Influences of humic acid on the bioavailability of phenanthrene and alkyl phenanthrenes to early life stages of marine medaka (Oryzias melastigma)

Mitigating phthalate toxicity: The protective role of humic acid and clay in zebrafish larvae

This research study aimed to explore the mitigating effects of humic acid and clay on the toxicity induced by three different phthalates (DBP, DEP, DEHP) on zebrafish larvae growth. Prolonged exposure to DBP resulted in a concerning 87.33% mortality rate, significantly reduced to 7.3% when co-administered with humic acid. A similar reduction in mortality was observed for the other two phthalates (DEP and DEHP). Additionally, the introduction of phthalates with humic acid, clay, or their combination led to a significant decrease in the malformation rate in larvae. High-Performance Liquid Chromatography (HPLC) analysis of phthalates in treatments revealed a noteworthy decline in their concentration when combined with humic acid and clay. This suggests a reduced bioavailability of phthalates to larvae, aligning with diminished toxicity, lower mortality, fewer malformations, and improved organ development, as well as less oxidative stress. Furthermore, measurements of larval length and morphological scoring affirmed the protective role of humic acid and clay in promoting the normal growth of zebrafish. This study underscores the potential of environment modulators, such as humic acid and clay, as effective bioremediation agents against phthalate toxicity. The generation of reactive oxygen species (ROS), indicative of oxidative stress, was markedly higher in larvae treated solely with phthalates compared to the control. Conversely, larvae treated with a combination of phthalates and humic acid or clay exhibited a significant decrease in ROS generation, signaling a decline in oxidative stress. Histopathological analysis of adult fish subjected to various treatments revealed significant damage to vital organs like the liver and intestine when treated with phthalates alone. However, when phthalates were introduced with humic acid, clay, or both, the morphology closely resembled that of the control, reinforcing the protective role of humic acid and clay in zebrafish development against administered phthalates.

Assessing the bioavailability of black carbon-derived dissolved organic matter for marine heterotrophic prokaryotes

Here we investigated the bioavailability of black carbon (BC)-derived dissolved organic matter (DOM) for a natural mixed community of marine heterotrophic prokaryotes. We ran an in vitro biodegradation experiment that took place over 3 months and exposed a community of organisms collected in the northwestern Mediterranean Sea (Bay of Marseille, France) to three different soluble fractions of BC prepared in the laboratory from various fossil fuel combustion particulates: standard diesel (DREF), oxidized diesel (DREF-OX), and natural samples of ship soot (DSHIP). Over the course of the three months, we observed significant decreases in the concentrations of dissolved organic carbon (DOC; from 9 to 21 %), dissolved BC (DBC; from 22 to 38 %) and dissolved polycyclic aromatic hydrocarbons (d-PAH; from 24 to 64 %) along with variability in the growth dynamics and activity of the heterotrophic prokaryotic community. The heterotrophic prokaryotic community exposed to DREF-OX treatment showed the highest values of respiration and production and the highest cell abundance, associated with the highest decrease in DOC (21 %) and d-PAH (64 %) concentrations. In the DREF and DSHIP treatments, prokaryotic activity was oriented towards anabolism. DREF treatment led to the highest decrease in DBC concentration (38 %). DSHIP treatment, which presented a substantially different d-PAH and dissolved metals content to the other two treatments, showed the lowest decreases in DOC, DBC and d-PAH concentrations, as well as the lowest prokaryotic activity and biomasses. Our results indicate that BC-derived DOM, including the most condensed fraction of this material, is partly bioavailable and therefore likely to be assimilated by marine prokaryotes. The origin of BC/soot deposited at the ocean surface turns out to be a key parameter that dictates the efficiency of biodegradation of its dissolved fraction by heterotrophic prokaryotes.

Transformation of potentially persistent and mobile organic micropollutants in column experiments

