Dissolved organic matter affects both bioconcentration kinetics and steady-state concentrations of polycyclic aromatic hydrocarbons in zebrafish (Danio rerio)

In vitro approach to refine bioconcentration and biotransformation predictions of organic persistent pollutants using cell lines

The application of the 3Rs (Replacement, Reduction, and Refinement) in animal experimentation has recently concentrated its efforts on utilizing cellular systems to predict toxicity in organisms. In this context, while refining the data obtained from cell lines, this study assesses their bioaccumulation potential and various methods for extrapolating the in vitro metabolization rate constant to support modelled bioaccumulation assessments for fish and their limitations. For this purpose, the concentrations of the parent compound, phenanthrene, and its major metabolites within the cells and in the medium at various exposure times were quantified. A chemical distribution model (mass balance) was applied to calculate the concentrations of the cell-bioaccessible compounds (Cfree) based on the experimentally determined concentrations. An elevated matching was observed between the in vitro bioconcentration factor (BCF) and the in vivo BCFs reported in the literature for zebrafish liver cells (ZFL). This study demonstrates the importance of further investigating in vitro biotransformation kinetics. The results obtained with the approach developed here provide valuable information to enhance current models. Additionally, it underscores the potential of cell lines as a strategy for rapid, simple, and cost-effective predictions without the need for animal experimentation.

Combined effects of pH and dissolved organic matter on the availability of pharmaceuticals and personal care products in aqueous environment

The interaction between pharmaceuticals and personal care products (PPCPs) with dissolved organic matter (DOM) can alter their bioavailability and toxicity. Nevertheless, little is known about how pH and DOM work together to affect the availability of PPCPs. This study investigated the impact of pH and DOM on the availability of seven PPCPs, namely Carbamazepine, Estrone, Bisphenol A, Testosterone Propionate, Triclocarban, 4-tert-Octylphenol and 4-n-Nonylphenol, using negligible depletion solid-phase microextraction (nd-SPME). The uptake kinetics of PPCPs by the nd-SPME fibers increased proportionally with DOM concentrations, likely due to enhanced diffusive conductivity in the unstirred water layer. At neutral pH, the partitioning coefficients of PPCPs for Humic Acid (log KDOC 3.87-5.25) were marginally higher than those for Fulvic Acid (log KDOC 3.64-5.11). Also, the log KDOC values correlated linearly with the log DOW (pH 7.0) values of PPCPs, indicating a predominant role for hydrophobic interactions in the binding of DOM and PPCPs. Additionally, specific interactions like hydrogen bonding, π-π, and electrostatic interactions occur for certain compounds, influenced by the polarity and spatial conformation of the compounds. For these ionizable PPCPs, the log DDOC values exhibit a strong dependence on pH due to the dual influence of pH on both DOM and PPCPs. The log DDOC values rose from pH 1.0 to 3.0, peaked at pH 5.0 to 9.0, and then (sharply) declined from 11.0 to 13.0. The reasons are that in strong acidic circumstances, the coiled and compressed shape of DOM inhibits the hydrophobic interaction, whereas in strong alkaline conditions, significant electrostatic repulsion reduces the sorption. This study reveals that the effects of DOM on the bioavailability of PPCPs are dependent on both pH and the specific compound involved.

Influence of humic acid on the bioaccumulation, elimination, and toxicity of PFOS and TBBPA co-exposure in Mytilus unguiculatus Valenciennes

Tetrabromobisphenol A (TBBPA) and Perfluorooctane sulfonate (PFOS) are emerging contaminants which coexist in marine environments, posing significant risks to ecosystems and human health. The behavior of these contaminants in the presence of dissolved organic matter (DOM), specifically the co-contamination of TBBPA and PFOS, is not well understood. The bioaccumulation, distribution, elimination, and toxic effects of TBBPA and PFOS on thick-shell mussels (Mytilus unguiculatus V.), with the absence and presence of humic acid (HA), a typical DOM, were studied. The results showed that the uptake of TBBPA decreased and the uptake of PFOS increased when exposed to 1 mg/L HA. However, at higher concentrations of HA (5 and 25 mg/L), the opposite effect was observed. Combined exposure to HA, TBBPA, and PFOS resulted in oxidative stress in the digestive gland, with the severity of stress dependent on exposure time and HA dose. Histological analysis revealed a positive correlation between HA concentration and tissue damage caused by TBBPA and PFOS. This study provides insights into the influence of HA on the bioaccumulation-elimination patterns and toxicity of TBBPA and PFOS in marine bivalves, offering valuable data for ecological and health risk assessments of combined pollutants in aquatic environments rich in DOM.

