The influence of calcium and pH on the uptake and toxicity of copper in Folsomia candida exposed to simplified soil solutions

Biotic Ligand Modeling for the Effect of Major Cations on the Uptake of Cadmium in Folsomia candida Exposed in a Sand-Solution Medium

Biotic ligand modeling (BLM) approaches are already applied to predict the bioavailability and possible risk of metals in surface water, but need further development for soils. The present study investigated the effect of major cations (Ca2+ , Mg2+ , Na+ , K+ , and H+ ) on cadmium bioaccumulation in the springtail Folsomia candida. To avoid the complexity of real soils and enable control of elemental speciation in the exposure medium, the animals were exposed to different cadmium concentrations in an inert quartz sand-solution medium. Accumulation of cadmium in the animals was measured after 7 days exposure at different cation concentrations. Among the cations, only Ca2+ significantly affected the uptake of cadmium in the springtails. Mg2+ also had higher effects compared with other selected cations. Using a BLM approach, the uptake of cadmium in the animals predicted by taking into account both Ca2+ and Mg2+ activities correlated well with the measured values (R2  = 0.68). The final estimated conditional binding constants for cadmium (log KCd-BL ), Ca (log KCa-BL ), and Mg (log KMg-BL ) of 1.06, 2.14, and 1.23 L/mol, respectively, were in agreement with previously reported values. The match between predicted and measured uptake data confirms the applicability and usefulness of the BLM for predicting the bioavailability of cadmium to springtails and opens the way for its application in soil. Environ Toxicol Chem 2024;00:1-7. © 2024 SETAC.

Hormesis and insects: Effects and interactions in agroecosystems

Insects in agroecosystems contend with many stressors - e.g., chemicals, heat, nutrient deprivation - that are often encountered at low levels. Exposure to mild stress is now well known to induce hormetic (stimulatory) effects in insects, with implications for insect management, and ecological structure and function in agroecosystems. In this review, we examine the major ecological niches insects occupy or guilds to which they belong in agroecosystems and how hormesis can manifest within and across these groups. The mechanistic underpinnings of hormesis in insects are starting to become established, explaining the many phenotypic hormetic responses observed in insect reproduction, development, and behavior. Whereas potential effects on insect populations are well supported in laboratory experiments, field-based hypothesis-driven research on hormesis is greatly lacking. Furthermore, because most ecological paradigms are founded within the context of communities, entomological agroecologists interested in hormesis need to 'level up' and test hypotheses that explore effects on species interactions, and community structure and functioning. Embedded in this charge is to continue experimentation on herbivorous pest species while shifting more focus towards insect natural enemies, pollinators, and detritivores - guilds that play crucial roles in highly functioning agroecosystems that have been understudied in hormesis research. Important areas for future insect agroecology research on hormesis are discussed.

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.

Comparison Among Test Substrates in Metal Uptake and Toxicity to Folsomia candida and Hordeum vulgare

The main aim of this short review was to assess the effect of test medium on the bioavailability of metals to the soil invertebrate Folsomia candida and the barley plant Hordeum vulgare. Solution-only exposures and sand-solution media were suitable media with control survival of > 80%. Comparing toxicity and accumulation data, LC₅₀ and/or EC₅₀ values as well as internal concentrations of cadmium (Cd) and copper (Cu) were similar in the tests with different porewater composition for springtails and barley plants. Similar results for toxicity and bioaccumulation of Cd and Cu using different test substrates, suggest the importance of physiological handling of the effects by the organisms rather than the influence of test medium composition.

Biotic ligand modeling approach: Synthesis of the effect of major cations on the toxicity of metals to soil and aquatic organisms

The biotic ligand model (BLM) approach is used to assess metal toxicity, taking into account the competition of other cations with the free metal ions for binding to the biotic ligand sites of aquatic and soil organisms. The bioavailable fraction of metals, represented by the free metal ion, is a better measure than the total concentration for assessing their potential risk to the environment. Because BLMs are relating toxicity to the fraction of biotic ligands occupied by the metal, they can be useful for investigating factors affecting metal bioaccumulation and toxicity. In the present review, the effects of major cations on the toxicity of metals to soil and aquatic organisms were comprehensively studied by performing a meta-analysis of BLM literature data. Interactions at the binding sites were shown to be species- and metal-specific. The main factors affecting the relationships between toxicity and conditional binding constants for metal binding at the biotic ligand appeared to be Ca(2+) , Mg(2+) , and protons. Other important characteristics of the exposure medium, such as levels of dissolved organic carbon and concentrations of other cations, should also be considered to obtain a proper assessment of metal toxicity to soil and aquatic organisms.

The relationship between metal toxicity and biotic ligand binding affinities in aquatic and soil organisms: a review

The biotic ligand model (BLM) is a theoretical, potentially mechanistic approach to assess metal bioavailability in soil and aquatic systems. In a BLM, toxicity is linked to the fraction of biotic ligand occupied, which in turn, depends on the various components of the solution, including activity of the metal. Bioavailability is a key factor in determining toxicity and uptake of metals in organisms. In this study, the present status of BLM development for soil and aquatic organisms is summarized. For all species and all metals, toxicity was correlated with the conditional biotic ligand binding constants. For almost all organisms, values for Ag, Cu, and Cd were higher than those for Zn and Ni. The constants derived for aquatic systems seem to be equally valid for soil organisms, but in the case of soils, bioavailability from the soil solution is greatly influenced by the presence of the soil solid phase.

