Toxic effects produced by microcystins from a natural cyanobacterial bloom and a Microcystis aeruginosa isolated strain on the fish cell lines RTG-2 and PLHC-1

Non-microcystin extracellular metabolites of Microcystis aeruginosa impair viability and reproductive gene expression in rainbow trout cell lines

Microcystis aeruginosa is a ubiquitous freshwater cyanobacterium best known for producing hepatotoxic microcystins; however, this common bloom-forming species also produces myriad biologically active and potentially deleterious other metabolites. Our understanding of the effects of these non-microcystin metabolites on fish is limited. In this study, we evaluated cytotoxicity of extracellular metabolites harvested from both microcystin-producing (MC+) and non-producing (MC-) strains of M. aeruginosa on rainbow trout (Oncorhynchus mykiss) cell lines derived from tissues of the brain, pituitary, heart, gonads, gills, skin, liver, and milt. We also examined the influence of M. aeruginosa exudates (MaE) on the expression of critical reproduction-related genes using the same cell lines. We found that exudates of the MC- M. aeruginosa strain significantly reduced viability in RTBrain, RTgill-W1, and RT-milt5 cell lines and induced significant cellular stress and/or injury in six of the eight cell lines-highlighting potential target tissues of cyanobacterial cytotoxic effects. Observed sublethal consequences of Microcystis bloom exposure occurred with both MC+ and MC- strains' exudates and significantly altered expression of developmental and sex steroidogenic genes. Collectively, our results emphasize the contributions of non-MC metabolites to toxicity of Microcystis-dominated algal blooms and the need to integrate the full diversity of M. aeruginosa compounds-beyond microcystins-into ecotoxicological risk assessments.

The sensitivity of multiple ecotoxicological assays for evaluating Microcystis aeruginosa cellular algal organic matter and contribution of cyanotoxins to the toxicity

Secondary metabolites of cyanobacteria and algae released during algal blooms often exhibit toxic effects, but only a small number of the metabolites are the subject of routine analytical screenings. Alternatively, ecotoxicological assays offer a better representation of the overall negative effects. The aim of this work was to compare multiple assays in their sensitivity towards cellular algal organic matter (COM) of the toxin-producing cyanobacterium Microcystis aeruginosa. Multiple endpoints were investigated: mortality, growth inhibition, bioluminescence inhibition, genotoxicity, endocrine-disrupting effects, oxidative stress, and the induction of ethoxyresorufin-O-deethylase (EROD). Three rainbow trout (Oncorhynchus mykiss) cell lines as well as representatives of bacteria, yeasts, algae, vascular plants, and crustaceans were employed, and the results were expressed per mg of dissolved organic carbon (DOC) in the COM. M. aeruginosa COM was toxic to the RTgill-W1, RTG-2, and RTL-W1 cell lines (EC₅₀ values ranging from 0.48 ± 0.02 to 1.9 ± 0.1 mg_DOC/L), to the crustacean Thamnocephalus platyurus (LC₅₀ = 20 ± 1 mg_DOC/L), and to Lepidium sativum (IC₅₀ = 241 ± 13 mg_DOC/L). In contrast, no effect was observed for bacteria and yeasts, and the growth of the alga Desmodesmus subspicatus was even stimulated. No genotoxicity, endocrine-disrupting effects or increase in oxidative stress or EROD activity was detected. The content of six microcystins (MC-LR, MC-RR, MC-YR, MC-LY, MC-LW, and MC-LF), anatoxin-a, cylindrospermopsin, and nodularin in the M. aeruginosa COM was determined by liquid chromatography-tandem mass spectrometry. An artificially prepared mixture of the detected cyanotoxins in the corresponding concentrations did not induce response in the O. mykiss cell lines and T. platyurus, suggesting that other cyanobacterial metabolites are responsible for the toxicity of M. aeruginosa.

