Toxicokinetics of copper and cadmium in the soil model Enchytraeus crypticus (Oligochaeta)

Toxicokinetics of selenate in earthworm sub-tissues and potential bio-accessibility assessment of earthworm-derived selenium

Selenium (Se) pollution is mainly caused by anthropogenic activities, and the resulting biosecurity concerns have garnered significant attention in recent years. Using one-compartmental toxicokinetic (TK) modelling, this study explored the kinetic absorption, sub-tissue distribution, and elimination processes of the main Se species (selenate, Se(VI)) in the cultivated aerobic soil of the earthworm Eisenia fetida. The bio-accessibility of earthworm-derived Se was assessed using an in vitro simulated gastrointestinal digestion test to evaluate its potential trophic risk. The results demonstrated that Se accumulated in the pre-clitellum (PC) and total tissues (TT) of earthworms in a time- and dose-dependent manner. The highest Se levels in the PC, post-clitellum (PoC), and TT were 70.54, 57.93, and 64.26 mg/kg during the uptake phase, respectively. The kinetic Se contents in the earthworms PC and TT were consistent with the TK model but not with PoC. The earthworm TT exhibited a faster uptake (Kus = 0.83-1.02 mg/kg/day) and elimination rate of Se (Kee = 0.044-0.049 mg/kg/day), as well as a shorter half-life time (LT1/2 = 15.88-14.22 days) than PC at low soil Se levels (≤5 mg/kg). Conversely, the opposite trend was observed with higher Se concentrations (10 and 20 mg/kg). These results are likely attributable to the tissue specificity and concentration of the toxicant. Earthworms PC and TT exhibited a higher kinetic Se accumulation factor (BAFk) than steady-state BAF (BAFss), with values ranging from 8 to 24 and 3-13, respectively. Furthermore, the bio-accessibility of earthworm-derived Se to poultry ranged from 66.25 % to 84.35 %. As earthworms are at the bottom of the terrestrial food chain, the high bio-accessibility of earthworm-derived Se poses a potential risk to predators. This study offers data support and a theoretical foundation for understanding the biological footprint of soil Se and its toxicological impacts and ecological hazards.

Multigenerational exposure of Ag materials (nano and salt) in soil - environmental hazards in Enchytraeus crypticus (Oligochaeta)

Because of its properties, silver is among the most used metals both as salt and as nanomaterials (NMs), hence reaching the environment. Multigenerational (MG) exposure testing is scarce, and especially so for NMs and soil invertebrates. In this study the MG effects of Ag NMs (Ag NM300K) and Ag salt (AgNO3) were assessed, using Enchytraeus crypticus in LUFA 2.2 soil. Survival, reproduction and internal Ag concentration in the animals were measured throughout 7 generations (5 generations (F0-F4) in spiked soil plus 2 (F5-F6) in clean soil) exposed to sublethal concentrations corresponding to the reproduction EC10 and EC50 obtained in standard toxicity tests (45 and 60 mg Ag per kg soil DW for AgNO3; 20 and 60 mg Ag per kg soil DW for Ag NM300K). MG exposure caused a dose-related decrease in reproduction for both Ag forms. Ag uptake peaked in the F1 (64 days) for AgNO3 and F2 (96 days) for Ag NM300K, after which it decreased. In agreement with toxicokinetic studies, a maximum body Ag concentration was reached (20 mg Ag per kg body DW (AgNO3) and 70 mg Ag per kg body DW (Ag NM300K)) and after which detoxification mechanisms seem to be activated with elimination of Ag accompanied by a decrease in reproduction. Transfer to clean soil allowed Ag to be (fully) eliminated from the animals. This MG study confirmed the effects determined in standard reproduction toxicity tests but further allowed to monitor the dynamics between exposure and effects of the Ag materials, and how the animals seem to cope with Ag for 7 generations by compensating between detoxification and reproductive output.