The concentration of dissolved organic matter impacts the neurobehavior in female zebrafish exposed to cyclophosphamide

Cyclophosphamide (CP) is a broad-spectrum anticancer drug for various cancers and frequently detected in aquatic environments, reaching concentrations up to 22 μg/L. However, there is limited understanding of the toxicities of CP with the presence of dissolved organic matter, a ubiquitous component in aquatic environments, in fish. In this study, we investigated the behaviors, morphological alterations of retina, and related gene transcripts in zebrafish exposed to CP (0 and 50 μg/L) and Humic acid (HA, a main component of DOM) at concentrations of 0, 3, 10, and 30 mg-C/L for 30 days. The results showed that, relative to the zebrafish in CP treatment, HA at 30 mg-C/L increased the locomotion (12.1 % in the light and 7.2 % in the dark) and startle response (9.7 %), while inhibiting the anxiety (12.5 %) and cognition of female zebrafish (24.6 %). The levels of transcripts of neurotransmitter- (tph1b and ache), neuroinflammation-(il-6 and tnfα) and antioxidant-(gpx) related genes in the brain of female adult were also altered by CP with the presence of HA. In addition, HA promoted the transgenerational effects of CP on the neurobehaviors. Therefore, HA can enhance potential neurotoxicity of CP in female fish through alteration neurotransmission related genes. Our findings provide new insights into the toxicity and underlying mechanisms of CP with the presence of dissolved organic matter, thereby contribute to a deeper understanding of the risks posed by CP in aquatic ecosystems.

Bioconcentration of polycyclic aromatic hydrocarbons in different tissues of zebrafish (Danio rerio) investigated with PBTK model

Due to the lipophilicity, polycyclic aromatic hydrocarbons (PAHs) are easily accumulated in fish. However, the research on PAH bioaccumulation process in different fish tissues and the relevant effect mechanisms are still deficient. The bioconcentration of PAHs (phenanthrene, anthracene, fluoranthene, and pyrene) in different zebrafish tissues (skin, fish muscle, gill, digestive tract, liver, gonad, and residual) was studied. It was found that there was a difference in the PAH concentrations in different zebrafish tissues. Compared with other tissues, the PAH concentration was highest in the skin and lowest in the fish muscle. For example, the steady-state concentration of phenanthrene in the skin was nearly five times higher than that in the muscle. PAH distribution was related with the lipid contents in different zebrafish tissues; however, the correlation was not significant (p > 0.05), indicating that the lipid content was not the determining factor for the PAH distribution. The distribution was also affected by the bioconcentration kinetics of PAHs in different zebrafish tissues, and the PAH hydrophobic properties. In addition, the physiological based toxicokinetic (PBTK) model showed good performance in predicting PAH internal concentrations, and it may be used to predict the concentrations of PAHs in different fish tissues in future.

Influence of dissolved organic carbon on multimedia distribution and toxicity of fipronil and its transformation products in lotic waterways

Environmental fate and ecological impacts of fipronil and its transformation products (FIPs) in aquatic environment have caused worldwide attention, however, the influence of dissolved organic carbon (DOC) on multimedia distribution, bioavailability, and toxicity of FIPs in field waterways was largely unknown. Here, we collected 11 companion water and sediment samples along a lotic stream in Guangzhou, South China. FIPs were ubiquitous with total water concentrations ranging from 1.22 to 43.2 ng/L (14.8 ± 12.9 ng/L) and fipronil sulfone was predominant in both water and sediment. More than 70% of FIPs in aqueous phase were bound to DOC and the K_DOC values of FIPs were approximately 1-2 orders of magnitude higher than K_d-s/K_OC, emphasizing the significance of DOC in phase partitioning and transport of FIPs in aquatic environment. Water and sediment samples were more toxic to Chironomus dilutus than Hyallela azteca, and FIPs (especially fipronil sulfone) pronouncedly contributed toxicity to C. dilutus. Toxic units (TU) based on freely dissolved concentrations in water determined by solid phase microextraction significantly improved toxicity estimation of FIPs to the invertebrates compared to TUs based on aqueous concentrations. The present study highlights the significance of DOC association on fate and ecological risk of hydrophobic insecticides in lotic ecosystem.

Estimation and potential ecological risk assessment of multiphase PAEs in mangrove wetlands in Dongzhai Harbor, Hainan

With the application of plastic products, phthalates now widely occur in various environmental media. A large number of ecological risk assessment experiments have only been carried out on a single medium such as water or sediment. There are few reports of ecological risk assessments based on the phase states of phthalic acid esters (PAEs) such as the free dissolved state and the dissolved organic carbon (DOC) adsorption state. In this study, the concentrations of the free dissolved state, the DOC adsorption state, and the easily released PAEs in the sediments, as well as the dissolved organic carbon release potential and their influencing factors were calculated in the Dongzhaigang water body. The potential ecological risks posed by state-of-the-art PAEs were investigated. The average concentration of six freely dissolved PAEs in water was 0.542 (0.226-1.115) μg/L, accounting for 76.3 % of the total PAEs. The PAEs with the highest concentrations in the free dissolved state were di-n-butyl phthalate (DBP, 0.383 μg/L), followed by Di(2-ethylhexyl) phthalate (DEHP, 0.094 μg/L). The average concentration of all six PAEs (∑6PAEs) adsorbed by the DOC in the water was 0.172 μg/L, accounting for 23.74 % of all of the PAEs. The DOC-adsorbed DEHP (0.148 μg/L) accounted for about 86 % of the six adsorbed PAEs. Sediment organic carbon may affect the release potential of the DOC through changing the soluble organic carbon concentration. Most types of PAEs in water posed low risk to organisms. However, DBP posed low and medium risk to algae and crustaceans, and medium risk to fish. Medium or high risk of DEHP to algae, crustaceans and fish was observed. The high ecological risk of PAEs related to sediments were only found at S13 and S14. Generally, the potential ecological risk of PAEs in sediment was more stable than that in water bodies.