Uptake and elimination kinetics of metals in soil invertebrates: a review

Uptake and elimination kinetics of metals in soil invertebrates are a function of both soil and organism properties. This study critically reviewed metal toxicokinetics in soil invertebrates and its potential use for assessing bioavailability. Uptake and elimination rate constants of different metals are summarized. Invertebrates have different strategies for essential and non-essential metals. As a consequence, different types of models must be applied to describe metal uptake and elimination kinetics. We discuss model parameters for each metal separately and show how they are influenced by exposure concentrations and by physiological properties of the organisms. Soil pH, cation exchange capacity, clay and organic matter content significantly affect uptake rates of non-essential metals in soil invertebrates. For essential metals, kinetics is hardly influenced by soil properties, but rather prone to physiological regulation mechanisms of the organisms. Our analysis illustrates that toxicokinetics can be a valuable measurement to assess bioavailability of soil-bound metals.

Oxidative stress biomarkers and metallothionein in Folsomia candida--responses to Cu and Cd

Folsomia candida (Collembola) is a standard soil ecotoxicological species; effect assessment includes survival and reproduction as endpoints. In the present study, and for the first time, a range of oxidative stress biomarkers measurement was optimized and validated. The antioxidant capacity was measured by the activities of catalase (CAT), glutathione reductase (GR), glutathione-s-transferase (GST) and content of total glutathione (TG). The oxidative damage in the lipid membranes was estimated by lipid peroxidation (LPO) and metallothionein (MT) levels. The exposure included the essential and non-essential metals Cu and Cd, in LUFA 2.2 natural standard soil, using a series of sampling times along a 10 days period (0, 2, 4, 6 and 10 days). Exposure concentrations were selected based on their reproduction EC50 values, 60 and 1000 mg/kg soil DW, for Cd and Cu respectively. The protocols were optimized and results show that oxidative stress biomarkers can be successfully used in F. candida, this being highly relevant as complementary information to the mechanistic level. The selected sampling times gave a good indication of the markers dynamic and can be reduced/adapted in future testing. Results showed that both metals caused an increase in the MT levels after 6 days but Cd acted as a stronger oxidant agent compared to Cu, i.e. causing higher damage. In sum, Cd mobilized/activated more antioxidant enzymes, but the increased activities were not enough to prevent LPO. This study confirms that the oxidative stress caused by Cd is higher despite the use of same reproduction EC50 indicating that toxicity seems more reversible for Cu than for Cd. Among others, GST and MT would be a good selection of biomarkers for Cd effect.

A combined toxicokinetics and toxicodynamics approach to assess the effect of porewater composition on cadmium bioavailability to Folsomia candida

The aim of the present study was to improve our understanding of cadmium bioavailability by linking toxicokinetics and toxicodynamics. The springtail Folsomia candida was exposed to different cadmium concentrations in solutions embedded in inert quartz sand. Survival and cadmium uptake in the animals were followed for 21 d. After 10 d, some animals were transferred to clean medium to assess cadmium elimination. Using a first-order one-compartment model, an overall uptake rate constant (k1) of 0.18 L kg(animal)(-1) d(-1) and an elimination rate constant (k(2-TK)) of 0.02 d(-1) were calculated. Survival decreased with time, resulting in an estimated final median lethal concentration (LC50) of 0.51 mM. A lethal body concentration (LBC) of 4.6 µmol Cd g(-1) dry body weight was estimated by multiplying the final LC50 by the bioconcentration factor (k1/k(2-TK)). The LC50(animal) values based on internal cadmium concentrations were between 3.56 µmol Cd g(-1) and 9.91 µmol Cd g(-1) dry body weight, with an overall value of 7.9 µmol Cd g(-1) dry body weight (95% confidence interval [CI]: 3.8-12.0 µmol Cd g(-1) dry body wt). Because the 95% CI of the LC50(animal) included the LBC, there was good agreement of cadmium toxicokinetics and toxicodynamics.

Toxicodynamics of copper and cadmium in Folsomia candida exposed to simulated soil solutions

To improve our understanding of metal bioavailability to soil-living invertebrates, the effect of porewater composition on the toxicodynamics of copper and cadmium in Folsomia candida (Collembola) was investigated. Assuming that porewater is the main exposure route, F. candida was exposed to simulated soil solutions of different composition. Toxicity of copper was slightly lower in a calcium-only solution than in a multication solution. With increasing copper concentrations from 0.005 mM to 1.37 mM, internal copper concentrations similarly increased in both exposure solutions, suggesting that a single cation nutrient solution is suitable for testing F. candida. In the second experiment, animals were exposed for 7 d to copper and cadmium in simplified soil solutions with different calcium (0.2 mM, 0.8 mM, 3.2 mM, 12.8 mM) and pH (5.0, 6.0, 7.0) levels. The median lethal concentration (LC50) values decreased with time in both the calcium and pH series. A hormetic-type effect was observed for copper in the second test, as well as in the calcium-only solution in the first experiment. Because of stronger hormesis, LC50s for copper were higher at lower calcium concentrations. For cadmium, LC50 values were higher at higher calcium concentrations, suggesting competition of calcium with the free cadmium ion. Toxicity of cadmium increased with decreasing pH, while copper was more toxic at intermediate pH. The results show that a toxicodynamics approach can help to improve the interpretation of metal toxicity to soil invertebrates, taking into account soil solution properties.