Toxicokinetics and toxicodynamics of Ag nanomaterials (NM300K) in the soil environment-impact on Enchytraeus crypticus (Oligochaeta)

Silver (Ag) is one of the most used elements in the nanomaterials (NMs) form, which upon release to the environment can be harmful to organisms. We compared the toxicokinetics (TK) and toxicodynamics (TD) of Ag from AgNO₃ (0, 15, 45, 135, 405 mg Ag/kg soil) and AgNM300K (0, 75, 150, 300, 600, 1200 mg Ag/kg soil) in the model organism Enchytraeus crypticus. Organisms were exposed in LUFA 2.2 soil, and besides body Ag concentrations, survival and reproduction were determined, in a time series (for 21 days). In the soil, the available (CaCl₂ extractable) Ag fraction from Ag NM300K increased from 0 to 21 days but did not consistently change for AgNO₃. Internal concentrations reached equilibrium in most exposures to both Ag forms. The organisms were able to internalize and eliminate Ag, but less when exposed to Ag NM300K. The overall uptake rate constants for Ag from AgNO₃ and Ag NM300K exposures were 0.05 and 0.06 kg soil/kg organism/day, respectively, the elimination rate constants 0.2 and 0.1 day^-1, respectively. For AgNO₃ the median lethal concentrations decreased steadily with time, while for Ag NM300K they remained constant during the first 10 days of exposure followed by a 2-fold decline in the last 7 days. The 21-d LC50s for both Ag forms were similar but the LC50_inter (based on internal concentrations) were 63 and 121 mg Ag/kg body DW (Dry Weight) for AgNO₃ and Ag NM300K, respectively, showing higher toxicity of AgNO₃. These results show the importance of assessing time to toxicity, a relevant factor in toxicity assessment, especially for NMs.

The effect of polystyrene microplastic and biosolid application on the toxicity and bioaccumulation of cadmium for Enchytraeus crypticus

Plastics smaller than 5 mm that end up in a soil environment are known as microplastics (MPs). Microplastics have become a common contaminant in agricultural areas in addition to metals. However, the effect of cadmium (Cd) on soil organisms has not been clearly defined in the presence of MPs. In addition to MPs, biosolid application as a soil amendment in agricultural lands is also leading to shifts in soil conditions, such as the concentrations of nutrients and organic matter. Therefore, the aim of this study is to investigate the toxicity and bioaccumulation of Cd for Enchytraeus crypticus in the presence of polystyrene (PS)-MPs and biosolids to provide insight into their possible interactions. The lethal toxic concentration (LC50) for Cd was higher than 650 mg Cd/kg dry soil for all conditions. The presence of PS-MPs increased the toxicity of Cd for which EC50 was 102 and 38 mg Cd/kg dry soil without and with Cd, respectively, which may be the result of an increased exposure rate through adsorption of Cd on PS-MPs. On the contrary, the presence of biosolids decreased the toxicity of Cd where EC50 was 193 and 473 mg Cd/kg dry soil for the sets applied with 0.6 and 0.9 g biosolids, respectively. Coexistence of biosolids and PS-MPs decreased the reproduction toxicity of Cd, which is similar to the biosolid effect (EC50 is 305 mg Cd/kg dry soil). Bioaccumulation of Cd only positively correlated with its initial concentration in soil and was not affected by the presence of PS-MPs or biosolids. Integr Environ Assess Manag 2023;19:489-500. © 2022 SETAC.

Toxicokinetics and toxicodynamics of chromium in the soil invertebrate Enchytraeus crypticus (Oligochaeta)

Chromium emissions led to increased concentrations in soil, where it can affect soil organisms to relevant levels. With the aim of better understanding the effects of Cr throughout time, its toxicokinetics-toxicodynamics (TKTD) were evaluated in the soil model organism Enchytraeus crypticus to assess the development of internal concentrations and consequent toxic effects. To achieve this goal, organisms were exposed in LUFA 2.2 soil spiked with increasing CrCl₃ concentrations. During the 21-day exposure period, survival, internal concentrations, and reproduction were evaluated at several time points up to 21 days. Uptake and elimination rate constants were 0.0044 kg soil/kg organism/day and 0.023 per day, respectively. Internal Cr concentrations increased with time, generally reaching equilibrium within 14 days with an estimated LC50_inter (based on internal metal concentrations) of 57.7 mg Cr/kg body DW. Internal Cr concentrations were regulated by the organisms up to exposure to 360 mg Cr/kg soil DW, where the elimination rate was highest, but at 546 mg Cr/kg soil DW the animals were no longer able to eliminate Cr, and the internal concentrations were well above the estimated LC50_inter. At day 21, exposure to 546 mg Cr/kg soil DW significantly reduced survival by 23 %, while reproduction EC50 was 344 mg Cr/kg soil DW. This study highlights the advantages of using a TKTD approach to understand the development of internal metal concentrations in time and relate it to the phenotypical effects observed. Toxicity is better understood when also taking into account time and not just exposure concentration alone.

