Bioavailability links mode of action can improve the long-term field risk assessment for tilapia exposed to arsenic

Transcriptomic and metabolomic analysis of the mechanisms underlying stress responses of the freshwater snail, Pomacea canaliculata, exposed to different levels of arsenic

Arsenic (As) pollution poses an important problem, but limited information is available about the physiological effects of As on freshwater invertebrates. Here, we investigated the physiological effects of chronic As exposure on Pomacea canaliculata, a freshwater invertebrate. High level of As (Ⅲ, 5 mg/L) inhibited the growth of P. canaliculata, whereas low level of As (Ⅲ, 2 mg/L) promoted growth. Pathological changes in shell and cellular ultrastructure due to As accumulation likely explain the growth inhibition at high As level. Low level of As simulated the expression of genes related to DNA replication and chitosan biosynthesis, potentially accounting for the growth promotion observed. High level of As enrichment pathways primarily involved cytochrome P450, glutathione, and arachidonic acid-mediated metabolism of xenobiotics. ATP-binding cassette (ABC) transporters, specifically the ABCB and ABCC subfamilies, were involved in As transport. Differential metabolites were mainly associated with the metabolism and biosynthesis of amino acids. These findings elucidate the dose-dependent effects of As stress on P. canaliculata growth, with low levels promoting and high levels inhibiting. Additionally, our findings also provide insights into As metabolism and transport in P. canaliculata.

Extension of biotic ligand model to account for the effects of pH and phosphate in accurate prediction of arsenate toxicity

Biotic ligand model (BLM) was extended to predict the toxicity of inorganic arsenate (iAs(V)) to the luminescent bacteria, Aliivibrio fischeri. As the pH increased from 5 to 9, the HAsO₄^2- form predominated more than the H₂AsO₄^- form did, and the EC₅₀[As]_T (50% effective iAs(V) concentration) decreased drastically from 3554 ± 393 to 39 ± 6 μM; thus, the HAsO₄^2- form was more toxic to A. fischeri than H₂AsO₄^-. As the HPO₄^2- activity increased from 0 to 0.44 mM, the EC₅₀{HAsO₄^2-} values (50% effective HAsO₄^2- activity) increased from 31 ± 6 to 859 ± 128 μM, indicating that the toxicity of iAs(V) decreased, owing to the competition caused by the structural similarity between iAs(V) and phosphate ions. However, activities of Ca²⁺, Mg²⁺, K⁺, SO₄^2-, NO₃^-, and HCO₃^- did not significantly affect the EC₅₀{HAsO₄^2-} values. The BLM was reconstructed to take into account the effects of pH and phosphate, and the conditional binding constants for H₂PO₄^-, HPO₄^2-, H₂AsO₄^-, and HAsO₄^2- to the active binding sites of A. fischeri were obtained; 3.424 for logK_XH2PO4, 4.588 for logK_XHPO4, 3.067 for logK_XH2AsO4, and 4.802 for logK_XHAsO4. The fraction of active binding sites occupied by iAs(V) to induce 50% toxicity (f_mix^50%) was found to be 0.616.

Toxicity-based toxicokinetic/toxicodynamic assessment for bioaccumulation of polystyrene microplastics in mice

While a large body of literature has shown that microplastics (MPs) are highly likely to be accumulated in marine organisms and terrestrial animals, information about toxicity of MPs in mammal from a mechanistic point of view is more limited. Our paper fills this knowledge gap by assessing polystyrene (PS)-MPs-mice system based on toxicity-based toxicokinetic/toxicodynamic (TBTK/TD) modeling to quantify organ-bioaccumulation and biomarker responses appraised with published dataset. The key TBTK-parameters for mice liver, kidney, and gut posed by 5 or 20 μm PS-MPs could be obtained. We found that gut had the highest bioaccumulation factor (BCF) of ∼8 exposed to 5 μm PS-MPs with a mean residence time of ∼17 days. We showed that threshold concentrations of 5 and 20 μm PS-MPs among the most sensitive biomarkers were 8 ± 5 (mean ± SE) and 0.71 ± 0.14 μg g^-1 bw, respectively, implicating that particle size was likely to affect TK/TD behavior in mice. The mice-based TK parameters and threshold criteria greatly assist in designing robust researches to evaluate MP consumption by humans. We establish a TBTK/TD framework for mechanistically assessing potential from mice size-specific MPs exposure that would offer a tool-kit for extrapolating to humans from health risk assessment perspective.

