Determination and occurrence of retinoic acids and their 4-oxo metabolites in Liaodong Bay, China, and its adjacent rivers

Toxic effects of exogenous retinoic acid on the neurodevelopment of zebrafish (Danio rerio) embryos

Endogenous retinoic acid (RA) is essential for embryonic development and maintaining adult physiological processes. Human-caused RA residues in the environment threaten the survival of organisms in the environment. We employed zebrafish as a model to explore the developmental impacts of excess RA. We used exogenous RA to raise the amount of RA signal in the embryos and looked at the effects of excess RA on embryonic morphological development. Upregulation of the RA signal significantly reduced embryo hatching and increased embryo malformation. To further understand the neurotoxic impact of RA signaling on early neurodevelopment, we measured the expression of neurodevelopmental marker genes and cell death and proliferation markers in zebrafish embryos. Exogenous RA disrupted stem cell (SC) and neuron marker gene expression and exacerbated apoptosis in the embryos. Furthermore, we looked into the links between the transcriptional coactivator RBM14 and RA signaling to better understand the mechanism of RA neurotoxicity. There was a negative interaction between RA signaling and the transcription coactivator RBM14, and the morpholino-induced RBM14 down-regulation can partially block the effects of RAR antagonist BMS493-induced RA signaling inhibition on embryonic malformation and cell apoptosis. In conclusion, exogenous RA causes neurodevelopmental toxicity, and RBM14 may be involved in this neurotoxic process.

Application of LTA zeolite-modified electrode for sensitive detection of retinoic acid in tap water

An electrochemical method based on a Linde Type-A zeolite-modified glass carbon electrode (LTA/GCE) was introduced for the determination of retinoic acid (RA). LTA zeolite could be synthesized through a hydrothermal method and served as a commercial electrochemical sensor with high stability and sensitivity in electrochemical progress. The as-synthesized product was characterized by scanning electron microscopy (SEM), differential thermal analysis (DTA), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Under optimal conditions, a detection limit of 0.8 μM was obtained for RA with a linear range of 0.8-20.1 μM. This electrochemical method for determining RA was simpler and cheaper than previously reported methods. Furthermore, the modified electrode could be applied to the detection of RA in tap water, achieving a linear range of 1.4-15.0 μM with a detection limit of 1.4 μM and good recovery. The modified electrode designed by this method provided good selectivity, stability, and reproducibility for RA determination and reliable application for the analysis of RA in environmental water.

Spatiotemporal variations of retinoic acids and their metabolites in the marine environment of Hong Kong

Excessive intake of retinoic acids (RAs) and the oxidative metabolites, 4-oxo-RAs, can lead to abnormal morphological development in animals. This study investigated spatiotemporal variations of concentrations and compositions of these compounds in Hong Kong's seawater and during algal blooms. Total concentrations of the studied compounds in seawater were up to 0.790 and 0.427 ng/L in dry and wet seasons, respectively, though no significant seasonal variation was observed. Spatially, the Deep Bay Water Control Zone was the most enriched area with the studied compounds owing to its semi-enclosed nature and influence from the Pearl River discharge. During algal blooms, the studied compounds were detected up to 4.74 ng/L. Based on calculated risk quotients, the ecological risk of the studied compounds to Hong Kong's marine ecosystems was low. Nevertheless, the occurrence and distribution of these chemicals in the marine environment should be closely monitored where algal blooms frequently occur.

Concentration-response of six marine species to all-trans-retinoic acid and its ecological risk to the marine environment

Being a class of vitamin A's main derivatives, retinoic acids (RAs) are important to animals' growth and development. Previous studies demonstrated that exposure of excessive amounts of RAs would lead to malformation and abnormal development in aquatic animals such as amphibians and fishes. Currently, there are only limited toxicity data of RAs available for freshwater species, while those for marine species are seriously lacking. This study aimed to fill such data gap by conducting toxicity tests on six marine species (i.e., one microalga, four invertebrates and one fish) towards the exposure to all-trans-RA (at-RA), which is the most widely distributed RA in the environment. Results showed that the embryo of medaka fish Oryzias melastigma was the most sensitive towards the exposure of at-RA while the gastropod Monodonta labio was the least sensitive. A species sensitivity distribution (SSD) was constructed based on the experimental results generated from the present study. An interim marine-specific predicted no-effect concentration (PNEC) of at-RA was derived at 2300 ng/L. By computing the hazard quotients using the interim marine-specific PNEC and available measured and predicted concentrations of RAs, we found the current levels of RAs posed no immediate risks to the marine environment of Hong Kong. The interim marine-specific PNEC was more than 500-fold of freshwater-specific PNEC (i.e., 3.93 ng/L), indicating that marine species were generally less sensitive than their freshwater counterparts towards RAs. This was the first study to document the concentration-response of various marine species towards at-RA exposure and construct the marine-specific SSD for assessing the ecological risk of at-RA towards the marine environment. Since various forms of RAs and their metabolites often coexist in aquatic environments, further studies should investigate their combined toxicity to an array of marine species of different trophic levels with consideration of chronic exposure scenarios.

