A systematic review on guanitoxin: General characteristics and ecological risks

Guanitoxin (GNT) is a potent cyanotoxin, with a relatively low number of publications (n = 51) compared to other cyanotoxins. Among the published studies, 35 % were on the effect of the toxin in animals, mainly in rodents and in vitro testing, followed by studies that identified species of cyanobacteria that produce GNT in aquatic systems and consequently accidental poisoning in wild and domestic animals (27 %). Studies that developed or tested methods for identifying the molecule, based on colorimetric and analytical techniques, represented 14 %, while 8 % were on GNT biosynthesis. Review articles and chemical isolation (6 %) and on the stability of the molecule (4 %) were the topics with the lowest number of publications. The results show the occurrence of GNT was identified mainly in eutrophic environments with a higher incidence in the American continent. Chemical characteristics of the molecule, such as short half-life in the environment, instability in solutions with alkaline pH values, temperature >23 °C, added to the lack of an analytical standard, are factors that make it difficult to identify and quantify it. However, GNT monitoring can be performed using LC-MS-MRM methods or genes specific to the newly discovered molecule.

Ecotoxicological assessment of guanitoxin-producing cyanobacteria in Danio rerio and Daphnia similis

Anthropogenic activity has dramatically deteriorated aquatic ecosystems in recent years. Such environmental alterations could change the primary producers' composition, exacerbating the proliferation of harmful microorganisms such as cyanobacteria. Cyanobacteria can produce several secondary metabolites, including guanitoxin, a potent neurotoxin and the only naturally occurring anticholinesterase organophosphate ever reported in the literature. Therefore, this study investigated the acute toxicity of guanitoxin-producing cyanobacteria Sphaerospermopsis torques-reginae (ITEP-024 strain) aqueous and 50% methanolic extracts in zebrafish (Danio rerio) hepatocytes (ZF-L cell line), zebrafish embryos (fish embryo toxicity - FET) and specimens of the microcrustacean Daphnia similis. For this, hepatocytes were exposed to 1-500 mg/L of the ITEP-024 extracts for 24 h, the embryos to 31.25-500 mg/L for 96 h, and D. similis to 10-3000 mg/L for 48 h. Non-target metabolomics was also performed to analyze secondary metabolites produced by the ITEP-024 using LC-MS/MS. Metabolomics indicated the guanitoxin presence just in the aqueous extract of the ITEP-024 and the presence of the cyanopeptides namalides, spumigins, and anabaenopeptins in the methanolic extract. The aqueous extract decreased the viability of zebrafish hepatocytes (EC(I)_50(24h) = 366.46 mg/L), and the methanolic extract was not toxic. FET showed that the aqueous extract (LC₅₀₍₉₆₎ = 353.55 mg/L) was more toxic than the methanolic extract (LC₅₀₍₉₆₎ = 617.91 mg/L). However, the methanolic extract had more sublethal effects, such as abdominal and cardiac (cardiotoxicity) edema and deformation (spinal curvature of the larvae). Both extracts immobilized daphnids at the highest concentration analyzed. However, the aqueous extract was nine times more lethal (EC(I)_50(48h) = 108.2 mg/L) than the methanolic extract (EC(I)_50(48h) = 980.65 mg/L). Our results showed an imminent biological risk for aquatic fauna living in an ecosystem surrounded by ITEP-024 metabolites. Our findings thus highlight the urgency of understanding the effects of guanitoxin and cyanopeptides in aquatic animals.