Integration of fish cell cultures in the toxicological assessment of effluents

The pollution by industrial and municipal effluents are major sources of concerns. Fish cell cultures were applied in different strategies of the evaluation of effluents, particularly whole toxicity, toxicity identification evaluation and mode of action studies based in adverse outcome pathways. Whole effluent toxicity was evaluated using a battery of five model systems from four trophic levels: Daphnia magna was the most sensitive system, followed by the hepatoma fish cell line PLHC-1, the bacterium Allivibrio fischeri, the fibroblastic fish cell line RTG-2 and the algae Chlorella vulgaris, detecting a risk of eutrofization. The uptake of neutral red was more sensitive than the content of protein assay. The main morphological alterations observed were cell loss, hydropic degeneration, and a general loss of lysosomes and of their perinuclear distribution. The toxicity was characterized in PLHC-1 cells through toxicity identification evaluation, in which a partial reduction with graduation at pH 11, filtration, aeration and addition of thiosulfate or EDTA was shown; on the other hand, a low sorption in solid phase extraction suggested that the main responsible were not organic compounds. Consequently, it was not necessary to apply an effect directed analysis HPLC fractionation. In the chemical identification phase, Zn, Cd, As, Cu and Pb were quantified in decreasing concentrations. In the toxicity confirmation phase, a reconstituted sample and individual solutions, presented decreasing toxicity: Zn > Pb > As⁺⁵ > Cd > Cu > As⁺³, the global toxicity being explained by response addition. In the last step, the mode of action was investigated using five specific biomarkers. While metallothionein and succinate dehydrogenase activity were increased, no changes occurred for lysosomal function, acetylcholinesterase and EROD activities, the responsibility of the toxicity for the elements found being confirmed.

Suitability of a cytotoxicity assay for detection of potentially harmful compounds produced by freshwater bloom-forming algae

Detecting harmful bioactive compounds produced by bloom-forming pelagic algae is important to assess potential risks to public health. We investigated the application of a cell-based bioassay: the rainbow trout gill-w1 cytotoxicity assay (RCA) that detects changes in cell metabolism. The RCA was used to evaluate the cytotoxic effects of (1) six natural freshwater lake samples from cyanobacteria-rich lakes in central Ontario, Canada; (2) analytical standards of toxins and noxious compounds likely to be produced by the algal communities in these lakes; and (3) complex mixtures of compounds produced by cyanobacterial and chrysophyte cultures. RCA provided a measure of lake water toxicity that could not be reproduced using toxin or noxious compound standards. RCA was not sensitive to toxins and only sensitive to noxious compounds at concentrations higher than reported environmental averages (EC₅₀≥10³nM). Cultured algae produced bioactive compounds that had recognizable dose dependent and toxic effects as indicated by RCA. Toxicity of these bioactive compounds depended on taxa (cyanobacteria, not chrysophytes), growth stage (stationary phase more toxic than exponential phase), location (intracellular more toxic than extracellular) and iron status (cells in high-iron treatment more toxic than cells in low-iron treatment). The RCA provides a new avenue of exploration and potential for the detection of natural lake algal toxic and noxious compounds.

Oxidative stress induced by microcystin-LR on PLHC-1 fish cell line

Increasing evidences suggest that oxidative stress may play a significant role in microcystins (MCs) toxicity not only in mammals, but also in fish. In this regard, many in vivo studies have been performed but little is still known about the alteration of oxidative stress biomarkers on fish cell lines so far. In this study, the toxic effects of MC-LR were investigated in the fish cell line PLHC-1, derived from a hepatocellular carninoma of the topminnow Poeciliosis lucida, after 48 h of exposure. The different response of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR) and glutathione-S-transferase (GST), as well as lipid peroxidation (LPO) as a biomarker of oxygen-mediated toxicity, were assessed in PLHC-1 cells. The increases in the antioxidant enzymatic activities (SOD, GPx, and GST) as well as in LPO values observed evidenced the oxidative stress induced by MC-LR exposure. Moreover, the enhancements of these enzymes could suggest an adaptative response to combat oxidative injure induced by MC-LR, confirming that this mechanism is involved in the damage induced by MCs on fish cells.

Cytotoxicity of quahog parasite unknown (QPX) toward hard clam ( Mercenaria mercenaria) haemocytes and interactions between different pathogen isolates and host strains

The ability of pathogens to neutralize host defence mechanisms represents a fundamental requisite in the successful establishment of an infection. Host-pathogen interactions between quahog parasite unknown (QPX) and its hard clam host are poorly understood. Our prior in vivo investigations have shown that different QPX isolates display varying levels of pathogenicity toward clams. Similarly, field investigations and laboratory transmission studies revealed some variations in the susceptibility of different hard clam stocks to QPX infection. An in vitro approach was developed in this study to evaluate the toxicity of QPX cells and extracellular products toward haemocytes using a neutral red uptake assay. Results demonstrated that QPX produces virulence factors that are cytotoxic to M. mercenaria haemocytes. This cytotoxicity appears to be induced by clam factors, suggesting that it may play an important role in supporting QPX infection and proliferation within the host. Moreover, application of this technique to different QPX isolates and clam broodstocks indicates variations of QPX cytotoxicity in agreement with previous in vivo experiments, strengthening the existence of different QPX strains.

