Neurotoxic cyanobacterial toxins

Uptake of a cyanotoxin, β-N-methylamino-L-alanine, by wheat (Triticum aestivum)

In order to study the uptake of the cyanobacterial neurotoxin β-N-methylamino-l-alanine (BMAA) into the crop plant Triticum aestivum during germination and primary growth imbibed grains and 7-day-old seedlings were irrigated with 100 and 1000µg l(-1) BMAA for 4 days and 100µg l(-1) BMAA for 28 days. Content of derivatized free and protein-associated BMAA in seedlings, root and shoot tissue, respectively, were analyzed by LC-MS/MS. Free BMAA was only detected in seedlings exposed to 1000µg l(-1) BMAA, whereas protein-associated BMAA was found at both exposure concentrations. Irrigation with 100µgl(-1) BMAA led to an uptake of the neurotoxin into roots and shoots and to immediate protein-association. In roots, protein-associated BMAA was detectable after 5 days with peaking amounts after 14 days. Longer exposure did not cause further accumulation in roots. In contrast, protein-associated BMAA was detected in shoot samples after only 1 day. In shoots the highest amounts of protein-associated BMAA were found after 28 days. In turn, in both plant compartments free BMAA was below the measurable concentration.

The effects of the toxic cyanobacterium Limnothrix (strain AC0243) on Bufo marinus larvae

Limnothrix (strain AC0243) is a cyanobacterium, which has only recently been identified as toxin producing. Under laboratory conditions, Bufo marinus larvae were exposed to 100,000 cells mL(-1) of Limnothrix (strain AC0243) live cultures for seven days. Histological examinations were conducted post mortem and revealed damage to the notochord, eyes, brain, liver, kidney, pancreas, gastrointestinal tract, and heart. The histopathological results highlight the toxicological impact of this strain, particularly during developmental stages. Toxicological similarities to β-N-Methylamino-L-alanine are discussed.

Cyanobacterial toxins: modes of actions, fate in aquatic and soil ecosystems, phytotoxicity and bioaccumulation in agricultural crops

The occurrence of harmful cyanobacterial blooms in surface waters is often accompanied by the production of a variety of cyanotoxins. These toxins are designed to target in humans and animals specific organs on which they act: hepatotoxins (liver), neurotoxins (nervous system), cytotoxic alkaloids, and dermatotoxins (skin), but they often have important side effects too. When introduced into the soil ecosystem by spray irrigation of crops they may affect the same molecular pathways in plants having identical or similar target organs, tissues, cells or biomolecules. There are also several indications that terrestrial plants, including food crop plants, can bioaccumulate cyanotoxins and present, therefore, potential health hazards for human and animals. The number of publications concerned with phytotoxic effects of cyanotoxins on agricultural plants has increased recently. In this review, we first examine different cyanotoxins and their modes of actions in humans and mammals and occurrence of target biomolecules in vegetable organisms. Then we present environmental concentrations of cyanotoxins in freshwaters and their fate in aquatic and soil ecosystems. Finally, we highlight bioaccumulation of cyanotoxins in plants used for feed and food and its consequences on animals and human health. Overall, our review shows that the information on the effects of cyanotoxins on non-target organisms in the terrestrial environment is particularly scarce, and that there are still serious gaps in the knowledge about the fate in the soil ecosystems and phytotoxicity of these toxins.

Detection of anatoxin-a and three analogs in Anabaena spp. cultures: new fluorescence polarization assay and toxin profile by LC-MS/MS

Anatoxin-a (ATX) is a potent neurotoxin produced by several species of Anabaena spp. Cyanobacteria blooms around the world have been increasing in recent years; therefore, it is urgent to develop sensitive techniques that unequivocally confirm the presence of these toxins in fresh water and cyanobacterial samples. In addition, the identification of different ATX analogues is essential to later determine its toxicity. In this paper we designed a fluorescent polarization (FP) method to detect ATXs in water samples. A nicotinic acetylcholine receptor (nAChR) labeled with a fluorescein derivative was used to develop this assay. Data showed a direct relationship between the amount of toxin in a sample and the changes in the polarization degree of the emitted light by the labeled nAChR, indicating an interaction between the two molecules. This method was used to measure the amount of ATX in three Anabaena spp. cultures. Results indicate that it is a good method to show ATXs presence in algal samples. In order to check the toxin profile of Anabaena cultures a LC-MS/MS method was also developed. Within this new method, ATX-a, retention time (RT) 5 min, and three other molecules with a mass m/z 180.1 eluting at 4.14 min, 5.90 min and 7.14 min with MS/MS spectra characteristic of ATX toxin group not previously identified were detected in the Anabaena spp. cultures. These ATX analogues may have an important role in the toxicity of the sample.