The occurrence of potentially persistent and mobile (PM) organic micropollutants (OMP) in the aquatic environment is recognized as a severe threat to water resources and drinking water suppliers. The current study investigated long-term fate (persistency and bio-transformation) of several emerging contaminants in a simulated bank filtration (BF) for the first time. In parallel, four sand column systems were operated with groundwater and continuously spiked with an average concentration of 1 μg/L for 24 OMP. Each column system consisted of two sand columns connected in series. Presumably, biological activities in the first column were higher than in the second column, as dissolved oxygen utilization, dissolved organic matter (DOM) and UV absorbance at 254 nm (UV₂₅₄) reduction rates were high in the first column. This study revealed that 9 out of 24 OMP were persistent and mobile throughout the study under oxic conditions and within a hydraulic retention time (HRT) of 12 days. However, 2 (out of 9) OMP were persistent but showed sorption behavior. 15 (out of 24) OMP displayed bio-transformation, 4 were eliminated entirely within 4.5 days of HRT. Others showed constant or improved degradation with the adaptation (or operation) time. Improved degradation with adaption was high in the bioactive sand columns. However, 8 OMP showed improved elimination at high HRT, even in low biologically active columns. In addition, no significant effect of the DOM on the eliminations of OMP was found except for 4-hydroxy-1-(2-hydroxyethyl)-2,2,6,6,-tetramethylpiperidine (HHTMP), 2-methyl-2-propene-1-sulfonic acid (MPSA) and sulfamethoxazole (SMX). The eliminations of HHTMP (Pearson's r > 0.80, p < 0.05), MPSA (Pearson's r > 0.70) and SMX (Pearson's r > 0.80) correlated with the removals of humic substances in the sand columns. Overall, adaptation time and HRT play a crucial role in the elimination of emerging OMP through BF, yet at the same time several OMP exhibit persistent behavior.

Comparing the sorption of pyrene and its derivatives onto polystyrene microplastics: Insights from experimental and computational studies

In this study, the sorption behaviors and mechanisms between polystyrene microplastics (micro-PS) and 4-rings polycyclic aromatic hydrocarbons (PAHs) pyrene (Pyr) and its derivatives (S-Pyr), including 1-methylpyrene (P-CH₃), 1-hydroxypyrene (P-OH), 1-aminopyrene (P-NH₂), 1-pyrenecarboxylic acid (P-COOH) were investigated at neutrality. The results revealed that the sorption rates of micro-PS for S-Pyr were higher than those for parent Pyr. Meanwhile, -CH₃ could slightly facilitate the sorption, whereas -OH, P-NH₂, and P-COOH intensively inhibit the sorption of S-Pyr onto micro-PS. The sorption capacities of Pyr/S-Pyr increased with decreasing size of micro-PS. Besides, the effects of salinity and temperature on the sorption characteristics of micro-PS for Pyr/S-Pyr depended on their substituents. Combined with experimental and computational methods, it could be concluded that the main sorption mechanisms were possibly hydrophobic interaction, π-π interaction and pore-filling. The observations reported here could improve predictions of environmental behaviors and bioavailability of PAHs and micro-PS.

Investigation on the interaction differences of algogenic dissolved organic matter released by Prorocentrum donghaiense with phenanthrene and 3-methylphenanthrene using spectroscopy

Despite the high algogenic dissolved organic matter (A-DOM) content in the marine environment, the impact of A-DOM on the environmental behavior of alkylated polycyclic aromatic hydrocarbons (A-PAHs) has rarely been reported. Thus, this study sought to investigate the changes in A-DOM and its interaction with phenanthrene (Phe) and 3-methylphenanthrene (3-MPhe). The results showed that the spectroscopic characteristics of A-DOM changed dynamically during the entire growth period of Prorocentrum donghaiense. The total proportions of humic-like components in A-DOM, including marine humic-like and longer wavelength humic-like components, increased from 26.30% at the initial phase to 40.42% at the decline phase, but those of protein-like components decreased. Humic-like components showed stronger interaction capabilities with Phe/3-MPhe than that of protein-like components. The interaction characteristics and increased the humic-like components content indicated their prominent role in the interaction of A-DOM and Phe/3-MPhe. 3-MPhe had stronger hydrophobic interactions with the humic-like components and the methyl group increased π-π interactions with A-DOM on day 28. Methyl group also increased the molecular weight and steric hindrance of 3-MPhe, resulting in a lower interaction capability with A-DOM than that of Phe on day 16. The diverse interaction behaviors of A-DOM fluorescent components with Phe and 3-MPhe were largely attributed to dynamic changes in the A-DOM characteristics. Methyl substitution varied the hydrophobicity and molecular weight of Phe and 3-MPhe, and further influenced the interactions with A-DOM fluorescent components. These results highlight the vital impact of dynamic A-DOM on the environmental behavior of PAHs, and the differences in the environmental behavior between parent and alkylated PAHs should also receive additional attention.

