Field cyanobacterial blooms producing retinoid compounds cause teratogenicity in zebrafish embryos

Cyanobacterial bloom-associated lipopolysaccharides induce pro-inflammatory processes in keratinocytes in vitro

Our previous studies have shown that CyanoHAB LPS (lipopolysaccharides) and LPS from cyanobacterial cultures induce pro-inflammatory effects on intestinal epithelial and immune cells in vitro. To expand our understanding, we investigated their impact on human keratinocytes, which are targeted during water recreational activities. LPS samples were isolated from CyanoHAB biomasses dominated by Microcystis, Aphanizomenon, Planktothrix, and Dolichospermum, or from axenic cultures of these genera. We identified two CyanoHAB biomasses containing a high proportion of Gram-negative bacteria, including potentially pathogenic genera. These biomasses showed the highest induction of interleukin (IL) 8, IL-6, C-C motif chemokine ligand (CCL) 2 (also known as MCP-1), and CCL20 production by HaCaT cells. Interestingly, all CyanoHAB-derived LPS and LPS from axenic cultures (except for Microcystis) accelerated cell proliferation and migration. Our findings highlight the role of G- bacteria composition and LPS structural disparities in influencing these effects, with implications for skin health during recreational activities.

Cyanobacterial Harmful Bloom Lipopolysaccharides Induce Pro-Inflammatory Effects in Immune and Intestinal Epithelial Cells In Vitro

Freshwater cyanobacterial harmful blooms (CyanoHABs) produce a variety of toxic and bioactive compounds including lipopolysaccharides (LPSs). The gastrointestinal tract can be exposed to them via contaminated water even during recreational activities. However, there is no evidence of an effect of CyanoHAB LPSs on intestinal cells. We isolated LPSs of four CyanoHABs dominated by different cyanobacterial species and LPSs of four laboratory cultures representing the respective dominant cyanobacterial genera. Two intestinal and one macrophage cell lines were used to detect in vitro pro-inflammatory activity of the LPS. All LPSs isolated from CyanoHABs and laboratory cultures induced cytokines production in at least one in vitro model, except for LPSs from the Microcystis PCC7806 culture. LPSs isolated from cyanobacteria showed unique migration patterns in SDS-PAGE that were qualitatively distinct from those of endotoxins from Gram-negative bacteria. There was no clear relationship between the biological activity of the LPS and the share of genomic DNA of Gram-negative bacteria in the respective biomass. Thus, the total share of Gram-negative bacteria, or the presence of Escherichia coli-like LPSs, did not explain the observed pro-inflammatory activities. The pro-inflammatory properties of environmental mixtures of LPSs from CyanoHABs indicate their human health hazards, and further attention should be given to their assessment and monitoring.

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.

Morphological and behavioral alterations in zebrafish larvae after exposure to contaminated river sediments collected in different weather conditions

Wastewater treatment plants (WWTPs) are the primary source of micropollutants in aquatic ecosystems. Many micropollutants tend to bind to sediments and persist until remobilizion by bioturbation or flood events. Advanced effluent treatment by ozonation has been proven to eliminate most micropollutants. The present study characterizes sediments' toxic potential regarding zebrafish embryo development, which highly complex nervous system is vulnerable to exposure to neurotoxic substances. Furthermore, behavioral changes can be induced even at low pollutant concentrations and do not cause acute toxicity. The study area includes stretches of the main waterbody, the Wurm River (sampling sites W1-W5), and its tributary the Haarbach River (sampling sites H1, and H2) in North-Rhine Westphalia, Germany. Both waterbodies serve as recipients of WWTPs' effluents. The effluent entering the Haarbach River is conventionally treated, while the Wurm River receives ozonated effluent from the Aachen-Soers WWTP. Seven sampling sites up- and downstream of the WWTPs were investigated in June of two subsequent years. The first sampling campaign in 2017 was characterized by prolonged dry weather. The second sampling campaign in 2018 occurred after prolonged rain events and the release of the rainwater overflow basin. Direct exposure of zebrafish embryos to native sediments using the sediment contact test represented an ecologically realistic scenario and showed no acute sublethal effects. Exposure of the zebrafish embryo to freeze-dried sediments representing the ecotoxicological status of sediments during flood events unfolded acute sublethal toxicity. Behavioral studies with zebrafish larvae were an essential part of environmental neurotoxicity testing. Zebrafish larvae exposed to sediments' concentrations causing no acute effects led to behavioral changes signalizing neurotoxic substances in sediments. Polyaromatic hydrocarbons, polychlorinated biphenyls, and nitroaromatic compounds were identified as potential toxicity drivers, whereby the rainwater overflow basin served as a possible source of pollution. Mixture toxicity, effect-directed analysis, and further sediment monitoring are needed.