Recent progress in neuroactive marine natural products

This review covers the isolation, chemical structure, biological activity, structure activity relationships including synthesis of chemical probes, and pharmacological characterization of neuroactive marine natural products; 302 references are cited.

Learning from 8 years of regional cyanobacteria observation in Brittany in view of sanitary survey improvement

Cyanobacteria and cyanotoxins have been more and more studied during the last decades with regard to environment and health issues. More recently the consequences of climate change reinforced the need for research in view of a better management of cyanobacteria blooms. In this context the exploitation of the water quality survey of 26 recreational lakes in Brittany (north western France) between 2004 and 2011 is reported in this paper in order to encompass spatial and interannual patterns of cyanobacteria development at a regional scale. Starting from weekly data principally acquired during summertime, the links between cyanobacteria cell densities, toxin occurrences and interannual meteorological factors can give insights on the potential evolution of cyanobacterial crisis in the future. This study is part of a project aiming at a better understanding of potentially toxic cyanobacteria crisis occurrences in recreational waters, in order to improve predictive monitoring routines.

Phormidium autumnale growth and anatoxin-a production under iron and copper stress

Studies on planktonic cyanobacteria have shown variability in cyanotoxin production, in response to changes in growth phase and environmental factors. Few studies have investigated cyanotoxin regulation in benthic mat-forming species, despite increasing reports on poisoning events caused by ingestion of these organisms. In this study, a method was developed to investigate changes in cyanotoxin quota in liquid cultures of benthic mat-forming cyanobacteria. Iron and copper are important in cellular processes and are well known to affect growth and selected metabolite production in cyanobacteria and algae. The effect of iron (40–4000 μg L⁻¹) and copper (2.5–250 μg L⁻¹) on growth and anatoxin-a quota in Phormidium autumnale was investigated in batch culture. These concentrations were chosen to span those found in freshwater, as well as those previously reported to be toxic to cyanobacteria. Anatoxin-a concentrations varied throughout the growth curve, with a maximum quota of between 0.49 and 0.55 pg cell⁻¹ measured within the first two weeks of growth. Growth rates were significantly affected by copper and iron concentrations (P < 0.0001); however, no statistically significant difference between anatoxin-a quota maxima was observed. When the iron concentrations were 800 and 4000 μg L⁻¹, the P.autumnale cultures did not firmly attach to the substratum. At 250 μg L⁻¹ copper or either 40 or 4000 μg L⁻¹ iron, growth was suppressed.

In vitro and in vivo toxicity evaluation of the freshwater cyanobacterium Heteroleiblenia kuetzingii

Cyanobacteria are prokaryotic organisms characterized by their ability to produce secondary metabolites with different biological activities. The aim of this work was to evaluate the in vitro and in vivo toxicity of the cosmopolitan freshwater cyanobacterium H. kuetzingii. An extract from H. kuetzingii and cyanobacterial growth media were assessed for presence of intracellular and extracellular toxins by in vitro tests using primary cell cultures from mouse kidney and fibroblasts, cell lines A549 and 3T3, a fish cell line RTgill-W1 as well as by a traditional in vivo mouse bioassay. The presence of toxicity was compared with the ELISA and HPLC data for corresponding cyanotoxins. In vitro tests showed pronounced cytotoxicity of the cyanobacterium extract and growth medium in which H. kuetzingii released potential extracellular toxic compounds as the mammalian cells were significantly more sensitive to exposure compared to the fish cells. Histopathological analyses of the liver and kidneys of treated mice showed pathological changes such as leukocyte infiltration and necrosis, changes in the proximal and distal convoluted tubules, lack of differentiation of Bowman’s space, enlarged Bowman’s capsules and massive hemorrhages. ELISA and HPLC analyses confirmed the presence of saxitoxins and microcystins at low concentrations. In addition, the histological analyses suggest that H. kuetzingii produces other, yet unknown toxic metabolites. Monitoring efforts are therefore required to evaluate the potential hazard for the freshwater aquatic systems and possible public health implications associated with this cyanobacterium.