Subcellular localization and compartment-specific toxicokinetics of cadmium, arsenic, and zinc in brandling worm Eisenia fetida

Awareness of toxicokinetics at the subcellular level is crucial to deciphering the underlying intoxication processes of metal(loid)s, although this information is often lacking. Here, the toxicokinetics of two non-essential metal(loid)s (Cd and As) and one essential metal (Zn) in both the whole body and subcellular fractions of earthworm (Eisenia fetida) were assessed. Earthworms were exposed to natural soils originating from a gradient of metal(loid) pollution for 14 days followed by a 14-day elimination phase in clean soil. Clearly distinct toxicokinetic patterns were found in the earthworms according to the metal(loid) considered. An obvious concentration-dependent increase was observed in earthworms or subcellular compartments where no equilibrium was reached (with slow or no elimination) for Cd and As throughout the experiment. As for Zn, the earthworms were able to retain a steady-state concentration of Zn in its body or each fraction without a clear intake behavior via the dynamic trade-off between uptake and elimination at different pollution levels. These differences in toxicokinetics at the subcellular level supported the observed differences in bioaccumulation patterns and were indicative of the strategy by which non-essential and essential elements are handled by earthworms. Notably, the concentration of Cd and As in subcellular compartments showed the same pattern as for Zn in the order of cellular cytosol > cellular debris > metal-rich granules, which might be associated with the binding of non-essential/essential elements with metallothionein enriched in the cytosol. Our findings enhance the understanding of the underlying mechanisms for metal(loid) accumulation kinetics in earthworms from the perspective of subcellular partitioning, and will be beneficial for accurate risk assessment of Cd, As, and Zn.

Toxicokinetics and toxicodynamics of copper and cadmium in the soil invertebrate Enchytraeus crypticus (Oligochaeta)

The aim of this study was to evaluate the toxicokinetics-toxicodynamics (TKTD) of Cu and Cd in the soil model organism Enchytraeus crypticus, and assess the development of internal effect concentrations over time. Animals were exposed in LUFA 2.2 soil spiked with increasing concentrations of Cu and Cd. Survival, reproduction and internal metal concentrations in the animals were evaluated at different points in time over a period of 21 days. Internal concentrations increased with time, for Cu reaching a steady state after c. 10 days, except for the highest test concentration, and for Cd continuing to increase after 21 days. Applying a one-compartment model to all data together, estimated uptake and elimination rate constants for Cu and Cd were 0.08 and 0.45 kg soil/kg organism/day and 0.4 and 0.04 per day, respectively. Median lethal concentrations, based on total soil concentrations, decreased with time for Cu and did not reach a steady state level, but they did not change with time for Cd. The LC50_inter (based on internal concentrations) was 75 mg Cu/kg body DW and > 800 mg Cd/kg body weight. Animals were able to regulate Cu internal concentrations, keeping them low, while for Cd internal concentrations continued to increase showing lack of regulation and also the importance of exposure time. This study highlights the advantages of using a TKTD approach to understand the relation between organism survival and internal Cu or Cd concentrations over time.