Comparisons in molecular weight distributions and size-dependent optical properties among model and reference natural dissolved organic matter

Humic acid (HA) and reference natural organic matter (NOM) have been widely used in environmental assessment, biogeochemistry, and ecotoxicity studies. Nevertheless, similarities and differences among the commonly used model/reference NOMs and bulk dissolved organic matter (DOM) have rarely been systematically evaluated. In this study, HA, SNOM (Suwannee River NOM) and MNOM (Mississippi River NOM), both from International Humic Substances Society, and freshly collected unfractionated NOM (FNOM) were concurrently characterized to evaluate their heterogeneous nature and size-dependent chemical properties. We found that molecular weight distributions, PARAFAC-derived fluorescent components, and size-dependent optical properties are NOM-specific and highly variable with pH. The < 1 kDa DOM abundance followed the order of HA < SNOM < MNOM < FNOM. In addition, FNOM was more hydrophilic and contained more protein-like and autochthonous components with a higher UV-absorbance ratio index (URI) and biological fluorescence index, whereas HA and SNOM contained more allochthonous, humic-like components with a higher aromaticity and lower URI. Significant differences in molecular composition and size spectra between FNOM and model/reference NOMs suggest that environmental role of NOMs should be evaluated at the levels of molecular weight and functionalities under the same experimental conditions and that HA and SNOM may not represent bulk NOM in the environment. This study provides new information about similarities and differences in DOM size-spectra and chemical properties between reference NOMs and in-situ NOM and highlights the need to better understand the heterogenous roles of NOMs in regulating the toxicity/bioavailability and environmental fate of pollutants in aquatic environments.

Dermal Uptake is an Important Pathway for the Bioconcentration of Hydrophobic Organic Compounds by Zebrafish (Danio rerio)

For bioconcentration of hydrophobic organic compounds (HOCs), most of studies assumed that fish absorb HOCs mainly through gills but often ignored the dermal uptake. In this study, deuterated polycyclic aromatic hydrocarbons (PAHs-d₁₀, phenanthrene-d₁₀, and pyrene-d₁₀) and polychlorinated biphenyls (PCB-153) were selected to study whether zebrafish can absorb freely dissolved and dissolved organic matter (DOM)-associated HOCs through dermal uptake. The results showed that the freely dissolved PAHs and PCBs could directly enter the body of zebrafish through its skin. However, PAHs and PCB-153 associated with DOM (~ 10 kDa) could not enter zebrafish through the skin. When gill and dermal exposure coexisted, dermal uptake contributed 2.9 ~ 7.6% and 31.9 ~ 38.4% of PAHs and PCB-153 bioconcentration after exposure for 6 h, respectively. The present study demonstrates that dermal uptake is an important pathway for the bioconcentration of HOCs by fish, which should be considered when studying the toxicodynamics and toxicokinetics of HOCs in organisms.

Underlying Mechanisms for Low-Molecular-Weight Dissolved Organic Matter to Promote Translocation and Transformation of Chlorinated Polyfluoroalkyl Ether Sulfonate in Wheat

Dissolved organic matter (DOM) such as fulvic acid (FA) and humic acid (HA) in soil considerably affects the fate of per- and polyfluoroalkyl substances (PFASs). However, the effect of DOM on their behavior in plants remains unclear. Herein, hydroponic experiments indicate that FA and HA reduce the accumulation of an emerging PFAS of high concern, 6:2 chlorinated polyfluoroalkyl ether sulfonate (6:2 Cl-PFESA), in wheat roots by reducing its bioavailability in the solution. Nevertheless, FA with low molecular weight (MW) promotes its absorption and translocation from the roots to the shoots by stimulating the activity and the related genes of the plasma membrane H⁺-ATPase, whereas high-MW HA shows the opposite effect. Moreover, in vivo and in vitro experiments indicate that 6:2 Cl-PFESA undergoes reductive dechlorination, which is regulated mainly using nitrate reductase and glutathione transferase. HA and FA, particularly the latter, promote the dechlorination of 6:2 Cl-PFESA in wheat by enhancing electron transfer efficiency and superoxide production. Transcriptomic analysis indicates that FA also stimulates catalytic activity, cation binding, and oxidoreductase activity, facilitating 6:2 Cl-PFESA transformation in wheat.

