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

Effects of Harmful Algal Blooms on Amphibians and Reptiles are Under-Reported and Under-Represented

Harmful algal blooms (HABs) are a persistent and increasing problem globally, yet we still have limited knowledge about how they affect wildlife. Although semi-aquatic and aquatic amphibians and reptiles have experienced large declines and occupy environments where HABs are increasingly problematic, their vulnerability to HABs remains unclear. To inform monitoring, management, and future research, we conducted a literature review, synthesized the studies, and report on the mortality events describing effects of cyanotoxins from HABs on freshwater herpetofauna. Our review identified 37 unique studies and 71 endpoints (no-observed-effect and lowest-observed-effect concentrations) involving 11 amphibian and 3 reptile species worldwide. Responses varied widely among studies, species, and exposure concentrations used in experiments. Concentrations causing lethal and sublethal effects in laboratory experiments were generally 1 to 100 µg/L, which contains the mean value of reported HAB events but is 70 times less than the maximum cyanotoxin concentrations reported in the environment. However, one species of amphibian was tolerant to concentrations of 10,000 µg/L, demonstrating potentially immense differences in sensitivities. Most studies focused on microcystin-LR (MC-LR), which can increase systemic inflammation and harm the digestive system, reproductive organs, liver, kidneys, and development. The few studies on other cyanotoxins illustrated that effects resembled those of MC-LR at similar concentrations, but more research is needed to describe effects of other cyanotoxins and mixtures of cyanotoxins that commonly occur in the environment. All experimental studies were on larval and adult amphibians; there were no such studies on reptiles. Experimental work with reptiles and adult amphibians is needed to clarify thresholds of tolerance. Only nine mortality events were reported, mostly for reptiles. Given that amphibians likely decay faster than reptiles, which have tissues that resist decomposition, mass amphibian mortality events from HABs have likely been under-reported. We propose that future efforts should be focused on seven major areas, to enhance our understanding of effects and monitoring of HABs on herpetofauna that fill important roles in freshwater and terrestrial environments. Environ Toxicol Chem 2024;00:1-14. Published 2024. This article is a U.S. Government work and is in the public domain in the USA. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Cyanobacterial anatoxin-a does not induce in vitro developmental neurotoxicity, but changes gene expression patterns in co-exposure with all-trans retinoic acid

Cyanobacterial blooms are increasing in frequency and intensity globally, and impacting recreational waters as well as waters used for drinking water provisioning. They are sources of bioactive metabolites including retinoids and the neurotoxin anatoxin-a. Here, we investigated the effects of anatoxin-a on a differentiating in vitro human neural stem cell model previously characterised with retinoic acids. Effects on protein and gene expression upon exposure for 9 or 18 days to anatoxin-a alone or in co-exposure with all-trans retinoic acid were evaluated using a panel of neural and glial differentiation biomarkers. Anatoxin-a did not cause distinct developmental neurotoxicity alone, or in co-exposure with retinoic acid. However, in line with its excitotoxicity, in co-exposure with 200 nM all-trans retinoic acid it reduced the differentiation of acetylcholinergic neuron subtypes in the culture at 1000 nM (highest tested concentration). While this could have substantial functional implications for the developing nervous system, there is no indication for developmental neurotoxicity beyond its (excito-)toxicity to acetylcholinergic neurons, which only occurred in co-exposure to all-trans retinoic acid.

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.

Effects of all-trans and 9-cis retinoic acid on differentiating human neural stem cells in vitro

Cyanobacterial blooms are known sources of environmentally-occurring retinoid compounds, including all-trans and 9-cis retinoic acids (RAs). The developmental hazard for aquatic organisms has been described, while the implications for human health hazard assessment are not yet sufficiently characterized. Here, we employ a human neural stem cell model that can differentiate in vitro into a mixed culture of neurons and glia. Cells were exposed to non-cytotoxic 8-1000 nM all-trans or 9-cis RA for 9-18 days (DIV13 and DIV22, respectively). Impact on biomarkers was analyzed on gene expression (RT-qPCR) and protein level (western blot and proteomics) at both time points; network patterning (immunofluorescence) on DIV22. RA exposure significantly concentration-dependently increased gene expression of retinoic acid receptors and the metabolizing enzyme CYP26A1, confirming the chemical-specific response of the model. Expression of thyroid hormone signaling-related genes remained mostly unchanged. Markers of neural progenitors/stem cells (PAX6, SOX1, SOX2, NESTIN) were decreased with increasing RA concentrations, though a basal population remained. Neural markers (DCX, TUJ1, MAP2, NeuN, SYP) remained unchanged or were decreased at high concentrations (200-1000 nM). Conversely, (astro-)glial marker S100β was increased concentration-dependently on DIV22. Together, the biomarker analysis indicates an RA-dependent promotion of glial cell fates over neural differentiation, despite the increased abundance of neural protein biomarkers during differentiation. Interestingly, RA exposure induced substantial changes to the cell culture morphology: while low concentrations resulted in a network-like differentiation pattern, high concentrations (200-1000 nM RA) almost completely prevented such network patterning. After functional confirmation for implications in network function, such morphological features could present a proxy for network formation assessment, an apical key event in (neuro-)developmental Adverse Outcome Pathways. The described application of a human in vitro model for (developmental) neurotoxicity to emerging environmentally-relevant retinoids contributes to the evidence-base for the use of differentiating human in vitro models for human health hazard and risk assessment.

