained 118,813 publications on bioavailability. The review summarizes the progress in bioavailability research and emerging trends, including exploring advanced analytical techniques, advancing modeling approaches, and integrating interdisciplinary approaches to better understand the fate and behavior of pollutants in complex environmental matrices. In particular, the review emphasizes the need for better integration of bioavailability concepts into soil environmental reference, risk assessment procedures, and environmental remediation strategies. Overall, this review emphasized the necessity of incorporating the concept of bioavailability into soil environmental reference, risk assessment procedures, and environmental remediation strategies.

A toxicokinetics approach using Enchytraeus crypticus to evaluate the efficiency of hydroxyapatite to remediate soils contaminated with rare earth elements

Extensive rare earth element (REE) mining activities pose threats to agricultural soils surrounding the mining areas. Here, low and high REE-contaminated soils from farmlands around mine tailings were remediated with hydroxyapatite. A toxicokinetic approach was applied to assess whether the use of hydroxyapatite reduced the bioavailability of REEs and thus inhibited their accumulation in the terrestrial organism Enchytraeus crypticus. Our results showed that addition of hydroxyapatite increased soil pH, DOC and anion contents. CaCl2-extractable REE concentrations significantly decreased, indicating the stabilization by hydroxyapatite. The influence of hydroxyapatite on the REE accumulation in enchytraeids was quantified by fitting a toxicokinetic model to dynamic REE body concentrations. The estimated uptake (Ku) and elimination rate constants (Ke), and bioaccumulation factor (BAF) for REEs were in the range of 0.000821 - 0.122 kgsoil/kgworm day-1, 0.0224 - 0.136 day-1, and 0.00135 - 1.96, respectively. Both Ku and BAF were significantly reduced by over 80% by hydroxyapatite addition, confirming the decreased REE bioavailability. Low atomic number REEs had higher BAFs in slightly contaminated soil, suggesting a higher bioaccumulation potential of light REEs in soil organisms. Overall, chemical stabilization with amendments can attenuate the bioavailability of REEs and reduce the potential ecological risk of contaminated agricultural soils near REE mining areas.

Molecular insights into rare earth element (REE)-mediated phytotoxicity and its impact on human health

Rare earth elements (REEs) that include 15 lanthanides, scandium, and yttrium are a special class of elements due to their remarkable qualities such as magnetism, corrosion resistance, luminescence, and electroconductivity. Over the last few decades, the implication of REEs in agriculture has increased substantially, which was driven by rare earth element (REE)-based fertilizers to increase crop growth and yield. REEs regulate different physiological processes by modulating the cellular Ca2+ level, chlorophyll activities, and photosynthetic rate, promote the protective role of cell membranes, and increase the plant's ability to withstand various stresses and other environmental factors. However, the use of REEs in agriculture is not always beneficial because REEs regulate plant growth and development in dose-dependent manner and excessive usage of them negatively affects plants and agricultural yield. Moreover, increasing applications of REEs together with technological advancement is also a rising concern as they adversely impact all living organisms and disturb different ecosystems. Several animals, plants, microbes, and aquatic and terrestrial organisms are subject to acute and long-term ecotoxicological impacts of various REEs. This concise overview of REEs' phytotoxic effects and implications on human health offers a context for continuing to sew fabric scraps to this incomplete quilt's many layers and colors. This review deals with the applications of REEs in different fields, specifically agriculture, the molecular basis of REE-mediated phytotoxicity, and the consequences for human health.

Interactions of molybdenum disulfide nanosheets with wheat plants under changing environments: More than meets the eye?

Molybdenum disulfide (MoS2) nanosheets are being increasingly employed in various applications. It is therefore imperative to assess their potential environmental implications in a changing world, particularly in the context of global warming. Here, we assessed the effects of MoS2 nanosheets on wheat Triticum aestivum L. under today's typical climatic conditions (22 °C) and future climatic conditions (30 °C), respectively. The results showed that MoS2 nanosheets (10 and 100 Mo mg/L) did not significantly affect wheat plant growth, root morphological traits, and chlorophyll fluorescence, regardless of dose and temperature. However, the metabolic processes were significantly altered in T. aestivum upon exposure to individual MoS2 nanosheets and to a combination of MoS2 nanosheets and future global warming. As a non-specific protective strategy, the wheat plants that were under stress conditions maintained the stability of cell membranes and thus relieved cell injury by accumulating more glycerophospholipids. Warming additionally influenced the nitrogen and carbon pool reallocation in wheat root. MoS2 nanosheets mainly depleted a range of antioxidant metabolites involved in phenylpropanoid biosynthesis and taurine and hypotaurine metabolism, while warming activated vitamin B6 cofactors related to vitamin B6 metabolism. Metabolites involved in glutathione metabolism were uniquely upregulated while most metabolites associated with nucleotide metabolisms were uniquely downregulated in combination-treated wheat. Overall, wheat plants regulated a wide range of growth-related processes, including carbohydrate, amino acids, lipid, vitamins, and nucleotide metabolism, to maintain optimal metabolite pool sizes and eventually global metabolic homeostasis upon different stress conditions. Our findings provide novel insights into MoS2 nanosheets-mediated crop responses under global warming.