Role of Seasons in the Fate of Dissolved Organic Carbon and Nutrients in a Large-Scale Surface Flow Constructed Wetland

The role of seasons in the removal of dissolved organic carbon (DOC), nutrients and in changes in the spectral properties of dissolved organic matter (DOM) in a large-scale surface flow constructed wetland (SF-CW) receiving reclaimed water and composed of three basins with different vegetation patterns was studied. Dissolved nitrogen removal efficiencies within the three basins in summer (>50%) and winter (<30%) were significantly different. SF-CW water is enriched in DOC in spring and summer with average outlet concentrations above 8 mg·L−1. UV-visible indices, such as the specific absorbance at 254 nm or the spectral slope between 275 and 295 nm, did not vary over the seasons; thus, the basins did not change DOM aromaticity and average molecular weight. Synchronous fluorescence spectra showed variations in terms of protein-like and humic-like substances, the latter being more sensitive to photodegradation. A lab-scale photodegradation experiment confirmed that radiation from the sun was responsible for this decrease, showing this process could alter the composition of DOM at full-scale. DOM variations result from a seasonal competition between release by vegetation and photodegradation. These results validate the necessity for long-term monitoring of SF-CWs, and the utility of rapid optical methods to monitor DOC.

Combined toxicity of polystyrene microplastics and sulfamethoxazole on zebrafish embryos

Despite extensive investigation on the toxicity of microplastics (MPs), an emerging global concern, little is known about the combined toxicity of MPs and co-occurring pollutants in aquatic environments. In this study, the combined toxicity of polystyrene MPs and sulfamethoxazole (SMZ) antibiotics was explored in zebrafish embryos in terms of the developmental, physiological, and endocrine toxicities. Exposure to PS and SMZ induced mortality (rate: 25.0 ± 7.5%) and malformation (rate: 20~35%) at multiple regions and stages of zebrafish development. Physiological toxicity was also induced as shown by the significant decrease in fetal movement (by 31.1~37.0%) and swimming frequency (by 26.9~36.8%) and the increase in heartbeat rate (by 19.0~20.9%). Finally, PS and SMZ exposure also induced extensive endocrine toxicities in zebrafish as confirmed by increases in various biomarkers including vitellogenin, 17β-estradiol, testosterone, and triiodothyronine. The combination index showed that antagonistic effects were present between PS and SMZ toxicity, which slightly decreased their combined toxicity. This study aims to further understand the combined toxicity of MPs and co-occurring pollutants in aquatic environments.

Acute toxicity of eucalyptus leachate tannins to zebrafish and the mitigation effect of Fe3+ on tannin toxicity

Fish ponds polluted by the black water of eucalyptus forests (formed by the complexation of eucalyptus tannins with Fe³⁺) have experienced fish deaths. However, the toxicity of the components of black water is still unclear. To study the acute toxicities of eucalyptus leachate tannins to fish, their changes in the presence of Fe³⁺, and the underlying mechanisms, the static bioassay test method was adopted for acute exposure testing of zebrafish. Zebrafish were exposed to three kinds of tannins, namely, tannic acid (TA), epigallocatechin gallate (EGCG) and tannins from fresh eucalyptus leaf leacheate (TFL), and to solutions of these tannins with different molar ratios of Fe³⁺, under both no-aeration and aeration conditions. The results showed that the 48 h LC50 values of TA, EGCG and TFL were respectively 92, 47, and 186 mg·L^-1, under no aeration, and 171, 86, and 452 mg·L^-1 under aeration. When Fe³⁺ at 2, 1, and 6 times the molar amount of tannin was added to LC100 solutions of TA, EGCG and TFL, zebrafish mortality in 24 h was reduced to 0-33%. Acute fish death in eucalyptus plantation areas is related to high concentrations of eucalyptus tannins in the water. However, with increasing dissolved oxygen and Fe³⁺ levels, the acute toxicity of tannins to fish can be reduced. Thus, the black water in eucalyptus plantation areas reflects a water quality phenomenon that reduces the acute toxicity of eucalyptus tannins to fish. The mechanism of tannin toxicity to fish may be related to the impairment of oxygen delivery by fish blood, but the mechanism needs further study. These results provide a scientific basis for the prevention and control of fish suffering from acute eucalyptus tannin poisoning in eucalyptus plantation areas and for the protection of water resources.

Simultaneous changes of exogenous dissolved organic matter treated by ozonation in properties and interaction behavior with sulfonamides

Effluent is often treated with ozone before being discharged into a natural water environment. This process will change the interaction between effluent organic matter and pollutants in aquatic environment. The impact of ozonation on complexation between dissolved organic matter in such wastewater and sulfadimidine often found in natural water was studied in laboratory experiments using four types of real wastewater. Ozonation was found to decrease the proportion of organic matter with a molecular weight greater than 5 kDa as well as protein-like, fulvic-like and humic-like components, but except the proportion of hydrophilic components. The aromaticity of the dissolved organic matter was also reduced after ozonation. The complexation of tryptophan and tyrosine with sulfadimidine mainly depends on their hydrophobicity and large molecular weight. Ozonation of fulvic and humic acid tends to produce small and medium molecular weight hydrophilics. The complexation of humic and fulvic acids with sulfadimidine was enhanced by ozonation. Dissolved organic matter, with or without oxidation, were found to weaken sulfadimidine's inhibition of microbial growth, especially for Aeromonas and Acinetobacter species. This finding will expand our understanding about the impact of advanced treatment processes on the dissolved organic matters' properties in effluent.

Can the hydrophobic organic contaminants in the filtrate passing through 0.45 μm filter membranes reflect the water quality?