Occurrence of retinoic acids and their metabolites in sewage and their removal efficiencies by chemically enhanced primary treatment and secondary biological treatment

Sewage treatment plants (STPs) are important in densely populated megacities like Hong Kong to control the release of harmful pollutants from households and industries into the receiving water bodies and maintain water quality for supporting various beneficial uses. This study investigated the occurrence of the teratogenic retinoic acids (RAs) and their oxidative metabolites in sewage and sludge of six selected STPs that treat about 87% of all sewage in Hong Kong annually, and compared the removal efficiencies of these compounds from sewage between two major sewage treatment processes, i.e., chemically enhanced primary treatment (CEPT) and secondary (biological) treatment. The total concentrations of the studied RAs in influent, effluent and sludge from the six selected STPs were found between 21.5 and 33.1 ng/L, 12.0-20.4 ng/L, and 4.33-7.02 ng/g dry weight, respectively. The compounds were dominated by all-trans-RA and 13-cis-RA, together accounting for 46.9-65.6%, 38.4-56.7%, and 62.8-82.8% of the total RAs in influent, effluent and sludge, respectively. The studied RAs could not be satisfactorily removed by both treatment processes with removal efficiencies ranging from 25.4% to 47.4% only, without significant difference in their removal between CEPT and secondary treatment. Based on the calculated hazard quotients of all-trans-RA equivalents (0.248-0.521), the treated effluents from all the six STPs exhibited medium ecological risks to the receiving coastal environment. Therefore, continuous monitoring of these compounds and enhancement of treatment technologies of STPs shall be considered in the future to improve the removal efficiencies of these compounds.

Neurobehavioral effects of cyanobacterial biomass field extracts on zebrafish embryos and potential role of retinoids

Cyanobacteria are known for their ability to produce and release mixtures of up to thousands of compounds into the environment. Recently, the production of novel metabolites, retinoids, was reported for some cyanobacterial species along with teratogenic effects of samples containing these compounds. Retinoids are natural endogenous substances derived from vitamin A that play a crucial role in early vertebrate development. Disruption of retinoid signalling- especially during the early development of the nervous system- might lead to major malfunctions and malformations. In this study, the toxicity of cyanobacterial biomass samples from the field containing retinoids was characterized by in vivo and in vitro bioassays with a focus on the potential hazards towards nervous system development and function. Additionally, in order to identify the compounds responsible for the observed in vitro and in vivo effects the complex cyanobacterial extracts were fractionated (C18 column, water-methanol gradient) and the twelve obtained fractions were tested in bioassays. In all bioassays, all-trans retinoic acid (ATRA) was tested along with the environmental samples as a positive control. Retinoid-like activity (mediated via the retinoic acid receptor, RAR) was measured in the transgenic cell line p19/A15. The in vitro assay showed retinoid-like activity by specific interaction with RAR for the biomass samples. Neurotoxic effects of selected samples were studied on zebrafish (Danio rerio) embryos using the light/dark transition test (Viewpoint, ZebraLab system) with 120 hpf larvae. In the behavioural assay, the cyanobacterial extracts caused significant hyperactivity in zebrafish at 120 hpf after acute exposure (3 h prior to the measurement) at concentrations below the teratogenicity LOEC (0.2 g dw L^-1). Similar effect was observed after exposure to fractions of the extracts with detected retinoid-like activity and additive effect was observed after combining the fractions. However, the effect on behaviour was not observed after exposure to ATRA only. To provide additional insight into the behavioural effects and describe the underlying mechanism gene expression of selected biomarkers was measured. We evaluated an array of 28 genes related to general toxicity, neurodevelopment, retinoid and thyroid signalling. We detected several affected genes, most notably, the Cyp26 enzymes that control endogenous ATRA concentration, which documents an effect on retinoid signalling.