Zn2+ induced molecular responses associated with oxidative stress, DNA damage and histopathological lesions in liver and kidney of the fish, Channa punctatus (Bloch, 1793)

Zn²⁺ is essential for normal physiological functioning of all organisms in small quantities, but when its concentration enhances in surrounding environment it acts as a toxicant to organisms. Common sources of Zn²⁺ pollution are electroplating, alloying, mining, and allied industrial operations. The present study aims to assess the biochemical, histopathological and genotoxicological implications under Zn²⁺ intoxication along with its accumulation patterns in prime biotransformation sites-liver and kidney, of a bottom feeder fish, Channa punctatus. Fish were chronically exposed to two different concentrations of Zn²⁺i.e., 5mg/L (permissible limit, T1) and 10mg/L (twice the permissible limit, T2). Simultaneous control was maintained. A significant (p<0.05) increment in Zn²⁺ bioaccumulation, antioxidant enzymes activities of SOD, CAT and GR and induction in micronuclei frequencies along with the significant (p<0.05) decrement in total protein and GSH were observed in all the exposed groups after 28 d. Altered biochemical parameters coupled with enhanced induction in micronuclei and accumulation of Zn²⁺ in liver and kidney of fish can be regarded as sensitive biomarkers of Zn²⁺ induced toxicological manifestations and thus, they may be effectively utilized for reliable ecotoxicological biomonitoring of aquatic regimes polluted with Zn²⁺.

Toxicity-based toxicokinetic/toxicodynamic assessment of bioaccumulation and nanotoxicity of zerovalent iron nanoparticles in Caenorhabditis elegans

Elucidating the relationships between the toxicity-based-toxicokinetic (TBTK)/toxicodynamic (TD) properties of engineered nanomaterials and their nanotoxicity is crucial for human health-risk analysis. Zerovalent iron (Fe⁰) nanoparticles (NPs) are one of the most prominent NPs applied in remediating contaminated soils and groundwater. However, there are concerns that Fe⁰NP application contributes to long-term environmental and human health impacts. The nematode Caenorhabditis elegans is a surrogate in vivo model that has been successfully applied to assess the potential nanotoxicity of these nanomaterials. Here we present a TBTK/TD approach to appraise bioaccumulation and nanotoxicity of Fe⁰NPs in C. elegans. Built on a present C. elegans bioassay with estimated TBTK/TD parameters, we found that average bioconcentration factors in C. elegans exposed to waterborne and food-borne Fe⁰NPs were ~50 and ~5×10^-3, respectively, whereas 10% inhibition concentrations for fertility, locomotion, and development, were 1.26 (95% CI 0.19-5.2), 3.84 (0.38-42), and 6.78 (2.58-21) μg·g^-1, respectively, implicating that fertility is the most sensitive endpoint in C. elegans. Our results also showed that biomagnification effects were not observed in waterborne or food-borne Fe⁰NP-exposed worms. We suggest that the TBTK/TD assessment for predicting NP-induced toxicity at different concentrations and conditions in C. elegans could enable rapid selection of nanomaterials that are more likely to be nontoxic in larger animals. We conclude that the use of the TBTK/TD scheme manipulating C. elegans could be used for rapid evaluation of in vivo toxicity of NPs or for drug screening in the field of nanomedicine.

Influences of size-fractionated humic acids on arsenite and arsenate complexation and toxicity to Daphnia magna

The intrinsic physicochemical properties of dissolved organic matter (DOM) may affect the mobility and toxicity of arsenic in aquatic environments. In the present study, the humic acid (HA) was ultra-filtered into five fractions according to molecular weight, and their physicochemical properties were characterized. Complexation of HA fractions with arsenite and arsenate was first determined by differential pulse polarography (DPP). The influences of HA fractions on arsenic toxicity were then examined using Daphnia magna as a model organism. As(V) had a higher affinity with HA than As(III), and their complexation was dependent on the total acidity and fluorescence characteristics of DOM. We demonstrated that the acidity and fluorescence also better explained the As toxicity to daphnids than UV absorbance and hydraulic diameter. Arsenic speciation determined by DPP significantly affected the toxicity of arsenite and arsenate. The results extended the free-ion activity model application to the case of arsenic. The present study clearly indicated that DOM with different molecular weights has distinct physicochemical properties, and could influence the speciation and toxicity of As to different extent.