In the traditional water quality assessment, the concentration of total dissolved hydrophobic organic compounds (HOCs) passing through 0.45 μm filter membranes is usually used to evaluate the influence of HOCs on water quality. However, the bioavailability of dissolved organic matter (DOM)-associated and particle-associated HOCs is not considered. In the present work, pyrene, fulvic acid, and natural suspended particles (SPS) were used to simulate natural water (raw water). The immobilization and pyrene content in the tissues of D. magna caused by total pyrene in the raw water and those caused by freely dissolved pyrene with the concentration equal to the total dissolved pyrene in the filtrate of raw water were compared to determine whether the total dissolved pyrene concentration can reflect the water quality. The results indicated that when the DOM concentration was 5 mg C L^-1 and the SPS concentration was higher than 0.2-0.4 g L^-1, the bioavailability of pyrene was underestimated by the traditional water quality assessment because of the SPS-associated pyrene, and it was underestimated by 23.6-63.9% when SPS concentration was higher than 0.6 g L^-1 due to the neglection of SPS-associated pyrene. Furthermore, the threshold value of SPS concentration was related to the SPS size and composition, and the effects of SPS and DOM on water quality were influenced by the concentration, size, and composition of SPS as well as the molecular weight of DOM. This study suggests that the traditional water quality assessment should be improved by comprehensively considering concentrations and characteristics of SPS and DOM.

Effects of natural organic matter on the photolysis of tetracycline in aquatic environment: Kinetics and mechanism

The residues of tetracycline in environment have raised increasing concern for the deleterious impact on ecological and human health. Natural organic matter (NOM), ubiquitous in natural waters, is unavoidable to encounter tetracycline, which might affect the fate of tetracycline in aquatic environment. In this study, we investigated the effect of natural organic matter (NOM) on the photolytic fate of tetracycline (TC). The photolysis kinetics of TC were evaluated with two representative NOM, tannic acid (TA) and gallic acid (GA). The presence of TA and GA obviously inhibited the removal of TC under UV irradiation with photolysis rate constant at 0.067 h^-1 and 0.071 h^-1, respectively, which were 32.3% and 28.3% less than that without TA and GA (0.099 h^-1). Furthermore, NOM exhibited different impacts on both indirect photolysis and direct photolysis. NOM promoted the formation of hydroxyl radical, induced the generation of triplet-excited state NOM and thus greatly enhanced the indirect photolysis of TC. However, direct photolysis was almost completely inhibited by NOM via inner filter effect and interacting with TC to form ground-state complex with low photoreactive. Moreover, similar intermediates were detected in the presence and absence of NOM, indicating that NOM exhibited limited influence on the degradation pathways of TC. This study reveals the multiple roles of NOM on tetracycline photolysis, contributing to better understand the photolytic fate of antibiotics in natural waters.

Effects of Dissolved Organic Carbon, Ultraviolet Light and their Co-Exposure on Deepwater Horizon crude oil acute toxicity to larval red drum (Sciaenops ocellatus)

In the aquatic environment, ubiquitous natural factors such as ultraviolet light (UV) and dissolved organic carbon (DOC) are likely to influence crude oil toxicity. The present study examined the interactive effects of DOC, UV, and DOC-UV co-exposure on the acute toxicity of Deepwater Horizon crude oil in larval red drum (Sciaenops ocellatus). Although DOC alone did not influence crude oil toxicity, it mildly reduced UV photo-enhanced toxicity. Environ Toxicol Chem 2020;39:2509-2515. © 2020 SETAC.

Elevated temperature enhances the bioavailability of pyrene to Daphnia magna in the presence of dissolved organic matter: Implications for the effect of climate warming

Dissolved organic matter (DOM) is an essential factor in natural waters to affect the bioavailability of hydrophobic organic compounds (HOCs). Climate warming may influence the partition of HOCs between DOM and water as well as the physiology of organisms. Thus, we hypothesized that elevated temperature might affect the bioavailability of HOCs in the presence of DOM. To test this hypothesis, the effect of temperature on the bioavailability of pyrene to Daphnia magna (D. magna) in water-DOM (fulvic acid) system was investigated. The results showed that, although the concentration of freely dissolved pyrene increased slightly with temperature in the presence of DOM when the level of total dissolved pyrene was kept constant, D. magna immobilization (increased by 50.0-167%) and internal body burden of pyrene (increased by 18.4-41.5%) increased significantly with every 4 °C increase in temperature (16, 20, 24 °C). The main reasonable explanation for this result is that elevated temperature promoted pyrene uptake by D. magna. It was found that the increase percentage of 1-hydroxypyrene (main metabolite of pyrene) concentrations with temperature was higher than that of pyrene concentrations in the body except gut of D. magna. This result indicated that increased temperature might enhance the metabolic rates of D. magna, thus leading to increased uptake rate of freely dissolved and DOM-associated pyrene. This study suggests that elevated temperature might enhance the bioavailability of HOCs in natural waters through influencing both the bioavailable fraction of HOCs and their uptake rates in aquatic organisms, and this should be considered for evaluating their eco-environmental risks under the context of climate warming.