Effect of straw-derived dissolved organic matter on the adsorption of sulfamethoxazole to purple paddy soils

The presence of sulfamethoxazole (SMX) in croplands has become an international concern. The environmental behavior and fate of SMX in agricultural soils are not well understood, especially when the adsorption behavior is disturbed by the dissolved organic matter (DOM) released by crop straw. As canola straw is one of the biomasses widely returned to farmlands, we characterized DOM derived from pristine and decomposed canola straw, and explored the effects and mechanisms of the DOMs on regulating SMX adsorption to purple paddy soils. The spectral analysis showed that the molecular weight, aromaticity, and hydrophobicity of canola straw-derived DOM increased as decomposition proceeded. These physicochemical properties collectively determined the effects of the DOM on SMX adsorption. The DOM derived from pristine canola straw increased SMX maximum adsorption capacity of the soils by approximately 2.6 times, but this positive effect gradually decreased to a steady state by day 90 in the straw decomposition period. Nevertheless, the SMX adsorption behavior in the soils was invariably determined by the DOM extracts. These adsorption processes of SMX were well fitted by the double-chamber kinetics model and the Langmuir and Freundlich thermodynamic models. Thermodynamic parameters indicated that SMX adsorption onto the soils was spontaneous and endothermic, and this adsorption characteristics was not significantly (p > 0.05) changed by the DOM extracts. However, the adsorption kinetics were altered by those DOMs, i.e., the fast and slow adsorption processes were both diminished. Correspondingly, co-adsorption and cumulative adsorption were identified as the main mechanisms determining SMX adsorption to the purple paddy soils in the presence of the straw-derived DOMs. These results collectively indicated that the DOMs released by straw in croplands may decrease the ecological risks of organic pollutants by inhibiting their migration processes.

Elucidating mechanisms of immunotoxicity by benzotriazole ultraviolet stabilizers in zebrafish (Danio rerio): Implication of the AHR-IL17/IL22 immune pathway

Benzotriazole ultraviolet stabilizers (BUVSs) are widely used additives in industrial materials and personal care products that protect products from ultraviolet damage. Due to their high production volume and potential to bioaccumulate, BUVSs are an environmental pollutant of concern. In this study, juvenile zebrafish (Danio rerio) were exposed to 4 BUVSs (UV-234, UV-326, UV-329, and UV-P) at 10 and 100 μg/L for 28 d. BUVSs induced hepatic vacuolization and nuclei pyknosis in the liver following 100 μg/L UV-234 and UV-329 exposure. Transcriptomic analysis in the liver uncovered pathways related to inflammation that were affected by BUVSs. Based upon these data, we measured the expression levels of 9 genes involved in AHR-IL17/IL22 pathway in zebrafish larvae exposed to each BUVSs at one dose of either 10 or 100 μg/L for 6 days in a second set experiment. Transcript levels of interleukins il17a and il22 were decreased, while il6 mRNA was increased with exposure to UV-234, UV-329, and UV-P. No change to targeted transcripts was observed with UV-326 treatments. Moreover, cyp1a1 and ahr2 levels were increased in larvae treated with 100 μg/L UV-329 or UV-P. Consistent with expression data, protein abundance of IL22 was decreased by 29% with exposure to 100 μg/L UV-P. Taken together, these results demonstrate that exposure to different benzotriazole congeners may be associated with immunotoxicity in zebrafish through the AHR-IL17/IL22 pathway, and this may be associated with hepatic damage with prolonged exposures. This study provides new insight into unique pathways perturbed by specific BUVSs congeners.

Neurodevelopmental toxicity assessments of alkyl phenanthrene and Dechlorane Plus co-exposure in zebrafish

Alkyl phenanthrene (A-Phen) and Dechlorane Plus (DP) are ubiquitous environmental pollutants that widely co-exist in the environment. It has been established that both A-Phen and DP elicit neurotoxicity, but the potential interactive toxicity of these contaminants is not well-known. To determine whether a mixture of A-Phen and DP would exhibit interactive effects on neurodevelopment, we co-exposed 3-methylphenanthrene (3-MP), a representative of A-Phen, with DP. Our results illustrated that exposure to 5 or 20 μg/L 3-MP alone or in combination with 60 μg/L DP caused neurobehavioral anomalies in zebrafish. In accordance with the behavioral deficits, 3-MP alone or co-exposed with DP significantly decreased axonal growth of secondary motoneurons, altered intracellular Ca²⁺ homeostasis and induced cell apoptosis in the muscle of zebrafish. Additionally, 3-MP alone or co-exposed with DP significantly increased reactive oxygen species (ROS) and the mRNA levels of apoptosis-related genes. These findings indicate that 3-MP alone or co-exposed with DP induces neurobehavioral deficits through the combined effects on neuronal connectivity and muscle function. Chemical analysis revealed significant increases in 3-MP and DP bioaccumulation in zebrafish co-exposed with 3-MP and DP. Elevated bioaccumulation resulting from mixture exposure may represent a significant contribution of the synergistic effects observed in combined chemical exposure.