Acute toxicity of single and combined rare earth element exposures towards Daphnia similis

The increasing use of Rare Earth Elements (REE) in emerging technologies, medicine and agriculture has led to chronic aquatic compartment contamination. In this context, this aimed to evaluate the acute toxic effects of lanthanum (La), neodymium (Nd) and samarium (Sm), as both single and binary and ternary mixtures on the survival of the microcrustacean Daphnia similis. A metal solution medium with (MS) and without EDTA and cyanocobalamin (MSq) as chelators was employed as the assay dilution water to assess REE bioavailability effects. In the single exposure experiments, toxicity in the MS medium decreased following the order La > Sm > Nd, while the opposite was noted for the MSq medium, which was also more toxic than the MS medium. The highest MS toxicity was observed for the binary Nd + La (1:1) mixture (EC₅₀ 48 h of 11.57 ± 1.22 mg.L^-1) and the lowest, in the ternary Sm + La + Nd (2:2:1) mixture (EC₅₀ 48 h 41.48 ± 1.40 mg.L^-1). The highest toxicity in the MSq medium was observed in the single assays and in the binary Sm + Nd (1:1) mixture (EC₅₀ 48 h 10.60 ± 1.57 mg.L^-1), and the lowest, in the ternary Sm + La + Nd (1:2:2) mixture (EC₅₀ 48 h 36.76 ± 1.54 mg.L^-1). Concerning the MS medium, 75 % of interactions were additive, 19 % antagonistic, and 6 % synergistic. In the MSq medium, 56 % of interactions were synergistic and 44 % additive. The higher toxicity observed in the MSq medium indicates that the absence of chelators can increase the concentrations of more toxic free ions, suggesting that the MS medium should be avoided in REE assays. Additive interactions were observed in greater or equivalent amounts in both media and were independent of elemental mixture ratios. These findings improve the understanding of environmental REE effects, contributing to the establishment of future guidelines and ecological risk calculations.

Increasing Temperatures Potentiate the Damage of Rare Earth Element Yttrium to the Crop Plant Triticum aestivum L

Given that increasing temperature may aggravate the toxicity of pollutants, it is a daunting challenge to evaluate the realistic risks of rare earth elements (REEs) under global warming. Here, we studied how elevated temperatures (27 and 32 °C) impact the effect of yttrium (Y) on wheat plants (Triticum aestivum L.) at concentrations not causing effects (0, 0.5, and 1 μM) at the control temperature (22 °C) in a hydroponic system. After 14 days of exposure, significant inhibition (p < 0.05, 29.5%) of root elongation was observed only at 1 μM of Y at 32 °C. Exposure to Y at 27 °C showed no visible effects on root length, but induced significant (p < 0.05) metabolic disorders of a range of carbohydrates and amino acids related to galactose, phenylalanine, and glutamate metabolisms. Such cases were even shifted to substantial perturbation of the nucleotide pool reallocation involved in the disruption of purine and pyrimidine metabolism at 32 °C. These observations were regulated by sets of genes involved in these perturbed pathways. Using weighted gene co-expression network analysis, the disorder of nucleotide metabolism was shown to be responsible for the aggravated Y phytotoxicity at the extreme high temperature. Although the temperature fluctuation considered seems to be in an extreme range, unexpected implications driven by high temperature cannot be neglected. Our findings thus reduce the gaps of knowledge in REE toxicity to plants under future climate warming scenarios and highlight the importance of incorporating environmental temperature into the framework of the risk assessment of REEs.

Phytotoxicity effect concentrations (ECx) for Ce, Nd and Eu added to soil relative to total and bioaccessible soil REE concentrations, and tissue REE accumulations

Toxicity thresholds (EC_x) for radish, tomato, and durum wheat growth endpoints (shoot length, shoot mass, root length) to Ce, Nd or Eu added to a black organic soil were determined from 14-day dose-response growth assays. EC₁₀ expressed as total soil [REE] had a more than twenty-fold range, from 337 mg/kg to more than >8000 mg/kg. Averaged over all REEs and endpoints, durum wheat was more tolerant than radish and tomato; and averaged over all endpoints, Eu appeared to be the most phytotoxic of the three REEs. Bioaccessibility of each REE was determined by extraction with 0.01 M CaCl₂, which for all three REEs in this soil was quite low, <0.10% of total. However, bioaccessibility of Eu was five or six times greater than that for Ce and Nd, and thus could explain its apparently greater toxicity, namely that Eu was more likely to be accumulated at the site of toxic action in the plant. To discern inherent toxicity from enhanced bioaccumulation, concentration of each REE in root and shoot tissues was determined, for a tissue-residue approach to toxicity assessment. The EC₁₀ expressed as tissue concentration was lower for Nd than for Ce and Eu, thus the most toxic of the three REEs. As for many cationic inorganic elements, toxicity varies with the chemistry of the exposure medium due to its effects on bioaccessibility. Simple methods to harmonize toxicity thresholds from different media enables greater integration into regulatory standards. When EC₂₅ from this and other studies were normalized to CaCl₂-extractable REE in their respective media, the range in Ce EC₂₅ was reduced from 20-fold to 2.5-fold, and the range for Eu EC₂₅ was reduced from 25-fold to 3-fold. This novel and low-input approach to meta-analysis of toxicity thresholds demonstrates the value of considering soil physico-chemical properties as modifiers of soil REE toxicity.