Current understanding of potential ecological risks of retinoic acids and their metabolites in aquatic environments

In animals, retinoic acids (RAs), one of the main derivatives of vitamin A, are crucial for a variety of physiological processes. RAs, including all-trans-RA, 9-cis-RA, 13-cis-RA, and their corresponding metabolites (i.e., all-trans-4-oxo-RA, 9-cis-4-oxo-RA and 13-cis-4-oxo-RA) can be excreted through urination from humans and animals. Sewage treatment plants (STPs) are a significant source of RAs and 4-oxo-RAs into aquatic environments. RAs and 4-oxo-RAs can be identified and quantified by use of liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). RAs and 4-oxo-RAs have been reported in various environmental matrices including rivers, lakes, reservoirs and coastal marine environments as well as in sewage effluents discharged from STPs. Greater concentrations of RAs and 4-oxo-RAs have been observed during blooms of cyanobacteria and microalgae, suggesting that cyanobacteria and microalgae are natural sources of RAs and 4-oxo-RAs in aquatic environments. These potential sources of RAs and 4-oxo-RAs raise concerns about their concentrations and risks in aquatic environments because excessive intake of these chemicals can result in abnormal morphological development in animals. Teratogenic effects were observed in amphibians, fish embryos, gastropods, mammals and birds when exposed to RAs. This review summarizes sources, concentrations, adverse effects and ecological risks of RAs and 4-oxo-RAs in aquatic environments. An interim, predicted no-effect concentration (PNEC) of RAs (in terms of at-RA) for freshwater environments was determined to be 3.93 ng/L at-RA equivalents. Based on limited data on concentrations of RAs in freshwater ecosystems, their hazard quotients were found to range from zero to 16.41, depending on the environmental conditions of receiving waters. Ecological risks of RAs in marine environments are yet to be explored due to the paucity of data related to both their concentrations in marine environment and toxic potencies to marine species. This review updates current knowledge of RAs and 4-oxo-RAs in aquatic environments and calls for more studies on their concentrations and fate in aquatic environments, especially estuarine and coastal marine environments with a view to enabling a comprehensive assessment of their ecological risks around the globe.

Field cyanobacterial blooms producing retinoid compounds cause teratogenicity in zebrafish embryos

Cyanobacteria routinely release potentially harmful bioactive compounds into the aquatic environment. Several recent studies suggested a potential link between the teratogenicity of effects caused by cyanobacteria and production of retinoids. To investigate this relationship, we analysed the teratogenicity of field-collected cyanobacterial bloom samples by means of an in vivo zebrafish embryo test, an in vitro reporter gene bioassay and by the chemical analysis of retinoids. Extracts of biomass from cyanobacterial blooms with the dominance of Microcystis aeruginosa and Aphanizomenon klebahnii were collected from water bodies in the Czech Republic and showed significant retinoid-like activity in vitro, as well as high degrees of teratogenicity in vivo. Chemical analysis was then used to identify a set of retinoids in ng per gram of dry weight concentration range. Subsequent fractionation and bioassay-based characterization identified two fractions with significant in vitro retinoid-like activity. Moreover, in most of the retinoids eluted from these fractions, teratogenicity with malformations typical for retinoid signalling disruption was observed in zebrafish embryos after exposure to the total extracts and these in vitro effective fractions. The zebrafish embryo test proved to be a sensitive toxicity indicator of the biomass extracts, as the teratogenic effects occurred at even lower concentrations than those expected from the activity detected in vitro. In fact, teratogenicity with retinoid-like activity was detected at concentrations that are commonly found in biomasses and even in bulk water surrounding cyanobacterial blooms. Overall, these results provide evidence of a link between retinoid-like activity, teratogenicity and the retinoids produced by cyanobacterial water blooms in the surrounding environment.