Negative Effects of Cyanotoxins and Adaptative Responses of Daphnia

The plethora of cyanobacterial toxins are an enormous threat to whole ecosystems and humans. Due to eutrophication and increases in lake temperatures from global warming, changes in the distribution of cyanobacterial toxins and selection of few highly toxic species/ strains are likely. Globally, one of the most important grazers that controls cyanobacterial blooms is Daphnia, a freshwater model organism in ecology and (eco)toxicology. Daphnia-cyanobacteria interactions have been studied extensively, often focusing on the interference of filamentous cyanobacteria with Daphnia's filtering apparatus, or on different nutritional constraints (the lack of essential amino acids or lipids) and grazer toxicity. For a long time, this toxicity only referred to microcystins. Currently, the focus shifts toward other deleterious cyanotoxins. Still, less than 10% of the total scientific output deals with cyanotoxins that are not microcystins; although these other cyanotoxins can occur just as frequently and at similar concentrations as microcystins in surface water. This review discusses the effects of different cyanobacterial toxins (hepatotoxins, digestive inhibitors, neurotoxins, and cytotoxins) on Daphnia and provides an elaborate and up-to-date overview of specific responses and adaptations of Daphnia. Furthermore, scenarios of what we can expect for the future of Daphnia-cyanobacteria interactions are described by comprising anthropogenic threats that might further increase toxin stress in Daphnia.

Exposure to toxic Microcystis via intact cell ingestion and cell crude extract differently affects small-bodied cladocerans

Cyanobacterial blooms are increasingly common in aquatic environments worldwide. These microorganisms cause concern due to their ability to produce cyanotoxins. Aquatic organisms, especially zooplankton, are exposed to cyanobacterial toxins by different routes, depending on the bloom phase. During cyanobacterial dominance, zooplankton is exposed to cyanotoxins through the ingestion of cyanobacterial cells, while at the bloom senescence, dissolved toxins are the most representative route. In this study, we assessed the effects of a microcystin-producing strain of Microcystis aeruginosa (NPLJ-4) on clones of the tropical small cladocerans Macrothrix spinosa (two clones) and Ceriodaphnia cornuta (one clone) exposed to intact cells and aqueous cell crude extracts. Short-term toxicity assays and life-table experiments were performed to assess the effects of the toxic M. aeruginosa on the survival and life history of the cladocerans. In the short-term toxicity assay, we found that cladocerans were more affected by intact cells. Both clones of M. spinosa were more affected when exposed to intact cells, while C. cornuta displayed about 5-fold more resistance. On the other hand, crude extracts had a low impact on cladocerans' survival. Also, we observed a significant decrease in survival, fecundity, and growth of animals exposed to sublethal and environmentally relevant concentrations of M. aeruginosa cellular biomass. However, even at high concentrations of dissolved microcystins, the crude extract did not have significant effects on the life history parameters of the cladocerans. Although they can be found during cyanobacterial bloom events, small-bodied cladocerans are still affected by toxic Cyanobacteria depending on the exposure route.

Reviewing Interspecies Interactions as a Driving Force Affecting the Community Structure in Lakes via Cyanotoxins

The escalating occurrence of toxic cyanobacterial blooms worldwide is a matter of concern. Global warming and eutrophication play a major role in the regularity of cyanobacterial blooms, which has noticeably shifted towards the predomination of toxic populations. Therefore, understanding the effects of cyanobacterial toxins in aquatic ecosystems and their advantages to the producers are of growing interest. In this paper, the current literature is critically reviewed to provide further insights into the ecological contribution of cyanotoxins in the variation of the lake community diversity and structure through interspecies interplay. The most commonly detected and studied cyanobacterial toxins, namely the microcystins, anatoxins, saxitoxins, cylindrospermopsins and β-N-methylamino-L-alanine, and their ecotoxicity on various trophic levels are discussed. This work addresses the environmental characterization of pure toxins, toxin-containing crude extracts and filtrates of single and mixed cultures in interspecies interactions by inducing different physiological and metabolic responses. More data on these interactions under natural conditions and laboratory-based studies using direct co-cultivation approaches will provide more substantial information on the consequences of cyanotoxins in the natural ecosystem. This review is beneficial for understanding cyanotoxin-mediated interspecies interactions, developing bloom mitigation technologies and robustly assessing the hazards posed by toxin-producing cyanobacteria to humans and other organisms.