Joint effect of nanoplastics and humic acid on the uptake of PAHs for Daphnia magna: A model study

Nanoplastics (NPs) are emerging pollutants which can adsorb large amounts of hydrophobic organic compounds (HOCs) and be ingested by aquatic organisms. NPs interact with dissolved organic matter (DOM) and result in significant impacts on the bioaccumulation of HOCs in the actual environment. For the first time, the joint effects of two complex matrices on the bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) to Daphnia magna (D. magna) were studied by modeling calculation. The complex matrices, nano-sized polystyrene (PS) and/or humic acid (HA), were under environmentally realistic concentrations. A biodynamic model was modified and the uptake fluxes from all exposure pathways were quantified using the experimental data. A flux estimation showed that the bioaccumulation amounts at equilibrium were mostly dependent on dermal uptake (≥99.3 % of the total). The PS matrix would retard the intestinal uptake process in D. magna, especially for the less hydrophobic PAHs; while the HA or the HA-PS matrix would facilitate the mass transfer of PAHs from the matrix to lipids in the gut. Moreover, the biota matrix accumulation factor (BMAF) were calculated to verify the biodynamic model. This work is helpful to clarify the bioaccumulation effects of PAHs in complex environmental systems.

Effect of suspended particles with different grain sizes on the bioaccumulation of PAHs by zebrafish (Danio rerio)

The physicochemical characteristics are different for suspended particles (SPS) with different sizes in rivers. Here, we studied the effect of SPS (1 g L^-1) with three different sizes (63-106 μm, 20-63 μm, and <20 μm) on the bioaccumulation of deuterated polycyclic aromatic hydrocarbons (phenanthrene-d10, anthracene-d10, fluoranthene-d10 and pyrene-d10) in zebrafish using passive dosing devices to maintain the freely dissolved concentrations of PAHs-d10 constant in water. The results showed that all the three grain size SPS could be ingested by zebrafish, and there was no significant difference in the amount of ingested SPS among the three grain sizes. The concentrations (lipid-normalized or not) of PAHs-d10 in zebrafish were promoted in the presence of the three different size SPS, and the PAH-d10 concentrations in zebrafish increased with decreasing particle size. Compared with the systems without SPS, the lipid-normalized concentrations of PAHs-d10 increased by 12%-72%, 34%-130%, and 60%-196%, respectively in zebrafish in systems with 63-106 μm, 20-63 μm, and <20 μm of SPS after exposure for 20 h. The stronger effect of SPS with smaller grain sizes was probably due to their lower organic carbon content, lower ratio of black carbon to organic carbon content, smaller particle size, and higher dissolved organic matter contents, which could promote the desorption of PAHs-d10 from ingested SPS and bioaccessibility of PAHs-d10 to zebrafish. This study suggests that in addition to SPS concentration, the suspended particle size should be considered in ecological risk assessment for hydrophobic organic compounds in aquatic environment.

Sorption kinetics of parent and substituted PAHs for low-density polyethylene (LDPE): Determining their partition coefficients between LDPE and water (KLDPE) for passive sampling

Low-density polyethylene (LDPE) has been widely used as a sorbent for passive sampling of hydrophobic organic contaminants (HOCs) in aquatic environments. However, it has seen only limited application in passive sampling for measurement of freely dissolved concentrations of parent and substituted PAHs (SPAHs), which are known to be toxic, mutagenic and carcinogenic. Here, the 16 priority PAHs and some typical PAHs were selected as target compounds and were simultaneously determined by gas chromatography-mass spectrometer (GC-MS). Some batch experiments were conducted in the laboratory to explore the adsorption kinetics of the target compounds in LDPE membranes. The results showed that both PAHs and SPAHs could reach equilibrium status within 19-38 days in sorption kinetic experiments. The coefficients of partitioning between LDPE film (50 μm thickness) and water (K_LDPE) for the 16 priority PAHs were in good agreement with previously reported values, and the values of K_LDPE for the 9 SPAHs are reported in this study for the first time. Significant linear relationships were observed, i.e., log K_LDPE = 0.705 × log K_OW + 1.534 for PAHs (R² = 0.8361, p < 0.001) and log K_LDPE = 0.458 × log K_OW + 3.092 for SPAHs (R² = 0.5609, p = 0.0077). The selected LDPE film was also proven to meet the condition of "zero sink" for the selected target compounds. These results could provide basic support for the configuration and in situ application of passive samplers.

Metabolic profiling of Daphnia magna exposure to a mixture of hydrophobic organic contaminants in the presence of dissolved organic matter