Assessment of the biotoxicity of lanthanides (La, Ce, Gd, and Ho) on zebrafish (Danio rerio) in different water environments

The expanding applications of lanthanides (Ln) in various aspects have raised concerns about their biosafety. Slight changes in the chemical composition of environmental media can significantly affect the biological effectiveness of poorly water-soluble Ln; however, the knowledge of the effects of environmental factors on Ln toxicity remains limited. Here, the effects of pH, HCO₃^-, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^-, and SO₄^2- on the bioefficacy and biotoxicity of Ln (La, Ce, Gd, and Ho) were comparatively studied using zebrafish (Danio rerio) as the test organism. In the standard water, the toxicity of Ln in zebrafish was significantly correlated with pH, HCO₃^-, and Ca²⁺-Mg²⁺ levels in the medium but not with the levels of Cl^-, Na⁺, K⁺, and SO₄^2-. At the beginning of the test, the four Ln were complexed with HCO₃^- in the medium to form precipitates. A decrease in pH or HCO₃^- concentration can promote the conversion of granular Ln to a soluble state, thus enhancing their bioavailability, biotoxicity, and bioaccumulation. At a pH of 5.0 and 0.2 mmol·L^-1 HCO₃^-, where Ln precipitates were not found, the four Ln showed a consistent trend of 96 h-LC50 in zebrafish. These data indicate that the differences in the toxicities of the four Ln in the standard water may be due to differences in the effective states of the individual elements rather than the different toxicities of the elements. Overall, in biological toxicity assessments, Ln can be regarded as a group of elements with additive patterns of toxicity until the differences in their biological toxicity mechanisms are revealed, and the effects of pH and carbonate should be considered.

Photosynthetic, antioxidative, and metabolic adjustments of a crop plant to elevated levels of La and Ce exposure

Rare earth elements (REEs) have been widely applied as fertilizers in farmland of China for decades to improve the yield and quality of crops. Unfortunately, adverse effects on plants have been observed due to overdosing with REEs. Until now, the toxicology of REEs was mainly evaluated based on phenotypic responses, but knowledge gaps still exist concerning their metabolic effects. Here, the physiological responses and nontargeted metabolomics studies were combined to systematically explore the potential effects of La and Ce on a crop plant, wheat Triticum aestivum. It was observed that REEs accumulated in the shoots of wheat, with significant reduction of the shoot biomass at higher exposure doses. The disturbance of photosynthesis and induced oxidative stress were identified by analyzing indicators of the photosynthetic (chlorophyll a/b, carotenoid and rubisco) and antioxidant systems (POD, CAT, SOD, GSH and MDA). Furthermore, the global metabolic profiles of REEs treatment groups and the non-exposed control group were screened and compared, and the metabolomic disturbance of REEs was dose-dependent. A high overlap of significantly changed metabolites and matched disturbed biological pathways was found between La and Ce treatments, indicating similarity of their toxicity mechanism in wheat shoots. Generally, the perturbed metabolomic pathways were mainly related to carbohydrate, amino acid and nucleotide/side metabolism, suggesting a disturbance of carbon and nitrogen metabolism, which finally affected the growth of wheat. We thus proved the potential adverse effect of inappropriate application of REEs in crop plants and postulated metabolomics as a feasible tool to identify the underlying toxicological mechanisms.

Toxic Effects of Two Representative Rare Earth Elements (La and Gd) on Danio rerio Based on Transcriptome Analysis

The expanding applications of rare earth elements (REEs) in various fields have raised concerns about their biosafety. However, previous studies are insufficient to elucidate their toxic effects and mechanisms of action and whether there are uniform or predictable toxicity patterns among REEs. Herein, we investigated the toxic effects of two representative REEs (lanthanum (La) and gadolinium (Gd)) on zebrafish (Danio rerio) through toxicity experiments and transcriptome analysis. The results of the toxicity experiments showed that the two REEs have similar lethality, with half-lethal concentrations (LC50) at micromolar levels and mixed toxicity showing additive effects. Differential expression gene screening and functional group enrichment analysis showed that La and Gd might affect the growth and development of Danio rerio by interfering with some biological molecules. The two REEs showed significant effects on the metabolic pathways of exogenous or endogenous substances, including glutathione sulfotransferase and acetaldehyde dehydrogenase. Moreover, some basic biological processes, such as DNA replication, the insulin signaling pathway, and the p53 signaling pathway, were significantly enriched. Overall, the toxicity patterns of La and Gd may affect some biological processes with different intensities; however, there are many similarities in their toxicity mechanisms and modes of action. The concentrations investigated in this study were comparable to those of REE residues at highly contaminated sites, thus mimicking the ecotoxicological effects at environmentally relevant concentrations.