Characterization of fluorescent dissolved organic matter from green macroalgae (Ulva prolifera)-derived biochar by excitation-emission matrix combined with parallel factor and self-organizing maps analyses

This study investigated the effects of various pyrolysis temperatures and extraction salinities on the fluorescence features of DOM from Ulva prolifera-derived biochar under aseptic conditions using fluorescence excitation-emission matrix (EEM) spectroscopy with parallel factor (PARAFAC) analysis and self-organizing maps (SOM). Four humic-like substances and one protein-like substance were identified by the PARAFAC model. The contents and compositions of PARAFAC components depended more on the pyrolysis temperature than on the extraction salinity. A high pyrolysis temperature could enhance the release of humic-like DOM from biochar. Coupling PARAFAC and SOM facilitates the visualization and interpretation of the relationship between the pyrolysis temperature and the fluorescence properties of DOM. These results are valuable for understanding the effects and processes of macroalgal biochar in the possible environmental and industrial applications.

Adsorption behaviors of phenanthrene and bisphenol A in purple paddy soils amended with straw-derived DOM in the West Sichuan Plain of China

The objectives of this study were to investigate the adsorption and transfer behaviors of phenanthrene (PHE) and bisphenol A (BPA) in purple paddy soils amended with dissolved organic matter (DOM) derived from rice and canola straw in the West Sichuan Plain of China. In the pristine soil, PHE was preferentially adsorbed on both pristine clayey (L) and sandy (T) paddy soils than BPA, indicating that the retention/adsorption by soils is closely dependent on the chemical properties of organic pollutants (OPs). The noticeably higher adsorption of PHE and BPA on smaller size fraction of the soils (L2 and T2) were observed, possibly due to their higher surface areas and higher content in organic matters with higher aromaticity and hydrophobicity in this soil fraction. The DOMs derived from rice (RDOM) and canola (CDOM) straws possessed remarkable differences in E₂/E₃ and SUV₂₅₄ measurements, which reflected that their chemical composition might be different. When CDOM was introduced in the studied soil T1, adsorption of BPA was doubled, but the augment in adsorption was much less impressive with RDOM, showing the nature of derived DOM played an important role. The study also demonstrated that in the fine fraction of clayey soil (L2), the retention of a same OP (PHE) was remarkably dropped when CDOM or RDOM was introduced, whereas in a sandy soil of the same size fraction (T2), the phenomenon was the opposite, suggesting a potential risk that, in certain types of soil, the introduction of straw derived DOMs may enhance the mobility of some OPs. The humification time of straw seems not to affect the adsorptions of OPs in most studied systems. Adsorption kinetics of PHE and BPA in the adsorption systems with derived DOMs were well fitted to the two-step first-order model with r_adj² values of 0.994-0.998. Results of this study will provide further comprehensive fundamental data for risk assessment and control of organic pollutants (OPs) in farmland ecosystems.

Aquatic organic matter: Classification and interaction with organic microcontaminants

Organic matter (OM) in aquatic system is originated from autochthonous and allochthonous natural sources as well as anthropogenic inputs, and can be found in dissolved, particulate or colloidal form. According to the type/composition, OM can be divided in non-humic substances (NHS) or humic substances (HS). The present review focuses on the main groups that constitute the NHS (carbohydrates, proteins, lipids, and lignin) and their role as chemical biomarkers, as well as the main characteristics of HS are presented. HS functions, properties and mechanisms are discussed, in addition to their association to the fate, bioavailability, and toxicity of organic microcontaminants in the aquatic systems. Despite the growing diversity and potential impacts of organic microcontaminants to the aquatic environment, limited information is available about their association with OM. A protective effect is, however, normally seen since the presence of OM (HS mainly) may reduce bioavailability and, consequently, the concentration of organic microcontaminants within the organism. It may also affect the toxicity by either absorbing ultraviolet radiation incidence and, then, reducing the formation of phototoxic compounds, or by increasing the oxygen reactive species and, thus, affecting the decomposition of natural and anthropogenic organic compounds. In addition, the outcome data is hard to compare since each study follows unique experimental protocols. The often use of commercial humic acid (Aldrich) as a generic source of OM in studies can also hinder comparisons since differences in composition makes this type of OM not representative of any aquatic environment. Thus, the current challenge is find out how this clear fragmentation can be overcome.