The hydrophobic organic contaminants triclosan, triphenyl phosphate (TPhP) and diazinon sorb to dissolved organic matter (DOM) and this may alter their bioavailability and toxicity. ¹H nuclear magnetic resonance (NMR)-based metabolomics was used to investigate how DOM at 1 and 5 mg organic carbon/L may alter the metabolome of Daphnia magna from exposure to equitoxic mixtures of triclosan, TPhP and diazinon. These contaminants have different modes of action toward D. magna. The contaminant concentrations in each mixture were an equal percentage of their lethal concentration to 50% of the population (LC₅₀) values, which equates to 1250 μg/L TPhP, 330 μg/L triclosan and 0.9 μg/L diazinon. The ternary mixture exposure at 1% LC₅₀ values did not alter the D. magna metabolome. Contaminant mixture exposures at 5%, 10%, and 15% LC₅₀ values decreased glucose, serine and glycine concentrations and increased asparagine and threonine concentrations, suggesting disruptions in energy metabolism. The contaminant mixture had a unique mode of action in D. magna and DOM at 1 and 5 mg organic carbon/L did not change this mode of action. The estimated sorption of triclosan, TPhP or diazinon to DOM at 1 or 5 mg organic carbon/L in this experimental design was calculated to be <50% for each contaminant. This suggests that the mode of action of the contaminant mixture was not altered by DOM because the two environmentally relevant concentrations of DOM may have not substantially altered contaminant bioavailability. Our results indicate that DOM may not inevitably mitigate or alter the sub-lethal toxicity of a mixture of hydrophobic organic contaminants. This indicates the complexity of predicting the molecular-level toxicity of environmental mixtures. For adequate risk assessment of freshwater ecosystems, it is vital to account for the combined sub-lethal toxicity of an environmental mixture of contaminants.

Inhibitory effects of dissolved organic matter on erythromycin bioavailability and possible mechanisms

Macrolides are widely used antibiotics with ubiquitous occurrence in aquatic environments. Unlike many emerging contaminants, macrolides are positively charged on their amine groups and are likely to interact with negative charge groups of dissolved organic matters (DOMs), which may alter macrolide bioaccumulation but yet to be explored. Here we evaluated the effects of different DOM (LeHA, PPHA, SRHA and SRFA) on erythromycin (an important macrolide) bioaccumulation into aquatic biota with ¹⁴C tracing. Results showed that ERY uptake in organisms was significantly inhibited by DOM (P < 0.05). In the presence of DOM at 20 mg L^-1, the averaged equilibrium concentration (C_e) decreased by 28.1-40.6% for zebrafish and 10.9-25.8% for duckweed, corresponding to reductions in the bioconcentration factor (BCF) by 15.9-32.8% and 10.9-18.5%, respectively. Likely due to their higher carboxyl group content, SRHA and SRFA exhibited stronger inhibitory effects than LeHA or PPHA. The possible interactions between ERY and DOM were explored and results suggested that DOM inhibited ERY bioavailability by forming DOM-ERY complexes via ionic bonding of -COO^- and ERY⁺, hydrogen bonding and hydrophobic partitioning. This study was the first to report on inhibitory effects of DOM on erythromycin bioavailability and has important implications for better understanding risks of macrolides.

Dietary Uptake Patterns Affect Bioaccumulation and Biomagnification of Hydrophobic Organic Compounds in Fish

Biomagnification of hydrophobic organic compounds (HOCs) increases the eco-environmental risks they pose. Here, we gained mechanistic insights into biomagnification of deuterated polycyclic aromatic hydrocarbons (PAHs- d₁₀) in zebrafish with carefully controlled water (ng L^-1) by a passive dosing method and dietary exposures using pre-exposed Daphnia magna and fish food. A new bioaccumulation kinetic model for fish was established to take into account discrete dietary uptake, while the frequently used model regards dietary uptake as a continuous process. We found that when freely dissolved concentrations of the PAHs- d₁₀ were constant in water, the intake amount of the PAHs- d₁₀ played an important role in affecting their steady-state concentrations in zebrafish, and there was a peak concentration in zebrafish after each dietary uptake. Moreover, considering the randomness of predation, the Monte Carlo simulation results showed that the probabilities of biomagnification of the PAHs- d₁₀ in zebrafish increased with their dietary uptake amount and frequency. This study indicates that in addition to the well-known lipid-water partitioning, the bioaccumulation of HOCs in fish is also a discontinuous kinetic process caused by the fluctuation of HOC concentration in the gastrointestinal tract as a result of the discrete food ingestion. The discontinuity and randomness of dietary uptake can partly explain the differences among aquatic ecosystems with respect to biomagnification for species at similar trophic levels and provides new insight for future analysis of experimental and field bioaccumulation data for fish.

Nanoparticle TiO2 size and rutile content impact bioconcentration and biomagnification from algae to daphnia

Little information is available about effect of particle size and crystal structure of nTiO₂ on their trophic transfer. In this study, 5 nm anatase, 10 nm anatase, 100 nm anatase, 20 nm P25 (80% anatase and 20% rutile), and 25 nm rutile nTiO₂ were selected to investigate the effects of size and crystal structure on the toxicity, bioconcentration, and trophic transfer of nTiO₂ to algae and daphnia. In the exposed daphnids, metabolic pathways affected by nTiO₂ and nTiO₂-exposed algae (nTiO₂-algae) were also explored. The 96 h IC₅₀ values of algae and the 48 h LC₅₀ values of daphnia were 10.3, 18.9, 43.9, 33.6, 65.4 mg/L and 10.5, 13.2, 37.0, 28.4, 60.7 mg/L, respectively, after exposed to nTiO₂-5A, nTiO₂-10A, nTiO₂-100A, nTiO₂-P25, and nTiO₂-25R, respectively. The bioconcentration factors (BCFs) for 0.1, 1, and 10 mg/L nTiO₂ in daphnia ranged from 21,220 L/kg to 145,350 L/kg. The nTiO₂ biomagnification factors (BMFs) of daphnia fed with 1 and 10 mg/L nTiO₂-exposed algae were consistently greater than 1.0 (5.7-122). The results show that the acute toxicity, BCF, and BMF all decreased with increasing size or rutile content of nTiO₂. All types of nTiO₂ were largely accumulated in the daphnia gut and were not completely depurated within 24 h. At the molecular level, 22 Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathways of daphnia were impacted by the nTiO₂ and nTiO₂-algae treatments, including glutathione metabolism, aminoacyl-tRNA biosynthesis, among others. Six and four KEGG metabolic pathways were significantly disturbed in daphnids exposed to nTiO₂ and nTiO₂-algae, respectively, indicating the presence of algae partially alleviated the negative impact of nTiO₂ on metabolism. These findings increase understanding of the impacts of physicochemical properties of nTiO₂ on the food chain from molecular scale to that of the whole organism, and provide new insight into the ecological effect of nanomaterials.