Assessment of the toxicity of a mixture of five rare earth elements found in aquatic ecosystems in Hydra vulgaris

Rare earth elements (REEs) are considered critical elements for technology and their extraction through mining activities is expected to increase in the future. Due to their chemical similarities, they often co-occur in minerals and thus their ecotoxicity should be assessed as a group/family. However, the available ecotoxicological studies focused mainly on the evaluation of the potential toxicological impacts of individual REEs rather than their mixtures. The aim of this study was therefore to evaluate the toxicity of a representative mixture of five REEs (La, Ce, Pr, Nd and Sm) spanning environmentally relevant concentrations ranging from 0.05X (29 µg REEs L^-1) to 5X (2926 µg REEs L^-1) to the test organism, Hydra vulgaris, at the morphological, reproductive and regenerative levels. The data showed that lethality occurred at concentrations near (2.5 fold) to those inducing sublethal effects after chronic exposure of 7 days. The mixture affected reproduction and head regeneration and even lethality at concentrations even below those reported at environmental concentration (0.5X = 293 µg REEs L^-1) in lakes. This suggests that REEs concentrations found in lakes near mining activities could disrupt regeneration and impair embryonic development. Our data also revealed that combining the 5 REEs results in an antagonistic effect, suggesting that those elements share the same receptor and that low molecular weight and high radius elements (approaching iron) were less toxic. Taken together, hydra could be used as a sensitive model organism for the assessment of aquatic ecotoxicological risks of REE mixtures but further analyses of biochemical and gene expressions should improve our understanding of the long-term effects of REEs mixtures.

Risk of Collapse in Water Quality in the Guandu River (Rio de Janeiro, Brazil)

The Guandu River, one of the main rivers in the state of Rio de Janeiro, provides water for more than nine million people in the metropolitan region. However, the Guandu has suffered from massive domestic and industrial pollution for more than two decades, leading to high levels of dissolved total phosphorus, cyanobacteria, and enteric bacteria observed during the summers of 2020 and 2021. The use of Phoslock, a palliative compound, was not effective in mitigating the levels of phosphorus in the Guandu River. Furthermore, potable water driven from the river had levels of 2-MIB/geosmin and a mud smell/taste. With all these problems, several solutions are proposed for improving the Guandu River water quality, including establishment of (i) sewage treatment plants (STPs), (ii) strict water quality monitoring, (iii) environmental recovery (e.g., reforestation), and (iv) permanent protected areas. The objective of this paper is to verify the poor water quality in the Guandu and the ineffectiveness and undesired effects of Phoslock.

Sensitivity of Ostracods to U, Cd and Cu: The Case of Cypridopsis vidua

The development of uranium mines has been necessary to obtain abundant and scarce uranium resources, but they also bring inevitable radioactive contamination to the surrounding soil, rivers and lakes. This paper explores the sensitivity of Cypridopsis vidua to the radioactive element uranium and the heavy elements cadmium and copper with single and combined acute toxicity experiments and combined toxicity model predictions. The results from the single toxicity experiments showed that the degree of toxic effects was cadmium > copper > uranium. The combined toxicity experiments showed that the compound toxicity of U-Cd and U-Cu was higher than the weakest component and lower than the strongest component, whereas the compound toxicity of Cd-Cu was higher than either of its components. When the overall proportion of a more toxic metal was increased, its mixed toxicity also increased, and vice versa. Combined toxicity predictions showed that the U-Cd combination was best described by the concentration additive (CA) model, the independent action (IA) model was more applicable to the Cd-Cu combination, and the most applicable model for the U-Cu combination changed depending on the concentration gradient. The acute toxicity data from this study provide a reference for the development of wastewater discharge standards for uranium mines, enriches the data related to the toxicity of uranium for ostracods and deepens the understanding of the threat of uranium pollution to aquatic ecosystems.

Distribution of rare earth elements (REEs) and their roles in plant growth: A review

The increasing use of rare earth elements (REEs) in various industries has led to a rise in discharge points, thus increasing discharge rates, circulation, and human exposure. Therefore, REEs have received widespread attention as important emerging pollutants. This article thus summarizes and discusses the distribution and occurrence of REEs in the world's soil and water, and briefly introduces current REEs content analysis technology for the examination of different types of samples. Specifically, this review focuses on the impact of REEs on plants, including the distribution and fractionation of REEs in plants and their bioavailability, the effect of REEs on seed germination and growth, the role of REEs in plant resistance, the physiological and biochemical responses of plants in the presence of REEs, including mineral absorption and photosynthesis, as well as a description of the substitution mechanism of REEs competing for Ca in plant cells. Additionally, this article summarizes the potential mechanisms of REEs to activate endocytosis in plants and provides some insights into the mechanisms by which REEs affect endocytosis from a cell and molecular biology perspective. Finally, this article discusses future research prospects and summarizes current scientific findings that could serve as a basis for the development of more sustainable rare earth resource utilization strategies and the assessment of REEs in the environment.