Comparative metabolomic analysis of exudates of microcystin-producing and microcystin-free Microcystis aeruginosa strains

Cyanobacterial harmful algal blooms (cHABs) dominated by Microcystis aeruginosa threaten the ecological integrity and beneficial uses of lakes globally. In addition to producing hepatotoxic microcystins (MC), M. aeruginosa exudates (MaE) contain various compounds with demonstrated toxicity to aquatic biota. Previously, we found that the ecotoxicity of MaE differed between MC-producing and MC-free strains at exponential (E-phase) and stationary (S-phase) growth phases. However, the components in these exudates and their specific harmful effects were unclear. In this study, we performed untargeted metabolomics based on liquid chromatography-mass spectrometry to reveal the constituents in MaE of a MC-producing and a MC-free strain at both E-phase and S-phase. A total of 409 metabolites were identified and quantified based on their relative abundance. These compounds included lipids, organoheterocyclic compounds, organic acid, benzenoids and organic oxygen compounds. Multivariate analysis revealed that strains and growth phases significantly influenced the metabolite profile. The MC-producing strain had greater total metabolites abundance than the MC-free strain at S-phase, whereas the MC-free strain released higher concentrations of benzenoids, lipids, organic oxygen, organic nitrogen and organoheterocyclic compounds than the MC-producing strain at E-phase. Total metabolites had higher abundance in S-phase than in E- phase in both strains. Analysis of differential metabolites (DMs) and pathways suggest that lipids metabolism and biosynthesis of secondary metabolites were more tightly coupled to growth phases than to strains. Abundance of some toxic lipids and benzenoids DMs were significantly higher in the MC-free strain than the MC-producing one. This study builds on the understanding of MaE chemicals and their biotoxicity, and adds to evidence that non-MC-producing strains of cyanobacteria may also pose a threat to ecosystem health.

Estrogenic and retinoid-like activity in stagnant waters with mass occurrence of water blooms

Stagnant freshwaters can be affected by anthropogenic pollution and eutrophication that leads to massive growth of cyanobacteria and microalgae forming complex water blooms. These can produce various types of bioactive compounds, some of which may cause embryotoxicity, teratogenicity, endocrine disruption and impair animal or human health. This study focused on potential co-occurrence of estrogenic and retinoid-like activities in diverse stagnant freshwaters affected by phytoplankton blooms with varying taxonomic composition. Samples of phytoplankton bloom biomass and its surrounding water were collected from 17 independent stagnant water bodies in the Czech Republic and Hungary. Total estrogenic equivalents (EEQ) of the most potent samples reached up to 4.9 ng·g^-1 dry mass (dm) of biomass extract and 2.99 ng·L^-1 in surrounding water. Retinoic acid equivalent (REQ) measured by in vitro assay reached up to 3043 ng·g^-1 dm in phytoplankton biomass and 1202 ng·L^-1in surrounding water. Retinoid-like and estrogenic activities at some sites exceeded their PNEC and effect-based trigger values, respectively. The observed effects were not associated with any particular species of cyanobacteria or algae dominating the water blooms nor related to phytoplankton density. We found that taxonomically diverse phytoplankton communities can produce and release retinoid-like compounds to surrounding water, while estrogenic potency is likely related to estrogens of anthropogenic origin adsorbed to phytoplankton biomass. Retinoids occurring in water blooms are ubiquitous signalling molecules, which can affect development and neurogenesis. Selected water bloom samples (both water and biomass extracts) with retinoid-like activity caused effects on neurodifferentiation in vitro corresponding to those of equivalent all-trans-retinoic acid concentrations. Co-occurrence of estrogenic and retinoid-like activities in stagnant water bodies as well as the potential of compounds produced by water blooms to interfere with neural differentiation should be considered in the assessment of risks associated with water blooms, which can comprise complex mixtures of natural and anthropogenic bioactive compounds.