Sublethal exposure to Microcystis aeruginosa extracts during the yolk-sac larval stage reduces aerobic swimming speed in juvenile zebrafish

This study examined whether the aerobic swimming capacity of zebrafish juveniles is affected by the exposure of the yolk-sac larvae to sublethal concentration of Microcystis aeruginosa extract (200 mg dw L^-1). Critical swimming speed significantly decreased in the pre-exposed fish (9.2 ± 1.0 vs 11.3 ± 1.4 TL s^-1 in the control group). Exposure did not have any significant effects on the shape of the heart ventricle, rate of skeletal abnormalities, and growth or survival rates. Decreased swimming performance due to the early and short exposure to M. aeruginosa could have negative impacts on fish in the wild.

The Origin of Teratogenic Retinoids in Cyanobacteria

Although information about the occurrence and distribution of retinoids in the environment is scarce, cyanobacterial water blooms have been identified as a significant source of these small molecules. Despite the confirmed presence of retinoids in the freshwater blooms dominated by cyanobacteria and their described teratogenic effects, reliable identification of retinoid producers and the mechanism of their biosynthesis is missing. In this study, the cultures of several taxonomically diverse species of axenic cyanobacteria were confirmed as significant producers of retinoid-like compounds. The consequent bioinformatic analysis suggested that the enzymatic background required for the biosynthesis of all-trans retinoic acid from retinal is not present across phylum Cyanobacteria. However, we demonstrated that retinal conversion into other retinoids can be mediated non-enzymatically by free radical oxidation, which leads to the production of retinoids widely detected in cyanobacteria and environmental water blooms, such as all-trans retinoic acid or all-trans 5,6epoxy retinoic acid. Importantly, the production of these metabolites by cyanobacteria in association with the mass development of water blooms can lead to adverse impacts in aquatic ecosystems regarding the described teratogenicity of retinoids. Moreover, our finding that retinal can be non-enzymatically converted into more bioactive retinoids, also in water, and out of the cells, increases the environmental significance of this process.

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.

Cyanobacterial secondary metabolites towards improved commercial significance through multiomics approaches

Cyanobacteria are ubiquitous photosynthetic prokaryotes responsible for the oxygenation of the earth's reducing atmosphere. Apart from oxygen they are producers of a myriad of bioactive metabolites with diverse complex chemical structures and robust biological activities. These secondary metabolites are known to have a variety of medicinal and therapeutic applications ranging from anti-microbial, anti-viral, anti-inflammatory, anti-cancer, and immunomodulating properties. The present review discusses various aspects of secondary metabolites viz. biosynthesis, types and applications, which highlights the repertoire of bioactive constituents they harbor. Majority of these products have been produced from only a handful of genera. Moreover, with the onset of various OMICS approaches, cyanobacteria have become an attractive chassis for improved secondary metabolites production. Also the intervention of synthetic biology tools such as gene editing technologies and a variety of metabolomics and fluxomics approaches, used for engineering cyanobacteria, have significantly enhanced the production of secondary metabolites.