Immune response induced by major environmental pollutants through altering neutrophils in zebrafish larvae

Environmental pollutants may cause adverse effects on the immune system of aquatic organisms. However, the cellular effects of pollutants on fish immune system are largely unknown. Here, we exploited the transgenic zebrafish Tg(lysC:DsRed2) larva as a preliminary screening system to evaluate the potential inflammatory effects of environmental pollutants. Tg(lysC:DsRED2) larvae aged 7-day-postfertilization (7 dpf) were treated with selected environmental chemicals for 24 h (24 h) and the number of neutrophils were quantified using both image analysis and fluorescence activated cell sorting (FACS). We found that the numbers of neutrophils in the Tg(lysC:DsRED2) larvae were significantly increased by most of the organic chemicals tested, including E2 (17β-estradiol), BPA (Bisphenol-A), NDEA (N-nitrosodiethylamine), 4-NP (4-Nitrophenol) and Lindane (γ-hexachlorocyclohexane). Neutrophil numbers were also increased by all the metals tested (Na2HAsO4· 7H2O, Pb(NO3)2, HgCl₂, CdCl2, CuSO₄·5H₂O, ZnSO4, and K2Cr2O7). The only exception was TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), which significantly reduced the number of neutrophils after exposure. Additionally, the transcription of genes (lyz, mpo, tnfα and il8) related to fish immune system were significantly modulated upon exposure to some of the selected chemicals such as E2, TCDD, Cu and Cd. This study revealed that representatives of major categories of environmental pollutants could cause an acute inflammatory response in zebrafish larvae as shown by alterations in the neutrophils, which may imply a common immunotoxicity mechanism for most environmental pollutants. This study has also demonstrated that Tg(lyz:DsRed2) transgenic zebrafish is an excellent tool for screening environmental chemicals with potential inflammatory effects through FACS-facilitated neutrophil counting.

Investigation of Daphnia magna Sub-Lethal Exposure to Organophosphate Esters in the Presence of Dissolved Organic Matter Using ¹H NMR-Based Metabolomics

Organophosphate esters (OPEs) are frequently detected in aquatic environments. Hydrophobic OPEs with high octanol-water partition coefficients (Log K_OW) will likely sorb to dissolved organic matter (DOM) and consequently alter OPE bioavailability and sub-lethal toxicity. ¹H nuclear magnetic resonance (NMR)-based metabolomics was used to evaluate how DOM (5 mg organic carbon/L) alters the metabolic response of Daphnia magna exposed to sub-lethal concentrations of three individual OPEs with varying hydrophobicity. D. magna exposed to the hydrophilic contaminant (Log K_OW = 1.43) tris(2-chloroethyl) phosphate (TCEP) did not have substantial metabolic changes and DOM did not alter the metabolic response. There were significant increases in amino acids and a decrease in glucose from exposure to the hydrophobic contaminant (Log K_OW = 3.65) tris(2-butoxyethyl) phosphate (TBOEP) which DOM did not mitigate, likely due to the high sub-lethal toxicity of TBOEP. Exposure to DOM and the hydrophobic contaminant (Log K_OW = 4.76) triphenyl phosphate (TPhP) resulted in a unique metabolic response which was unlike TPhP only exposure, perhaps because DOM may be an additional stressor with TPhP exposure. Therefore, Log K_OW values may not always predict how sub-lethal contaminant toxicity will change with DOM and there should be more consideration to incorporate DOM in sub-lethal ecotoxicology testing.

The effects of dissolved organic matter and feeding on bioconcentration and oxidative stress of ethylhexyl dimethyl p-aminobenzoate (OD-PABA) to crucian carp (Carassius auratus)