Lanthanum and cerium disrupt similar biological pathways and interact synergistically in Triticum aestivum as revealed by metabolomic profiling and quantitative modeling

The industrial and agricultural applications of rare earth elements (REEs) lead to considerable REE emissions into environment. Yet, little is known about the molecular-level effects and interactions of REEs in terrestrial plants. Herein, the individual and joint effects of La and Ce in Triticum aestivum were investigated using mass spectrometry-based metabolomics. Metabolic effect level index (MELI) was utilized as a readable endpoint for quantifying mixture interactions. Exposure to single La/Ce at environmentally relevant levels induced significant dose-dependent metabolic changes. The highly overlap of differential metabolites and perturbed pathways of La and Ce suggested their similar mode of action. Exposure to La-Ce mixtures did not induce additional metabolic pathway perturbation. Specifically, metabolism of amino sugar and nucleotide sugar, starch and sucrose, fructose and mannose, glycerophospholipid and purine were disrupted for both single and binary exposures. These results, together with physiological indicators, point to REE-induced oxidative stress, energy expenditure, DNA damage and membrane disturbance. The MELI calculations showed that La and Ce interacted synergistically at the overall metabolic level, which could be causally linked to synergistic interaction at the individual level (root elongation). This work proved metabolomics could be an important and effective strategy for interpreting toxicity and interactions of REE mixtures.

Transfer of La, Ce, Sm and Yb to alfalfa and ryegrass from spiked soil and the role of Funneliformis mosseae

Rare earth elements (REEs) are widely used in high-tech industries, and REE waste emissions have become a concern for ecosystems, food quality and human beings. Arbuscular mycorrhizal fungi (AMF) have repeatedly been reported to alleviate plant stress in metal-contaminated soils. To date, little information is available concerning the role of AMF in REE-contaminated soils. We recently showed that there was no transfer of Sm to alfalfa by Funneliformis mosseae, but only a single REE was examined, while light and heavy REEs are present in contaminated soils. To understand the role of AMF on the transfer of REEs to plants, we carried out an experiment using alfalfa (Medicago sativa) and ryegrass (Lolium perenne) in compartmented pots with separate bottom compartments that only were accessible by F. mosseae fungal hyphae. The bottom compartments contained a mixture of four REEs at equal concentrations (La, Ce, Sm and Yb). The concentration of REEs in plants was higher in roots than in shoots with higher REE soil-root than root-shoot transfer factors. Moreover, significantly higher light-REEs La and Ce were transferred to ryegrass shoots than Sm and the heavy-REE Yb, but this was not observed for alfalfa. Alfalfa dry weight was significantly increased by F. mosseae inoculation, but not ryegrass dry weight. For both plant species, there was significantly higher P uptake by the mycorrhizal plants than the nonmycorrhizal plants, but there was no significant transfer of La, Ce, Sm or Yb to alfalfa and ryegrass roots or shoots due to F. mosseae inoculation.

Estimating the synergistic and antagonistic effects of dual antibiotics on plants through root elongation test

Antibiotics are recently recognized as a group of emerging environmental contaminants that are frequently detected in various environmental matrixes. Relative root elongation (RRE) test is a rapid and effective strategy to evaluate the water/soil quality and the toxic effects of environmental contaminants on plants. In the present study, we examine the toxicity effect of ciprofloxacin (CIP), norfloxacin (NOR), and tetracycline (TET) to pakchoi individually and in combinations. Both independent action (IA) and concentration addition (CA) models are used for toxicity assessment. Results showed that the EC₅₀ values of CIP, NOR, and TET are 193.59, 60.81, and 40.37 μM, respectively. Combinations of TET + CIP and TET + NOR caused more inhibitory effects on root elongation than those of CIP + NOR. Toxic Unit (TU) and Synergistic Ratio (SR) analysis showed that the relatively lower (higher) EC values are observed in the combinations with lower (higher) antibiotic concentrations, suggesting an effect of low-dose synergism and high-dose antagonism. The reliability of the simulation results from IA and CA models to predict that combined toxicity is highly dependent upon the results from the analysis of TU or SR.

Particle-Specific Toxicity of Copper Nanoparticles to Soybean (Glycine max L.): Effects of Nanoparticle Concentration and Natural Organic Matter