Sublethal exposure to Microcystis aeruginosa extracts during embryonic development reduces aerobic swimming capacity in juvenile zebrafish

In the last decades, cyanobacterial harmful algal blooms (CyanoHABs) pose an intensifying ecological threat. Microcystis aeruginosa is a common CyanoHAB species in freshwater ecosystems, with severe toxic effects in a wide range of organisms. In the present paper we examined whether transient and short (48 h) exposure of fish embryos to sublethal levels of M. aeruginosa crude extract (200 mg biomass dw L^-1) affects swimming performance at later life stages (end of metamorphosis, ca 12 mm TL, 22,23 days post-fertilization). Pre-exposed metamorphosing larvae presented a significant decrease in swimming performance (9.7 ± 1.6 vs 11.4 ± 1.7 TL s^-1 in the control group, p < 0.01), and a significant decrease in the ventricle length-to-depth ratio (1.23 ± 0.15 vs 1.42 ± 0.15 in control fish, p < 0.05). In addition, extract-exposed fish presented significantly elevated rates of vertebral abnormalities (82 ± 13% vs 7 ± 4% in the control group), mainly consisting of the presence of extra neural and haemal processes. No significant differences between groups were detected in survival and growth rates. Results are discussed in respect to the mechanisms that might mediate the detected cyanobacterial effects. This is the first evidence of a direct link between sublethal exposure to M. aeruginosa during the embryonic period and swimming performance at later life-stages. Decreased swimming performance, altered cardiac shape, and elevated vertebral abnormalities in response to early exposure to M. aeruginosa could have significant effects on fish populations in the wild.

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.

Variability in retinoid-like activity of extracellular compound mixtures produced by wide spectra of phytoplankton species and contributing metabolites

Some phytoplankton species were shown to produce teratogenic retinoids. This study assessed the variability in the extracellular production of compounds with retinoid-like activity for 50 independent cultivations of wide spectra of species including 12 cyanobacteria (15 strains) and 4 algae of different orders. Extracellular retinoid-like activity was detected for repeated cultivations of six cyanobacteria. The results were consistent for some species including Microcystis aeruginosa and Aphanizomenon gracile. The detected retinoid-like activities ranged from below the limit of quantification of 16 ng/L to over 6 µg all-trans retinoic acid (ATRA) equivalent/L. Nontargeted virtual fractionation together with suspect screening approach enabled to identify some retinoid-like compounds in exudates, including ATRA, 9/13-cis retinoic acid, all-trans 5,6-epoxy retinoic acid, 4keto-ATRA, 4keto-retinal, 4hydroxy-ATRA, and retinal. Most of them were for the first time repeatedly detected in exudates of all studied algae (at ng/L levels) and cyanobacteria. Their relative potencies ranged from 0.018 (retinal) to 1 compared to ATRA. They accounted for less than 0.1-50% of total detected retinoid-like activity. The high detected activities and concentrations of retinoids in some samples and their direct accessibility from exudates document potential risk of developmental toxicity for organisms in proximity of massive water blooms.

Cytotoxic and Genotoxic Effects of Cyanobacterial and Algal Extracts-Microcystin and Retinoic Acid Content

In the last decade, it has become evident that complex mixtures of cyanobacterial bioactive substances, simultaneously present in blooms, often exert adverse effects that are different from those of pure cyanotoxins, and awareness has been raised on the importance of studying complex mixtures and chemical interactions. We aimed to investigate cytotoxic and genotoxic effects of complex extracts from laboratory cultures of cyanobacterial species from different orders (Cylindrospermopsis raciborskii, Aphanizomenon gracile, Microcystis aeruginosa, M. viridis, M. ichtyoblabe, Planktothrix agardhii, Limnothrix redekei) and algae (Desmodesmus quadricauda), and examine possible relationships between the observed effects and toxin and retinoic acid (RA) content in the extracts. The cytotoxic and genotoxic effects of the extracts were studied in the human hepatocellular carcinoma HepG2 cell line, using the MTT assay, and the comet and cytokinesis-block micronucleus (cytome) assays, respectively. Liquid chromatography electrospray ionization mass spectrometry (LC/ESI-MS) was used to detect toxins (microcystins (MC-LR, MC-RR, MC-YR) and cylindrospermopsin) and RAs (ATRA and 9cis-RA) in the extracts. Six out of eight extracts were cytotoxic (0.04–2 mg_DM/mL), and five induced DNA strand breaks at non-cytotoxic concentrations (0.2–2 mg_DM/mL). The extracts with genotoxic activity also had the highest content of RAs and there was a linear association between RA content and genotoxicity, indicating their possible involvement; however further research is needed to identify and confirm the compounds involved and to elucidate possible genotoxic effects of RAs.

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.