Spectroscopy of Retinoic Acid at the Air-Water Interface

The spectroscopy of all-trans-retinoic acid (ATRA), an important molecule of biological origin that can be found in nature, is investigated at the air-water interface using UV-Vis and IR reflection spectroscopy. We employ a UV-Vis reflection absorption spectroscopy (RAS) experiment along with infrared reflection absorption spectroscopy (IR-RAS) to probe ATRA at the air-water interface. We elucidate the factors influencing the spectroscopy of ATRA at the air-water interface and compare its spectra at the water surface with results of bulk samples obtained with conventional spectroscopic methods and computational chemistry. Monolayers of pure ATRA as well as mixed ATRA with stearic-d₃₅ acid were prepared, and the spectroscopy reveals that ATRA forms J-aggregates with itself, causing a significant redshift of its S₀ to S₁ electronic transition. Pure ATRA monolayers are found to be unstable at the air-water interface and are lost from the surface over time due to the formation of aggregates. The mixture of ATRA and stearic-d₃₅ acid has been shown to stabilize the monolayers and inhibit the loss of surface ATRA. On the basis of our observations, we propose that ATRA could be a significant photosensitizer in natural aqueous environments.

Constructing an MCF-7 breast cancer cell-based transient transfection assay for screening RARα (Ant)agonistic activities of emerging phenolic compounds

The screening of compounds with endocrine disrupting effects has been attracting increasing attention due to the continuous release of emerging chemicals into the environment. Testing the (ant)agonistic activities of these chemicals on the retinoic acid receptor α (RARα), a vital nuclear receptor, is necessary to explain their perturbation in the endocrine system in vivo. In the present study, MCF-7 breast carcinoma cells were transiently transfected with a RARα expression vector (pEF1α-RARα-RFP) and a reporter vector containing a retinoic acid reaction element (pRARE-TA-Luc). Under optimized conditions, the performance of the newly constructed system was evaluated for its feasibility in screening the (ant)agonistic effects of emerging phenolic compounds on RARα. The results showed that this transient transfection cell model responded well to stimulation with (ant)agonists of RARα, and the EC₅₀ and IC₅₀ values were 0.87 nM and 2.67 μM for AM580 and Ro41-5253, respectively. Its application in testing several emerging phenolic compounds revealed that triclosan (TCS) and tetrabromobisphenol A (TBBPA) exerted notable RARα antagonistic activities. This newly developed bioassay based on MCF-7 is promising in identifying the agonistic or antagonistic activities of xenobiotics on RARα and has good potential for studying RARα signaling-involved toxicological effects of emerging chemicals of concern.

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.

Teratogenicity of retinoids detected in surface waters in zebrafish embryos and its predictability by in vitro assays

Retinoids are newly detected compounds in aquatic ecosystems associated with cyanobacterial water blooms. Their potential health risks are only scarcely described despite numerous detections of all-trans retinoic acid (ATRA) and its derivatives in the environment. Besides the known teratogen ATRA there is only little or no information about their potency and namely their effects in vivo. We characterize ATRA and 8 other retinoids reported to occur in the environment for their bioactivity and teratogenicity using four in vitro reporter gene assays and zebrafish (Danio rerio) embryotoxicity assay. Our results document the ability of these compounds to interfere with retinoid signalling and cause teratogenicity at environmentally relevant levels with EC₅₀ values at nM (hundreds of ng/L) levels and teratogenic indexes ranging from 2.8 (9cis retinoic acid) to 15.8 (retinal). The relative potency of individual compounds for teratogenicity ranged from 0.059 (retinal) to 0.96 (5,6-epoxy ATRA) when compared to ATRA. An environmentally relevant mixture of retinoids was tested showing good predictability of teratogenicity from the in vitro activities and additive toxicity of the mixture. The high teratogenicity of the newly described compounds associated with cyanobacteria presents a concern for developmental stages due to high conservation of the retinoid signalling across vertebrates.

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.