For the soluble metallic nanoparticles (NPs), which forms (particles [NP(particle) ] vs. dissolved ions [NP(ion) ]) are the main cause of toxicity of the NP suspension (NP(total) ) remains uncertain. In the present study, soybean was exposed to Cu NPs in a hydroponic system to determine how natural organic matter (NOM; 10 mg/l) and concentration of Cu NP(total) (2-50 mg/l) affect the relative contributions of Cu NP(particle) and Cu NP(ion) to the overall toxicity. We found that NOM mitigated the phytotoxicity of Cu NP(particle) more significantly than that of Cu salt. When no NOM was added, Cu NP(particle) rather than Cu NP(ion) was the main contributor to the observed toxicity regardless of the concentration of Cu NP(total) . However, NOM tended to reduce the relative contribution of Cu NP(particle) to the toxicity of Cu NP(total) . Especially at a low concentration of Cu NP(total) (2 mg/l), the toxicity of Cu NP(total) mainly resulted from Cu NP(ion) in the presence of NOM (accounting for ≥70% of the overall toxicity). This might be attributable to the combined effects of increased dissolution of Cu NPs and steric-electrostatic hindrance between Cu NP(particle) and the soybean roots caused by NOM. Fulvic acids (FAs) tended to reduce the role of Cu NP(particle) in the overall toxicity more effectively than humic acids (HAs), which might partially be due to the higher extent of Cu NP dissolution on FA treatment than in HA treatment. Our results suggest that because of the relatively low metallic NP concentration and the presence of NOM in natural water, NP(ion) are likely problematic, which can inform management and mitigation actions. Environ Toxicol Chem 2021;40:2825-2835. © 2021 SETAC.

Using quantitative ion character-activity relationship (QICAR) method in evaluation of metal toxicity toward wheat

It is important to assess the toxic effects posed by soil pollutants toward plants. However, plant toxicology experiments normally involve a considerable amount of manpower, consumables and time. Therefore, the use of metal toxicity prediction models, independent of toxicity tests, is critical. In this study, we investigated the toxicity of different metal ions to wheat using hydroponic experiments. We employed the methods of soft-hard ion grouping, soft-hard ligand theory and K (conditional binding constant based on the biotic ligand model principle) in combination with hydroponic experiments to explore the application of quantitative ion character-activity relationships in predicting phytotoxicity. The results showed that the toxicity of the 19 metal ions tested varied significantly, with EC₅₀ ranging from 0.27 μM to 4463.36 μM. The linear regression relationships between the toxicity of these metal ions and their physicochemical properties were poor (R² = 0.237-0.331, p < 0.05). These relationships were improved after grouping the metals according to the soft-hard theory (R² = 0.527-0.744 and p < 0.05 for soft ions; R² = 0.445-0.743 and p < 0.05 for hard ions). The application of soft-hard ligand theory, based on the binding affinity of the metals to the ligands, showed poor prediction of the phytotoxicity of metals, with R² = 0.413 (p = 0.024) for the softness consensus scale (σ_Con) and R² = 0.348 (p = 0.218) for the normalized hard ligands scale (HLScale). However, the method of K provided the closest fit in predicting toxicity (R² = 0.803, p < 0.001). Our results showed that the application of soft-hard ion grouping and log K can improve prediction of the phytotoxicity of metals relatively well, which can potentially be used for deriving the toxicity of elements with limited toxicity data.

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

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

Do essential elements (P and Fe) have mitigation roles in the toxicity of individual and binary mixture of yttrium and cerium to Triticum aestivum?

Essential elements can affect the bioavailability, uptake, and toxicity of metals. However, hardly any research has focused on the roles of essential elements on the toxicity of rare earth metals. Here we examined how P and Fe modified the individual and binary toxicity of Y and Ce to Triticum aestivum, respectively. Standard root elongation tests were used to quantify the toxicity of both single and binary mixtures at three levels of P addition (1, 5, and 10 μM) and Fe addition (0.1, 1, and 5 mM). Our results showed that both P and Fe can alleviate individual toxicity of Y or Ce irrespective of the dose indicators as suggested by the enhanced EC50 values. Both P and Fe might mitigate Y/Ce toxicity by limiting Y/Ce uptake into roots and improving nutritional status of wheats, whereas P can also decrease free Y/Ce ion activities in the exposure media. As for the mixture toxicity of Y and Ce, only improved P, but not Fe can exhibit approximately additive mixture toxicity, which can be adequately predicted by the simple Concentration Addition model. Our results suggested the important roles of P and Fe in assessing Y and Ce toxicity accurately.

Possible sources of rare earth elements near different classes of road in Poland and their phytoextraction to herbaceous plant species

The growing use of rare earth elements (REE) in industry determines their increased transport to the environment. The higher concentration of this group of elements in soils near roads may also suggest that traffic plays a significant role in their distribution. The aim of this study was to examine the content of REEs in selected consumables (car parts, asphalt) and environmental samples (plants, soils) in order to estimate the extent to which these elements derive from traffic and also to analyze their phytoextraction from soil by selected herbaceous plants species. Research materials were car parts (5 brake pads, 10 new tires - summer and winter), 20 samples of asphalt and road dust settled on its surface; soil, and 7 plants species growing at a distance of 1 m from the edge of the 5 roads located in the Wielkopolska Voivodeship, Poland. The content of REEs in the collected samples was determined using inductively coupled plasma optical emission spectrometer. The content of REEs in asphalt and brake pads was similar and significantly higher than in tires. According to the mass of particular stripped materials, the main source of these elements was asphalt. The amount of REEs released from tires to the environment was found to be much lower than REEs released from asphalt but generally higher than from brake pads. The content of REEs in the soil was found to increase in accordance with traffic intensity, but chemical composition of soil was the main determinant of the uptake these elements, mainly via the root systems of plants. The obtained results suggest that densely vegetated roadsides and verges could be an effective strategy for decontamination of soils polluted with REEs, although the most effective remedy would involve significant changes in the production technologies of automotive parts and asphalt that would limit the emission of elements to environment.