Detection and Prioritization of Developmentally Neurotoxic and/or Neurotoxic Compounds Using Zebrafish

The standard methods for toxicity testing using rodent models cannot keep pace with the increasing number of chemicals in our environment due to time and resource limitations. Hence, there is an unmet need for fast, sensitive, and cost-effective alternate models to reliably predict toxicity. As part of Tox21 Phase III's effort, a 90-compound library was created and made available to researchers to screen for neurotoxicants using novel technology and models. The chemical library was evaluated in zebrafish in a dose-range finding test for embryo-toxicity (ie, mortality or morphological alterations induced by each chemical). In addition, embryos exposed to the lowest effect level and nonobservable effect level were used to measure the internal concentration of the chemicals within the embryos by bioanalysis. Finally, considering the lowest effect level as the highest testing concentration, a functional assay was performed based on locomotor activity alteration in response to light-dark changes. The quality control chemicals included in the library, ie, negative controls and replicated chemicals, indicate that the assays performed were reliable. The use of analytical chemistry pointed out the importance of measuring chemical concentration inside embryos, and in particular, in the case of negative chemicals to avoid false negative classification. Overall, the proposed approach presented a good sensitivity and supports the inclusion of zebrafish assays as a reliable, relevant, and efficient screening tool to identify, prioritize, and evaluate chemical toxicity.

Impact of post-hatching maturation on the pharmacokinetics of paracetamol in zebrafish larvae

Zebrafish larvae are increasingly used in pharmacological and toxicological studies, but it is often overlooked that internal exposure to exogenous compounds, rather than the incubation medium concentration, is driving observed effects. Moreover, as the zebrafish larva is a developing organism, continuous physiological changes impact pharmacokinetic or toxicokinetic processes like the absorption and elimination of exogenous compounds, influencing the interpretation of observations and conclusions drawn from experiments at different larval ages. Here, using paracetamol as paradigm compound, mathematical modelling is used to quantify absorption and elimination rates from internal exposure over time profiles after waterborne treatment, as well as changes in these parameters in post-hatching larvae of 3, 4, and 5 days post fertilisation (dpf). An increase of 106% in absorption rate was observed between 3 and 4 dpf, but no further increase at 5 dpf, and an increase of 17.5% in elimination rate for each dpf. Paracetamol clearance, determined from elimination rate constants and reported total larval volumes of 253, 263, and 300 nL at 3, 4, and 5 dpf respectively, correlates best with higher vertebrates at 5 dpf. This suggests that when studying direct effects of exogenous compounds, experiments with zebrafish larvae are best performed at 5 dpf.

Bioavailability of Pyrene Associated with Different Types of Protein Compounds: Direct Evidence for Its Uptake by Daphnia magna

The protein-like dissolved organic matter (DOM) is ubiquitous in aquatic environments. However, the bioavailability of protein-like DOM-associated hydrophobic organic compounds (HOCs) is not well-understood, and in particular, the direct evidence of their uptake by organisms is scarce. In the present work, tryptone (2000 Da), bovine serum albumin (BSA; 66 000 Da), and phycocyanin (120 000 Da) were chosen as model protein-like DOM, which were labeled by commercial fluorescein (cy5) to investigate the uptake mechanisms of protein compound-associated pyrene (a typical HOC) by Daphnia magna. The pyrene concentration in the tissues except the gut and immobilization of D. magna were detected to calculate the bioavailable fraction of protein compound-associated pyrene when the freely dissolved pyrene concentration was controlled through passive dosing devices. The results demonstrated that the tryptone could permeate cellular membrane and directly enter the tissues of  D. magna from the exposure solutions, whereas BSA and phycocyanin might indirectly enter the tissues from the gut. A part of pyrene associated with protein compounds was bioavailable to D. magna; the order of their bioavailable fractions was trypone (54.6-58.1%) > phycocyanin (21.6-32.8%) > BSA (17.7-26.8%). The difference was principally related to the uptake mechanisms of pyrene associated with different types of protein. This work suggests that the protein compound-associated HOCs should be considered to evaluate the bioavailability and eco-environmental hazard of HOCs in natural waters.