Retinoids and oestrogenic endocrine disrupting chemicals in saline sewage treatment plants: Removal efficiencies and ecological risks to marine organisms

Discharge of partially treated effluent from sewage treatment plants (STPs) is a significant source of chemical contaminants, such as retinoids and oestrogenic endocrine disrupting chemicals (EDCs), which are continuously input into the marine environments of densely populated and urbanized coastal cities. In this study, we successfully developed three analytical methods to detect and qualify retinoic acids (at-RA, 13c-RA & 9c-RA), their metabolites (at-4-oxo-RA, 13c-4-oxo-RA & 9c-4-oxo-RA), and oestrogenic EDCs using high pressure liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Using these methods, we found that the total concentrations of retinoids in the influents and effluents of three saline STPs in Hong Kong were 7.1-29 ng/L and 3.7-9.1 ng/L, respectively, and those of EDCs were 3107-5829 ng/L and 1225-2638 ng/L, respectively. Retinoids were dominated by at-4-oxo-RA or 13c-4-oxo-RA in wastewater, whereas at-RA and 13c-RA were the most abundant in sludge. Alkylphenols and bisphenol A were the dominant EDCs in wastewater, whilst alkylphenols, triclosan, and triclocarban were dominant in sludge. Overall, the sewage treatment processes in the STPs of Hong Kong were not highly efficient in the removal of retinoids and EDCs from wastewater influents, with removal efficiencies in the aqueous phase of 41-82% and 31-79%, respectively. The removals were attributed mainly to sorption and degradation. Due to such limited removal, the effluents from STPs and the adjacent seawaters (i.e., receiving water bodies) still exhibited relatively high concentrations of retinoids (2.0-4.3 ng/L in seawaters) and EDCs (71-260 ng/L in seawaters), which posed medium ecological risks to the coastal marine ecosystem of Hong Kong (i.e., hazard quotients: 0.1-1).

Retinoid-like compounds produced by phytoplankton affect embryonic development of Xenopus laevis

Teratogenic effects, which were remarkably similar to those induced by retinoic acids, have been seen in wild frogs indicating possible source of retinoids in the environment. Recent studies indicate that some cyanobacterial species can contain teratogenic retinoic acids (RAs) and their analogues. Retinoids are known to regulate important processes such as differentiation, development, and embryogenesis. The study investigated the effects of exudates (extracellular compounds) of two cyanobacteria species with retinoic-like activity and one algae species on embryonic development of amphibians. The retinoid-like activity determined by in vitro reporter gene assay reached 528ng retinoid equivalents (REQ)/L and 1000ng REQ/L in exudates of Cylindrospermopsis raciborskii and Microcystis aeruginosa, respectively, while algal exudates showed no detectable activity. Total mean of retinoid-like copounds into exudate was 35.6ng ATRA/mil.cells for M.aeruginosa and 6.71ng ATRA/mil.cells for C.raciborskii, respectively. Toxicity tests with amphibian embryos up to 96h of development were carried out according to the standard guide for the Frog Embryo Teratogenesis Assay Xenopus. Lowest observed effect concentrations (LOEC) of malformations (2.5-2.6µg/L REQ) were two times lower than LOEC for ATRA (5µg/L). The exudates of both cyanobacteria were indeed provoking diverse teratogenic effects (e.g. tail, gut and eyes deformation) and interference with growth in frogs embryos, while such effects were not observed for the algae. Xenopus embryos were also exposed to all-trans retinoic acid (ATRA) in concentration range (1-40µg/L) equivalent to the REQs detected in cyanobacterial exudates. ATRA (10µg/L) caused similar teratogenic phenotypes at corresponding REQs as cyanobacterial exudates. The study confirms the ability of some species of cyanobacteria to produce retinoids naturally and excrete them directly into the environment at concentrations which might have adverse influence on the development of amphibians.