Bioavailability and phytotoxicity of rare earth metals to Triticum aestivum under various exposure scenarios

It is a daunting challenge to predict toxicity and accumulation of rare earth metals (REMs) in different exposure scenarios (e.g., varying water chemistry and metal combinations). Herein, we investigated the toxicity and uptake of La and Ce in the presence of various levels of Ca, Mg, Na, K, and at different pH values, as well as the combined effects of La and Ce in wheat Triticum aestivum. Major cations (Ca²⁺ and Mg²⁺) significantly mitigated the toxicity and accumulation of La³⁺/Ce³⁺. Toxicity and uptake of La, Ce, and La-Ce mixtures could be well quantified by the multi-metal biotic ligand model (BLM) and by the Langmuir-type uptake model with the consideration of the competitive effects of Ca²⁺ and Mg²⁺, with more than 85.1% of variations explained. The derived binding constants of Ca, Mg, La, and Ce to wheat root were respectively 3.87, 3.59, 6.97, and 6.48 on the basis of toxicity data, and 3.23, 2.84, 6.07, and 5.27 on the basis of uptake data. The use of the alternative WHAM-F_tox approach, requiring fewer model parameters than the BLM but with similar Akaike information criterion (AIC) values, successfully predicted the toxicity and accumulation of La/Ce as well as toxicity of La-Ce mixtures, with at least 76.4% of variations explained. However, caution should be taken when using this approach to explain the uptake of La-Ce mixtures. Our results provided promising tools for delineating REMs toxicity/uptake in the presence of other toxicity-modifying factors or in mixture scenarios.

Toxicity of three rare earth elements, and their combinations to algae, microcrustaceans, and fungi

Rare earth elements (REEs) are naturally distributed in the environment, and are increasingly being used in agriculture and high technology materials worldwide, thereby increasing anthropogenic contamination and environmental risks. There exists scarce and contradictory toxicity information about REEs; hence, more studies are required, especially on their mixtures. Thus, this study aimed to assess the toxicities of La³⁺, Nd³⁺, Sm³⁺, and the combinations of these elements (binary 1:1 and ternary 1:1:1), to organisms from different trophic levels: producers (the microalgae Chlorella vulgaris and Raphidocelis subcapitata), primary consumers (the microcrustaceans Daphnia similis and Artemia salina), and decomposers (the fungi Penicillium simplicissimum and Aspergillus japonicus). Ecotoxicological bioassays were performed, and toxic concentrations were determined. Thereafter, toxicities of single and mixture REEs were classified as slightly to highly toxic according to their toxic units. Finally, a concentration addition (CA) model was used to estimate how REEs interact upon combining. Nd³⁺ was the most toxic element for all organisms, especially D. similis (48 h LC₅₀ 9.41 mg.L^-1), and was therefore classified as highly toxic. Sm³⁺ promoted cell agglomeration in Chlorella vulgaris and was the most toxic of the tested elements for this organism (72 h IC₅₀ 25.78 mg.L^-1). The CA model revealed synergistic responses for most of the combinations, principally Nd³⁺ + Sm³⁺, which was the most toxic combination for the tested organisms. Both fungi were the most resistant organisms, and A. japonicus produced exudate and sclerotia, which help in the detoxification of chemicals. Owing not only to the fact that fungi displayed a higher resistance to REEs, but also due to the absence of regulations for REEs released from the agricultural or industrial sector, and the lack of methods to treat effluents or to dispose of technological items containing REEs, these organisms should be considered as a model for the biosorption or bioremediation of REEs. Finally, the toxic effects of REEs, particularly Nd³⁺, on the biota and human health should be the focus of future studies due to their increased use in technology.

Coupling mixture reference models with DGT-perceived metal flux for deciphering the nonadditive effects of rare earth mixtures to wheat in soils

The risk assessment of mixtures of rare earth elements (REEs) is hampered by a lack of fundamental understanding of their interactions in different soil types. Here, we assessed mixture interactions and toxicity to Triticum aestivum of Y and Ce in four different soils in relation to their bioavailability. Mixture toxicity was modelled by concentration addition (CA) and independent action (IA), in combination with different expressions of exposure: three equilibrium-based doses (total soil concentrations [M]_tot, free ion activity in soil solution {M³⁺}, and the fraction (f) of metal ions bound to the biotic ligands (BLs)) and one kinetically controlled dose ([M]_flux) metrics. Upon single exposure, REE toxicity was increasingly better described when using exposure expressions based on deepened understanding of their bioavailability: [M]_flux > f > {M³⁺} > [M]_tot. The mixture analyses based on [M]_tot and {M³⁺} displayed deviations from additivity depending on the soil type. With the parameters derived from single exposures, the BLM approach gave better predictions of mixture toxicity (R² ~ 0.70) than when using CA and IA based on either [M]_tot or {M³⁺} (R² < 0.64). About 30% of the variance in toxicity remained unexplained, challenging the view that the free metal ion is the main bioavailable form under the BLM framework based on thermodynamic equilibrium. Toxicity was best described when accounting for changes in the size of the labile metal pool by using a kinetically controlled dose metric (R² ~ 0.80). This suggests that dynamic bioavailability analysis could provide a robust basis for modeling and reconciling the interplays and toxicity of metal mixtures in different soils.