Daphnia's Adaptive Molecular Responses to the Cyanobacterial Neurotoxin Anatoxin-α Are Maternally Transferred

Cyanobacterial blooms are an omnipresent and well-known result of eutrophication and climate change in aquatic systems. Cyanobacteria produce a plethora of toxic secondary metabolites that affect humans, animals and ecosystems. Many cyanotoxins primarily affect the grazers of phytoplankton, e.g., Daphnia. The neurotoxin anatoxin-α has been reported world-wide; despite its potency, anatoxin-α and its effects on Daphnia have not been thoroughly investigated. Here, we investigated the effects of the anatoxin-α-producing Tychonema on life-history parameters and gene expression of nicotine-acetylcholine receptors (NAR), the direct targets of anatoxin-α, using several D. magna clones. We used juvenile somatic growth rates as a measure of fitness and analyzed gene expression by qPCR. Exposure to 100% Tychonema reduced the clones' growth rates and caused an up-regulation of NAR gene expression. When 50% of the food consisted of Tychonema, none of the clones were reduced in growth and only one of them showed an increase in NAR gene expression. We demonstrate that this increased NAR gene expression can be maternally transferred and that offspring from experienced mothers show a higher growth rate when treated with 50% Tychonema compared with control offspring. However, the addition of further (anthropogenic) stressors might impair Daphnia's adaptive responses to anatoxin-α. Especially the presence of certain pollutants (i.e., neonicotinoids), which also target NARs, might reduce Daphnia's capability to cope with anatoxin-α.

Availability of Guanitoxin in Water Samples Containing Sphaerospermopsis torques-reginae Cells Submitted to Dissolution Tests

Guanitoxin (GNT) is a potent neurotoxin produced by freshwater cyanobacteria that can cause the deaths of wild and domestic animals. Through reports of animal intoxication by cyanobacteria cells that produce GNT, this study aimed to investigate the bio-accessibility of GNT in simulated solutions of the gastrointestinal content in order to understand the process of toxicosis promoted by GNT in vivo. Dissolution tests were conducted with a mixture of Sphaerospermopsis torques-reginae (Cyanobacteria; Nostocales) cultures (30%) and gastrointestinal solutions with and without proteolytic enzymes (70%) at a temperature of 37 °C and rotation at 100 rpm for 2 h. The identification of GNT was performed by LC-QqQ-MS/MS through the transitions [M + H]⁺m/z 253 > 58 and [M + H]⁺m/z 253 > 159, which showed high concentrations of GNT in simulated gastric fluid solutions (p-value < 0.001) in comparison to simulated solutions of intestinal content. The gastric solution with pepsin promoted the stability of GNT (p-value < 0.05) compared to the simulated solution of gastric fluid at the same pH without the enzyme. However, the results showed that GNT is also available in intestinal fluids for a period of 2 h, and solutions containing the pancreatin enzyme influenced the bio-accessibility of the toxin more compared to the intestinal medium without enzyme (p-value < 0.05). Therefore, the bio-accessibility of the toxin must be considered both in the stomach and in the intestine, and may help in the diagnosis and prediction of exposure and risk in vivo through the oral ingestion of GNT-producing cyanobacteria cells.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Use of cholinesterase activity as an ecotoxicological marker to assess anatoxin-a(s) exposure: Responses of two cladoceran species belonging to contrasting geographical regions

The specificity of cholinesterase (ChE) activity to detect the presence of anatoxin-a(s) and sublethal effects of a 7-day exposure to Anabaena spiroides extract containing anatoxin-a(s) were assessed in two freshwater cladoceran species. Activities of ChE of both Pseudosida ramosa and Daphnia magna can be used to indicate the presence of the neurotoxin anatoxin-a(s), but not for the hepatotoxic microcystin. Activity of ChE of P. ramosa, however, performed better as a biomarker of exposure to A. spiroides than that of D. magna. Furthermore, sublethal exposure to A. spiroides extract significantly inhibited the ChE activity in P. ramosa and negatively affected both individual and population endpoints. For D. magna, the inhibition of ChE activity was not related to effects at higher levels of biological organization, since no direct effect was recorded on the individual and population endpoints. The activity of ChE in P. ramosa also proved to be a good predictor of chronic effects of the A. spiroides extract at higher levels of biological organization, since 48-h ChE inhibition was linked to the sublethal effects on the individual and population. These relationships could not be established for D. magna. Since relationships between the effects of A. spiroides extract at different levels of biological organization were species-specific, it can be concluded that the choice of test organism interferes with the accuracy of the environment risk assessment of this neurotoxin and, hence, the use of native species is recommended for its assessment.