Retinoid-like activity and teratogenic effects of cyanobacterial exudates

Retinoic acids and their derivatives have been recently identified by chemical analyses in cyanobacteria and algae. Given the essential role of retinoids for vertebrate development this has raised concerns about a potential risk for vertebrates exposed to retinoids during cyanobacterial blooms. Our study focuses on extracellular compounds produced by phytoplankton cells (exudates). In order to address the capacity for the production of retinoids or compounds with retinoid-like activity we compared the exudates of ten cyanobacteria and algae using in vitro reporter gene assay. Exudates of three cyanobacterial species showed retinoid-like activity in the range of 269-2,265 ng retinoid equivalents (REQ)/L, while there was no detectable activity in exudates of the investigated algal species. The exudates of one green alga (Desmodesmus quadricaudus) and the two cyanobacterial species with greatest REQ levels, Microcystis aeruginosa and Cylindrospermopsis raciborskii, were selected for testing of the potential relation of retinoid-like activity to developmental toxicity in zebrafish embryos. The exudates of both cyanobacteria were indeed provoking diverse teratogenic effects (e.g. tail, spine and mouth deformation) and interference with growth in zebrafish embryos, while such effects were not observed for the alga. Fish embryos were also exposed to all-trans retinoic acid (ATRA) in a range equivalent to the REQ concentrations detected in exudates by in vitro bioassays. Both the phenotypes and effective concentrations of exudates corresponded to ATRA equivalents, supporting the hypothesis that the teratogenic effects of cyanobacterial exudates are likely to be associated with retinoid-like activity. The study documents that some cyanobacteria are able to produce and release retinoid-like compounds into the environment at concentrations equivalent to those causing teratogenicity in zebrafish. Hence, the characterization of retinoid-like and teratogenic potency should be included in the assessment of the potential adverse effects caused by the release of toxic and bioactive compounds during cyanobacterial blooms.

Unexpected phenotypes of malformations induced in Xenopus tropicalis embryos by combined exposure to triphenyltin and 9-cis-retinoic acid

Xenopus tropicalis embryos were exposed for 48 hr to the mixtures of 5 μg Sn/L triphenyltin (TPT), which is a well-known endocrine disruptor, and 0.25-5 μg/L 9-cis retinoic acid (9c-RA), which is the natural ligand of retinoid X receptor. The phenotypes induced by combined exposure were more variable than those resulting from single exposure to either TPT or 9c-RA. The prominent phenotypes included underdeveloped head structures, abnormal eyes, narrow fins, enlarged proctodaeum, etc. Especially, combined exposure induced unexpected notochord malformations, which ranged from small swellings of the surface of the tails to the extension and extrusion of notochord out of the posterior tails. Compared with the 5 μg Sn/L TPT-treated group, the index of fin deficiency was not affected, and the index of axis deficiency was significantly increased with increasing RA concentrations in the mixtures. Our results suggest that combined exposure to TPT and 9c-RA induced not only more variable phenotypes of malformations than exposure to single compound but also some new and unexpected phenotypes.

A new approach to highly sensitive determination of retinoic acid isomers by preconcentration with CdSe quantum dots

Unusual amounts of retinoic acid (RA) isomers play an important role in abnormal morphological development of mammals; such as rat embryos. Each isomer of RA has a unique function in first steps of embryonic life. In the current study, a new method for preconcentration and simultaneous determination of all-trans retinoic acid, 13-cis retinoic acid, 9-cis retinoic acid and 9,13-di-cis retinoic acid in rat whole rudimentary embryo culture (RWEC) has been developed. RA isomers were extracted from samples by conjugation to appropriate amount of surface modified CdSe quantum dots (QDs) prior to HPLC/UV determination. In order to quickly release of the analytes with unchanged form, separated RA-QD conjugation were irradiated by intensive near infrared wavelength (NIR). Low energy NIR irradiation results in maintaining the primary forms of RA isomers during the release. The conjugation and release mechanisms were described and experimental parameters were investigated in detail. Under optimized conditions, the method was linear in the range of 0.040-34.600 pmol g(-1) for all-trans RA (R(2)=0.9996), 0.070-34.200 pmol g(-1) for 13-cis RA (R(2)=0.9992), 0.050-35.300 pmol g(-1) for 9,13-di-cis RA (R(2)=0.9998) and 0.050-32.900 pmol g(-1) for 9-cis RA (R(2)=0.9990). The present method can be useful for retinoic acid monitoring in clinical studies.

Determination and occurrence of retinoids in a eutrophic lake (Taihu Lake, China): cyanobacteria blooms produce teratogenic retinal