Bioconcentration of UV filters in organisms is an important indicator for the assessment of environmental hazards. However, bioconcentration testing rarely accounts for the influence of natural aquatic environmental factors. In order to better assess the ecological risk of organic UV filters (OUV-Fs) in an actual water environment, this study determined the influences of dissolved organic matter (DOM) (0, 1, 10, and 20 mg/L) and feeding (0, 0.5, 1, and 2% body weight/d) on bioconcentration of ethylhexyl dimethyl p-aminobenzoate (OD-PABA) in various tissues of crucian carp (Carassius auratus). Moreover, oxidative stress in the fish liver caused by the OD-PABA was also investigated by measuring activities of superoxide dismutase (SOD), catalase (CAT) and glutathione S-transferase (GST), and levels of glutathione (GSH) and malondialdehyde (MDA). The bioconcentration of OD-PABA in the fish tissues was significantly decreased with the presence of DOM indicating a reduction of OD-PABA bioavailability caused by DOM. The bioconcentration factors (BCFs) decreased by 28.00~50.93% in the muscle, 72.67~96.74% in the gill, 37.84~87.72% in the liver, and 10.32~79.38% in the kidney at different DOM concentrations compared to those of the non-DOM treatments. Significant changes in SOD, CAT, GST, GSH, and MDA levels were found in the DOM- and OD-PABA-alone treatments. However, there were no significant differences in the SOD, CAT, GST, and MDA levels found when co-exposure to OD-PABA and DOM. Feeding led to lower OD-PABA concentrations in the fish tissues, and the concentrations were decreased with increasing feeding ratios. BCFs in various tissues reduced by 39.75~72.52% in the muscle, 56.86~79.73% in the gill, 66.41~87.50% in the liver, and 75.88~89.10% in the kidney, respectively. In the unfed treatments, the levels of SOD and MDA were significantly higher than those of the fed ones while GST and GSH levels were remarkably inhibited indicating the enhanced effect of starvation to oxidative stress. There was no markedly alternation of the biomarker levels observed between different fed treatments. In conclusion, our study indicated that both DOM and feeding reduced bioconcentration of OD-PABA and alleviated oxidative stress to some extent in the crucian carp.

Quantifying Bioavailability of Pyrene Associated with Dissolved Organic Matter of Various Molecular Weights to Daphnia magna

Dissolved organic matter (DOM) is a key environmental factor for the bioavailability of hydrophobic organic compounds (HOCs) in natural waters. However, the bioavailability of DOM-associated HOCs is not clear. In this research, pyrene was selected as a model HOC, and its freely dissolved concentration (C_free) was maintained by passive dosing systems. The immobilization and pyrene content in the tissues excluding gut of Daphnia magna were examined to quantify the bioavailability of DOM-associated pyrene. The results indicated that DOM promoted the bioavailability of pyrene when the C_free of pyrene was kept constant, and the bioavailability of pyrene associated with DOM of various molecular weights was ordered as middle molecular weight (5 000-10 000 Da) DOM > lower molecular weight (<1 000, 1 000-3 000, and 3 000-5 000 Da) DOM > higher molecular weight (>10 000 Da) DOM. The influencing mechanisms of DOM molecular weight were related with the partition of pyrene between DOM and water, the uptake routes of DOM by D. magna, and the desorption or release of pyrene from DOM in the gut of D. magna. The findings obtained in this research suggest that the bioavailability of DOM-associated HOCs should be taken into account for the eco-environmental risk assessment of HOCs in water systems.

Microbial transformation of intracellular dissolved organic matter from Microcystis aeruginosa and its effect on the binding of pyrene under oxic and anoxic conditions

The environmental behaviour and the bioavailability of polycyclic aromatic hydrocarbons (PAHs) are strongly affected by dissolved organic matter (DOM) in aquatic environments. In this study, we investigated the dynamics of the bioavailability and character of the intracellular DOM (IDOM) from Microcystis aeruginosa (M. aeruginosa-IDOM) during 10 days oxic and anoxic incubations by spectroscopy. Subsequently, the binding affinity of pyrene with the initial/altered M. aeruginosa-IDOM was estimated by fluorescence quenching method. The incubation results indicated that changes in dissolved organic carbon (DOC) concentration and selected spectral descriptors of the M. aeruginosa-IDOM under oxic condition were different from those of anoxic condition. The microbial transformation of the M. aeruginosa-IDOM resulted in an enhancement of the organic carbon-normalized binding coefficient (K _DOC) of pyrene in both oxic and anoxic treatments. Moreover, only for the oxic condition, Pearson correlation analysis demonstrated that aromaticity (specific UV absorbance at 254 nm, SUVA₂₅₄), humification degree (humification index, HIX) and the percent distribution of humic-like component 2 (%C2) presented significantly positive correlations with the pyrene K _DOC, while the percent distribution of protein-like component 1 (%C1) exhibited a negative correlation with the K _DOC. However, no significant correlation was observed between any spectral descriptor and the K _DOC under anoxic condition. This result suggested that the binding affinity of pyrene may be primarily influenced by the altered M. aeruginosa-IDOM characteristics associated with the biological transformation. Hence, our results provided potential evidence for resolving the inconsistency in the relationships between DOM characteristics and the binding affinities of PAHs.