Carotenoid modifying enzymes in metazoans

Carotenoids constitute an essential dietary component of animals and other non-carotenogenic species which use these pigments in both their modified and unmodified forms. Animals utilize uncleaved carotenoids to mitigate light damage and oxidative stress and to signal fitness and health. Carotenoids also serve as precursors of apocarotenoids including retinol, and its retinoid metabolites, which carry out essential functions in animals by forming the visual chromophore 11-cis-retinaldehyde. Retinoids, such as all-trans-retinoic acid, can also act as ligands of nuclear hormone receptors. The fact that enzymes and biochemical pathways responsible for the metabolism of carotenoids in animals bear resemblance to the ones in plants and other carotenogenic species suggests an evolutionary relationship. We will explore some of the modes of transmission of carotenoid genes from carotenogenic species to metazoans. This apparent relationship has been successfully exploited in the past to identify and characterize new carotenoid and retinoid modifying enzymes. We will review approaches used to identify putative animal carotenoid enzymes, and we will describe methods used to functionally validate and analyze the biochemistry of carotenoid modifying enzymes encoded by animals.

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.

Norm index-based QSAR models for acute toxicity of organic compounds toward zebrafish embryo

Zebrafish embryos are highly sensitive to toxicant exposure and have been used to evaluate the potential eco-toxicity caused by organic pollutants in the aquatic environment. This study was to develop four quantitative structure-activity relationship (QSAR) models based on norm descriptors for acute toxicity of different exposure times toward zebrafish embryo of organic compounds with various structures. Norm descriptors were obtained by calculating the norm index of the atomic distribution matrix, which was composed of atomic spatial distribution and atomic properties. These norm index-based QSAR models presented satisfactory results with R² of 0.8549, 0.9162, 0.8335 and 0.8119 for 48, 96, 120 and 132 h, respectively. Validation results including cross validation, external validation, Y-randomized test and applicability domain analysis indicated that the proposed models were stable, robust and reliable. Accordingly, these norm descriptors might be effective in predicting the acute toxicity of various organics to zebrafish embryos, which might be useful for evaluating the potential hazards of organic pollutants to aquatic environment.

Occurrence, toxicity and ecological risk of larvicidal oil in the coastal marine ecosystem of Hong Kong

Application of larvicidal oil (LO) is the most common practice in Hong Kong to control mosquitos, and hence prevent mosquito-borne diseases and protect human health. Globally, this study represented the first comprehensive assessment of toxicity and risk posed by LO to marine organisms. We found concentrations of LO ranged from 0.08 to 0.66 mg/L in coastal seawaters of Hong Kong. Waterborne exposure to water-accommodated fractions of LO resulted in growth inhibition to two microalgal species (72-h EC₅₀: 1.92-2.90 mg/L) and acute mortality to three marine animals (96-h LC₅₀: 3.41-8.10 mg/L). From these toxicity results, a concentration that considered to be hazardous to 5% of species (HC₅) was predicted at 1.45 mg/L, while the predicted no-effect concentration was determined to be 0.29 mg/L. The hazard quotient of LO exceeded 1 at 9 out of 15 sites, indicating moderate-to-high ecological risk to exposure of LO in the marine environment of Hong Kong.

Removal of emerging contaminants from wastewater during chemically enhanced primary sedimentation and acidogenic sludge fermentation

A novel wastewater treatment process, which couples chemically enhanced primary sedimentation (CEPS) of sewage with acidogenic fermentation of sludge in tandem, has recently been developed to improve the removal of pollutants and nutrients, and recover valuable resources such as phosphorus and organics. This study represented the first laboratory-based examination on the level and removal of the emerging contaminants, including retinoids (i.e., retinoic acids (RAs) and their metabolites) and oestrogenic endocrine disrupting chemicals (EDCs; e.g., 4-nonylphenol, bisphenol A, etc.), in sewage, sludge and its supernatant during this novel wastewater treatment process. The results showed that 65% of retinoids and 73% of EDCs were removed from sewage after aluminum (Al) based CEPS, while 80% of retinoids and 72% of EDCs were removed after iron (Fe) based CEPS. After acidogenic fermentation of the CEPS sludge, 50% and 58% of retinoids, and 50% and 47% of EDCs were further removed in the supernatants of Al-sludge and Fe-sludge, respectively. While there were comparable removals for these two classes of emerging contaminants during Al- and Fe-based CEPS and sludge fermentation, Fe-based CEPS of sewage and sludge fermentation should be preferentially considered, given the relatively lower production of Fe-sludge and lower accumulation of retinoids in Fe-sludge. The levels of retinoids and EDCs in the supernatant and sludge changed during acidogenic fermentation of Fe-sludge. The removals of at-4-oxo-RA (i.e., the dominant retinoid) and bisphenol A (i.e., the dominant EDC) in the supernatant followed the pseudo first-order reaction model, with a half-life of 1.62 days (in the first two days) and 1.55 days (in the whole experiment of seven days), respectively. The results demonstrated the effective removal of emerging contaminants from the sewage and the supernatant during the CEPS and acidogenic sludge fermentation.