Besides retinoic acids (RAs), some retinoids such as retinal (RAL) and retinol (ROH), which are considered as RA precursors in vertebrates, are also reported to be teratogenic agents. In this study we investigated four RA precursors including RAL, ROH, retinyl palmitate, and β-carotene in the eutrophic Taihu Lake, China, by developing a sensitive analytical method. RAL and β-carotene were widely detected in natural cyanobacteria blooms and lake water. Intracellular concentrations of RAL and β-carotene in blooms were 9.4 to 6.9 × 10(3) and 3.4 to 1.8 × 10(5) ng L(-1), respectively, and their concentrations in lake water were up to 1.4 × 10 ng L(-1) (RAL) and 9.8 × 10(2) ng L(-1) (β-carotene). The good correlation between intracellular concentrations of RAL and RAs implied that RAL was involved in the production of RAs by cyanobacteria blooms. Further examination of 39 cyanobacteria and algae species revealed that most species could produce RAL and β-carotene. The greatest amount of RAL was found in Chlamydomonas sp. (FACHB-715; 1.9 × 10(3) ng g(-1) dry weight). As the main cyanobacteria in Taihu Lake, many Microcystis species could produce high amounts of RAL and were thought to greatly contribute to the production of RAL measured in the blooms. Productions of RAL and β-carotene by cyanobacteria were associated with species, origin location, and growth stage. The results in this study present the existence of a potential risk to aquatic animals living in a eutrophic environment from a high concentration of RAL in cyanobacteria blooms and also provide a clue for further investigating the mechanism underlying the biosynthetic pathway of RAs in cyanobacteria and algae.

Cyanobacteria blooms produce teratogenic retinoic acids

Deformed amphibians have been observed in eutrophic habitats, and some clues point to the retinoic acids (RAs) or RA mimics. However, RAs are generally thought of as vertebrate-specific hormones, and there was no evidence that RAs exist in cyanobacteria or algae blooms. By analyzing RAs and their analogs 4-oxo-RAs in natural cyanobacteria blooms and cultures of cyanobacteria and algae, we showed that cyanobacteria blooms could produce RAs, which were powerful animal teratogens. Intracellular RAs and 4-oxo-RAs with concentrations between 0.4 and 4.2 × 10(2) ng/L were detected in all bloom materials, and extracellular concentrations measured in water from Taihu Lake, China, were as great as 2.0 × 10 ng/L, which might pose a risk to wildlife through chronic exposure. Further examination of 39 cyanobacteria and algae species revealed that 32 species could produce RAs and 4-oxo-RAs (1.6-1.4 × 10(3) ng/g dry weight), and the dominant cyanobacteria species in Taihu Lake, Microcystis flos-aquae and Microcystis aeruginosa, produced high amounts of RAs and 4-oxo-RAs with concentrations of 1.4 × 10(3) and 3.7 × 10(2) ng/g dry weight, respectively. Most genera of cyanobacteria that could produce RAs and 4-oxo-RAs, such as Microcystis, Anabaena, and Aphanizomenon, often occur dominantly in blooms. Production of RAs and 4-oxo-RAs by cyanobacteria was associated with species, origin location, and growth stage. These results represent a conclusive demonstration of endogenous production of RAs in freshwater cyanobacteria blooms. The observation of teratogenic RAs in cyanobacteria is evolutionarily and ecologically significant because RAs are vertebrate-specific hormones, and cyanobacteria form extensive and highly visible blooms in many aquatic ecosystems.

Detection of retinoic acid receptor agonistic activity and identification of causative compounds in municipal wastewater treatment plants in Japan

Retinoic acid (RA) receptor (RAR) agonists are potential toxicants that can cause teratogenesis in vertebrates. To determine the occurrence of RAR agonists in municipal wastewater treatment plants (WWTPs), we examined the RARα agonistic activities of influent and effluent samples from several municipal WWTPs in Osaka, Japan, using a yeast two-hybrid assay. Significant RARα agonistic activity was detected in all the influent samples investigated, suggesting that municipal wastewater consistently contains RAR agonists. Fractionations using high-performance liquid chromatography, directed by the bioassay, found several bioactive peaks from influent samples. The RAR agonists, all-trans RA (atRA), 13-cis RA (13cRA), 4-oxo-atRA, and 4-oxo-13cRA, possibly arising from human urine, were identified by liquid chromatography ion trap time-of-flight mass spectrometry. Quantification of the identified compounds in municipal WWTPs confirmed that they were responsible for the majority of RARα agonistic activity in WWTP influents, and also revealed they were readily removed from wastewater by activated sludge treatment. Simultaneous measurement of the RARα agonistic activity revealed that although total activity typically declined concomitant with the reduction of the four identified compounds, it remained high after the decline of RAs and 4-oxo-RAs in one WWTP, suggesting the occurrence of unidentified RAR agonists during the activated sludge treatment.