Detoxification and bioregulation are critical for long-term waterborne arsenic exposure risk assessment for tilapia

Long-term metal exposure risk assessment for aquatic organism is a challenge because the chronic toxicity of chemical is not only determined by the amount of accumulated chemical but also affected by the ability of biological regulation or detoxification of biota. We quantified the arsenic (As) detoxification ability of tilapia and developed a biologically based growth toxicity modeling algorithm by integrating the process of detoxification and active regulations (i.e., the balance between accumulated dose, tissue damage and recovery, and the extent of induced toxic effect) for a life span ecological risk prediction. Results showed that detoxification rate (k (dex)) increased with increasing of waterborne As when the accumulated metal exceeded the internal threshold level of 19.1 μg g( - 1). The k (dex) values were comparable to or even higher than the rates of physiological loss and growth dilution in higher exposure conditions. Model predictions obtained from the proposed growth toxicity model were consistent with the measured growth data. The growth toxicity model was also used to illustrate the health condition and growth trajectories of tilapia from birth to natural death under different exposure scenarios. Results showed that temporal trends of health rates and growth trajectories of exposed fish in different treatments decreased with increasing time and waterborne As, revealing concentration-specific patterns. We suggested that the detoxification rate is critical and should be involved in the risk assessments framework. Our proposed modeling algorithm well characterizes the internal regulation activities and biological response of tilapia under long-term metal stresses.

Subcellular partitioning links BLM-based toxicokinetics for assessing cadmium toxicity to rainbow trout

The purpose of this article was to develop an integrated-scale toxicological model to investigate the impact of cadmium (Cd) toxicity on rainbow trout (Oncorhynchus mykiss) based on recent published experimental data. This model was generated from three different types of functional relationship: biotic ligand model (BLM), damage assessment model (DAM), and subcellular partitioning model (SPM), both of key toxicological determinants involved and of functional connections between them. Toxicokinetic parameters of uptake rate constant (k(1)) and elimination rate constant (k(2)) in gill, liver, and subcellular fractions were derived. A negative correlation between gill binding fraction of Cd and bioaccumulation factor was found. Detoxifying ability (% detoxified in liver metabolically detoxified pool (MDP)) and k(2) were negatively correlated, indicating that increasing % detoxified in MDP can compensate for lower k(2). This finding suggests a potential tradeoff between the abilities of elimination and detoxification for Cd. Yet, compensation between the ability to eliminate Cd and the ability to recover Cd-induced damage was not found. However, changes in k(2) and recovery rate constant (k(r)) can shift the dynamics of Cd susceptibility probability. This analysis implicates that once k(2) is determined experimentally, the values of k(r) and % detoxified in MDP can be predicted by the proposed k(2)-k(r) and k(2)-% detoxified relationships. This study suggests that the mechanistic linking of BLM-based DAM and SPM can incorporate the organ- and cell-scale exposure experimental data to investigate the mechanisms of ecophysiological response for aquatic organisms exposed to metal stressors.

Trade-offs between elimination and detoxification in rainbow trout and common bivalve molluscs exposed to metal stressors

The purpose of this paper was to examine trade-offs between elimination and detoxification in rainbow trout and three common bivalve molluscs (clam, oyster, and scallop) exposed to cadmium (Cd), copper (Cu), and zinc (Zn) based on recent reported experimental data. We incorporated metal influx threshold with subcellular partitioning to estimate rate constants of detoxification (k(d)) and elimination (k₂). We found that the relationships between k₂ and k(d) were negative for rainbow trout and positive for bivalve molluscs. However, the relationships between k(d) and % metal in metabolically detoxified pool were found positive for rainbow trout and negative for bivalve molluscs. Our results also indicated that rainbow trout had higher accumulation (~60-90%) in metabolically active pool when exposed to essential metals of Cu and Zn and had only 10-50% accumulation in response to non-essential metal of Cd. Based on a cluster analysis, this study indicated that similarity of physiological regulations among study species was found between Cd and Zn. Our study suggested that detoxification can be predicted by an elimination-detoxification scheme with the known elimination rate constant. We concluded that quantification of trade-offs between subcellular partitioning and detoxification provides valuable insights into the ecotoxicology of aquatic organisms and enhances our understanding of the subcellular biology of trace metals.