Toxicokinetics of metals in the soil invertebrate Enchytraeus crypticus exposed to field-contaminated soils from a mining area

Toxicokinetics may help assessing the risk of metal-contaminated soils by quantifying the development of internal metal concentrations in organisms over time. This study assessed the toxicokinetics in Enchytraeus crypticus of non-essential (Pb and Cd) and essential elements (Zn and Cu) in metal-contaminated field soils from a mining area, containing 3.49-24.3 mg Cd/kg dry soil, 433-1416 mg Pb/kg dry soil, 15.7-44.9 mg Cu/kg dry soil and 1718-6050 mg Zn/kg dry soil. Three different uptake-elimination patterns in E. crypticus were found. Both essential elements (Zn and Cu) showed fast increasing internal concentrations reaching equilibrium within 2 d in the uptake phase, without hardly any elimination after transfer to clean soil. The non-essential Cd showed a slow linear accumulation and excretion with body concentrations not reaching steady state within 21 d. Internal Pb concentrations, however, reached equilibrium within 7 d in the uptake phase. Longer exposure times in ecotoxicological tests, therefore, are required for elements like Cd. Porewater pH and dissolved organic carbon (DOC) levels were the dominant factors controlling Cd uptake from the test soils. The 21-d body Cd and Pb concentrations were best explained from 0.01 M CaCl₂-extractable soil concentrations. Steady-state Cu and Zn body concentrations were independent of soil exposure concentrations. Bioaccumulation factors (BAF) were low for Pb (<0.1 kg_soil/kg_worm), but high for Cd at 1.78-24.3 kg_soil/kg_worm, suggesting a potential risk of Cd biomagnification in the terrestrial food chain of the mining area ecosystem.

Toxicokinetics of Chromium in Enchytraeus crypticus (Oligochaeta)

Chromium is naturally occurring, but emission from anthropogenic sources can lead to increased soil concentrations. Information on its toxicokinetics is essential in order to understand the time needed to reach toxicity and the mechanisms of uptake/elimination. In this study the toxicokinetics of Cr(III) was evaluated using the soil standard species Enchytraeus crypticus. The animals were exposed to 180 mg Cr/kg dry soil, a sublethal concentration, in LUFA 2.2 natural soil. OECD guideline 317 was followed, with a 14-day uptake phase in spiked soil followed by a 14-day elimination in clean soil. Exposure to Cr led to fast uptake and elimination, with K_u = 0.012 kg_soil/kg_organism/day and K_e = 0.57 day⁻¹. The bioaccumulation factor was 0.022, and DT₅₀ for elimination was 1.2 days. The concentration of Cr reached an internal equilibrium in the animals after 10 days. Transfer to clean soil allowed body Cr concentrations to return to background levels after approximately 7 days. E. crypticus seemed able to efficiently regulate internal Cr concentrations by actively eliminating Cr (an essential element). Although K_u and K_e deviated from the values reported in other studies for other soil invertebrates, the bioaccumulation factors were similar. These findings show the importance of toxicokinetic studies in evaluating toxicity based on internal metal concentrations that can more accurately represent the bioavailable concentration.

Reveal the metal handling and resistance of earthworm Metaphire californica with different exposure history through toxicokinetic modeling

Toxicokinetic (TK) model provides a new approach to mechanistically elucidate the natural variation of metal handling strategy by adaptive and sensitive earthworm populations. Here, TK model was applied to explore the metal handling and resistance strategy of wild Metaphire californica with different historical exposure history through a 12-day re-exposure and another 12-day elimination incubation. M. californica populations showed different kinetic strategies for non-essential metals (Cd and Pb) and essential metals (Zn and Cu), which were closely related to their exposure history. M. californica from the most serious Cd-contaminated soil showed the fastest kinetic rates of both Cd uptake (K₁ = 0.78 g_soil/g_worm/day) and elimination (K₂ = 0.23 day^-1), and also had the lowest Cd half-life (t_1/2 = 3.01 day), which demonstrated the potential Cd-resistance of wild M. californica from Cd-contaminated soils. Besides, the comparative experiment showed totally different metal kinetics of laboratory Eisenia fetida from field M. californica, suggesting the impacts of distinct exposure history and species-specifical sensitivities. These findings provide a novel approach to identify and quantify resistance using TK model and also imply the risk of overlooking existing exposure background and interspecies extrapolation in eco-toxicological studies and risk assessments.

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.