Acute Effects of Microcystins MC-LR and MC-RR on Acid and Alkaline Phosphatase Activities and Pathological Changes in Intraperitoneally Exposed Tilapia Fish (Oreochromis sp.)

Microcystins (MC) are frequently present in cyanobacterial blooms in rivers and lakes, increasing the risk of toxicity to both animals and humans. There more than eighty reported microcystins, and the present study was undertaken to determine whether MC-LR and MC-RR can induce different enzyme alterations and histopathological changes in tilapia fish ( Oreochromis sp.) exposed to a single intraperitoneal (i.p.) injection of the pure standards (MC-LR and MC-RR) at a dose of 500 μg/kg; the tilapia fish were then observed for seven days. The two MC variants caused significant changes in the activities of acid and alkaline phosphatases (ACP and ALP) in vital organs, showing a different response pattern. The livers and kidneys of fish injected with MC-LR were particularly affected. MC-RR induced a very pronounced increase of ACP in the kidney and a significant increase of ALP in the liver. Both MC variants caused pathological lesions in hepatic tissues, such as megalocytosis, necrotic process, and microvesicular steatosis, particularly in fish treated with MC-LR, and degenerative renal changes, glomerulopathy, were more severe in tilapias exposed to MC-RR. In addition, both microcystins also caused significant myopathy in the heart. In contrast, the gills did not show any change in enzyme activity or histopathological injury.

Acute and subacute toxic effects produced by microcystin-YR on the fish cell lines RTG-2 and PLHC-1

Approximately 80 microcystins (MCs) variants have been isolated in surface water worldwide. The toxicity of the most frequently MCs are encountered, MC-LR and MC-RR, has been extensively studied in humans and animals. However, studies dealing with MC-YR toxicity are still scarce. In this work, the toxic effects of MC-YR were investigated in the fish cell line PLHC-1, derived from a hepatocellular carcinoma of the topminnow Poeciliopsis lucida, and RTG-2 fibroblast-like cells derived from the gonads of rainbow trout Oncorhynchus mykiss. After 48 h, morphological and biochemical changes (total protein content, neutral red uptake and methylthiazol tetrazolium salt metabolization) were determined. The most sensitive endpoint for both cell lines was the reduction of total protein content, with EC(50) values of 35 microM for PLHC-1 cells and 67 microM for the RTG-2 cell line. Lysosomal function and methylthiazol tetrazolium salt metabolization were stimulated at low concentrations, while they decreased at high doses. Increase of piknotic cells, rounding effects, reduction in cell number and cell size, hydropic degeneration, and death mainly by necrosis but also by apoptosis were observed in the morphological study. Furthermore, PLHC-1 cells are more sensitive than RTG-2 cells to MC-YR exposure. These results were similar to those obtained when both cell lines were exposed for 24h to a Microcystis aeruginosa isolated strain extract containing MC-LR.

Toxicity of complex cyanobacterial samples and their fractions in Xenopus laevis embryos and the role of microcystins

This work evaluated the effects of various cyanobacterial fractions in Frog Embryo Teratogenesis Assay Xenopus (FETAX) with African clawed frog embryos. Fractions were prepared from five biomasses with different dominant genera (Microcystis, Aphanizomenon, Anabaena, Planktothrix) and different microcystin content. Effects of following fractions were investigated: (I) homogenate of complex cyanobacterial biomass, (II) cell debris (pellet) after centrifugation of complex biomass, (III) supernatant after centrifugation of complex biomass (= crude aqueous extract), (IV) permeate after passing of crude extract through C-18 column (fraction devoid of microcystins), and (V) eluate from C-18 column (containing microcystins, if present). Besides classical parameters evaluated in 96 h FETAX (mortality, growth inhibition, malformations), we have also assessed the effects on biochemical markers of oxidative stress and detoxification (glutathione pool, GSH; activity of glutathione peroxidase, GPx; glutathione reductase, GR; activity of glutathione-S-transferase, GST). Complex biomass (I) and aqueous extract (III) were generally the most toxic fractions in terms of mortality and growth inhibition, whereas eluates containing microcystins (V) were generally less toxic. On the other hand, the same fraction (eluates) induced significant malformations in low concentrations but the effects were not related to the content of microcystins. Biomarkers were affected in variable manner but no significant effect or clear relation to microcystin content was observed. Our data support the hypothesis that microcystins are not the only or major toxic compounds in the complex cyanobacterial samples (at least for some species) and that more attention should be paid to other components of complex cyanobacterial biomass including non-specific parameters such as oxygen content or toxic ammonia released during bacterial decay of organic material.