Do toxicokinetic and toxicodynamic processes hold the same for light and heavy rare earth elements in terrestrial organism Enchytraeus crypticus?

The widespread use of rare earth elements (REEs) in numerous sectors have resulted in their release into the environment. Existing knowledge about the effects of REEs were acquired mainly based on toxicity tests with aquatic organisms and a fixed exposure time, Here, the dynamic accumulation and toxicity of REEs (La, Ce, and Gd) in soil organism Enchytraeus crypticus were determined and modeled by a first-order one-compartment model and a time-toxicity logistic model, respectively. Generally, the accumulation and toxicity of REEs were both exposure level- and time-dependent. The overall uptake rate constants were 2.97, 2.48, and 2.38 L kg^-1d^-1 for La, Ce, and Gd, respectively. The corresponding elimination rate constants were 0.99, 0.78, and 0.56 d^-1, respectively. The worms exhibited faster uptake and elimination ability for light REEs (La and Ce) than for heavy REEs (Gd). For all three REEs, the LC50 values based on exposure concentrations decreased with time and reached ultimate values after approximately 10 d exposure. The estimated ultimate LC50 values (LC50∞) were 279, 334, and 358 mg L^-1 for Ce, Gd, and La, respectively. When expressed as body concentration, the LC50_inter value was almost constant with time, demonstrating that internal body concentration could be a better indicator of dynamic toxicity of REEs than external dose. This study highlights that specific REE and exposure time should be taken into account in accurately assessing risk of REEs.

Model-based rationalization of mixture toxicity and accumulation in Triticum aestivum upon concurrent exposure to yttrium, lanthanum, and cerium

Rare earth elements (REEs) often co-exist in the environment, but predicting their 'cocktail effects' is still challenging, especially for high-order mixtures with more than two components. Here, we systematically investigated the toxicity and accumulation of yttrium, lanthanum, and cerium mixtures in Triticum aestivum following a standardized bioassay. Toxic effects of mixtures were predicted using the reference model of Concentration Addition (CA), Ternary model, and Ternary-Plus model. Interactions between the REEs in binary and ternary mixtures were determined based on external and internal concentrations, and their magnitude estimated from the parameters deviated from CA. Strong antagonistic interactions were found in the ternary mixtures even though there were no significant interactions in the binary mixtures. Predictive ability increased when using the CA model, Ternary model, and Ternary-Plus model, with R²= 0.78, 0.80, and 0.87 based on external exposure concentrations, and R²= 0.72, 0.73, and 0.79, respectively based on internal concentrations. The bioavailability-based model WHAM-F_TOX explained more than 88 % and 85 % of the toxicity of binary and ternary REE treatments, respectively. Our result showed that the Ternary-Plus model and WHAM-F_TOX model are promising tools to account for the interaction of REEs in mixtures and could be used for their risk assessment.

The cation competition and electrostatic theory are equally valid in quantifying the toxicity of trivalent rare earth ions (Y3+ and Ce3+) to Triticum aestivum

There is a lack of appropriate models to delineate the toxicity of rare earth elements (REEs) while taking into account the factors that affect bioavailability. Here, standardized wheat (Triticum aestivum L.) root elongation tests were conducted to examine the impact of exposure conditions (i.e., varying Ca, Mg, Na, K and pH levels) on Y and Ce toxicity. Cation competition and electrostatic theory were examined for their applicability in explaining the observed variations in toxicity. Only Ca²⁺ and Mg²⁺ significantly alleviated the toxicity of Y³⁺ and Ce³⁺, while Na⁺, K⁺ and H⁺ showed no significant effects. Based on the cation competition, the derived binding constants for the hypothetical biotic ligands of wheat logK_CaBL, logK_MgBL, logK_YBL, and logK_CeBL were 3.87, 3.59, 6.70, and 6.48, respectively. The biotic ligand model (BLM) succeeded in predicting toxicities of Y and Ce, with more than 93% of the variance in toxicity explained. Given the BLM requires large data sets for deriving model parameters, attempts were further made to explore a simpler electrostatic based model to quantify REEs toxicity. The results demonstrated that the predictive capacity of the electrostatic approach, which considers ion activities at the plasma membrane surface, was comparable to that of BLM with at least 87% of the variations in toxicity explained. This suggested that the electrostatic theory can serve as a surrogate to BLM in modeling Y and Ce toxicities. Therefore, we recommend the BLM and electrostatic-based model as effective approaches to incorporate bioavailability in quantifying REEs toxicity in the presence of various levels of other major cations.