Assessing the impact of waterborne and dietborne cadmium toxicity on susceptibility risk for rainbow trout

The purpose of this study was to use a risk-based integrated-scale toxicological model to examine the impact of waterborne and dietborne cadmium (Cd) toxicity on rainbow trout (Oncorhynchus mykiss) susceptibility appraised with recent published data. A probabilistic assessment model was performed to estimate Cd susceptibility risk. The dose-response models were constructed based on two endpoints of % Cd in metabolically active pool (MAP) and susceptibility time that causes 50% effect (ST50). We further constructed an elimination-detoxification-recovery scheme to enhance the model predictive ability. We found a 95% probability of % Cd in gill and liver MAP exceeding 47-49% and it was likely (70% probability) to have exceeded 52-55%, but it was unlikely (30% probability) to have exceeded 56-60%. In contrast to gill and liver, gut had a relative lower Cd susceptibility risk (15-17% Cd in MAP) with a longer ST50. We suggested that the proposed probabilistic risk assessment framework can incorporate the elimination-detoxification-recovery scheme to help government based biomonitoring and bioassessment programs to prevent potential aquatic ecosystems and human health consequences.

Dynamic features of ecophysiological response of freshwater clam to arsenic revealed by BLM-based toxicological model

The objective of this study was to use a quantitative process-based damage assessment model (DAM) associated with biotic ligand model (BLM) to examine the ecophysiological responses of freshwater clam Corbicula fluminea to waterborne arsenic. We carried out a 14-day exposure experiment to obtain bioaccumulation parameters and a 7-day acute toxicity bioassay to obtain survival data. To investigate the survival dynamics, we examined 2 key parameters characterizing bioaccumulation and damage regulation: capacity to eliminate body arsenic burden and reversible ability to recover the damage. Results show that uptake rate constant of 2.075 + or - 0.442 (mean + or - SE) ml g(-1) d(-1) during uptake phase and elimination rate constant was estimated to be 0.1995 + or - 0.022 d(-1). The derived bioconcentration factor of 10.401 ml g(-1) suggests that arsenic has a high potential for bioaccumulation in C. fluminea. Our results show that a negative association between bioaccumulation and the fraction of arsenic binding in biotic ligand at 50% mortality, indicating that C. fluminea with higher arsenic binding in gill biotic ligand at 50% mortality level gives a lower capacity to accumulate bioavailable arsenic. We found a linearly positive correlation between elimination rate and recovery rate constants. Yet, a potential tradeoff between ability to eliminate arsenic and ability to recover the damage is not found. We showed that an ecophysiological significance of C. fluminea exposed to arsenic can be revealed by the elimination-recovery regime. This research may also provide mechanistic insights into the development of biomonitoring organism such as C. fluminea mimicking metal bioaccumulation in a real situation.

Systems-level modeling the effects of arsenic exposure with sequential pulsed and fluctuating patterns for tilapia and freshwater clam

The purpose of this paper was to use quantitative systems-level approach employing biotic ligand model based threshold damage model to examine physiological responses of tilapia and freshwater clam to sequential pulsed and fluctuating arsenic concentrations. We tested present model and triggering mechanisms by carrying out a series of modeling experiments where we used periodic pulses and sine-wave as featured exposures. Our results indicate that changes in the dominant frequencies and pulse timing can shift the safe rate distributions for tilapia, but not for that of freshwater clam. We found that tilapia increase bioenergetic costs to maintain the acclimation during pulsed and sine-wave exposures. Our ability to predict the consequences of physiological variation under time-varying exposure patterns has also implications for optimizing species growing, cultivation strategies, and risk assessment in realistic situations.