Shellfish toxins targeting voltage-gated sodium channels

Voltage-gated sodium channels (VGSCs) play a central role in the generation and propagation of action potentials in excitable neurons and other cells and are targeted by commonly used local anesthetics, antiarrhythmics, and anticonvulsants. They are also common targets of neurotoxins including shellfish toxins. Shellfish toxins are a variety of toxic secondary metabolites produced by prokaryotic cyanobacteria and eukaryotic dinoflagellates in both marine and fresh water systems, which can accumulate in marine animals via the food chain. Consumption of shellfish toxin-contaminated seafood may result in potentially fatal human shellfish poisoning. This article provides an overview of the structure, bioactivity, and pharmacology of shellfish toxins that act on VGSCs, along with a brief discussion on their pharmaceutical potential for pain management.

State of knowledge and concerns on cyanobacterial blooms and cyanotoxins

Cyanobacteria are ubiquitous microorganisms considered as important contributors to the formation of Earth's atmosphere and nitrogen fixation. However, they are also frequently associated with toxic blooms. Indeed, the wide range of hepatotoxins, neurotoxins and dermatotoxins synthesized by these bacteria is a growing environmental and public health concern. This paper provides a state of the art on the occurrence and management of harmful cyanobacterial blooms in surface and drinking water, including economic impacts and research needs. Cyanobacterial blooms usually occur according to a combination of environmental factors e.g., nutrient concentration, water temperature, light intensity, salinity, water movement, stagnation and residence time, as well as several other variables. These environmental variables, in turn, have promoted the evolution and biosynthesis of strain-specific, gene-controlled metabolites (cyanotoxins) that are often harmful to aquatic and terrestrial life, including humans. Cyanotoxins are primarily produced intracellularly during the exponential growth phase. Release of toxins into water can occur during cell death or senescence but can also be due to evolutionary-derived or environmentally-mediated circumstances such as allelopathy or relatively sudden nutrient limitation. Consequently, when cyanobacterial blooms occur in drinking water resources, treatment has to remove both cyanobacteria (avoiding cell lysis and subsequent toxin release) and aqueous cyanotoxins previously released. Cells are usually removed with limited lysis by physical processes such as clarification or membrane filtration. However, aqueous toxins are usually removed by both physical retention, through adsorption on activated carbon or reverse osmosis, and chemical oxidation, through ozonation or chlorination. While the efficient oxidation of the more common cyanotoxins (microcystin, cylindrospermopsin, anatoxin and saxitoxin) has been extensively reported, the chemical and toxicological characterization of their by-products requires further investigation. In addition, future research should also investigate the removal of poorly considered cyanotoxins (β-methylamino-alanine, lyngbyatoxin or aplysiatoxin) as well as the economic impact of blooms.

Zebrafish locomotor capacity and brain acetylcholinesterase activity is altered by Aphanizomenon flos-aquae DC-1 aphantoxins

Aphanizomenon flos-aquae (A. flos-aquae) is a source of neurotoxins known as aphantoxins or paralytic shellfish poisons (PSPs) that present a major threat to the environment and to human health. Generally, altered neurological function is reflected in behavior. Although the molecular mechanism of action of PSPs is well known, its neurobehavioral effects on adult zebrafish and its relationship with altered neurological functions are poorly understood. Aphantoxins purified from a natural isolate of A. flos-aquae DC-1 were analyzed by HPLC. The major analogs found in the toxins were the gonyautoxins 1 and 5 (GTX1 and GTX5; 34.04% and 21.28%, respectively) and the neosaxitoxin (neoSTX, 12.77%). Zebrafish (Danio rerio) were intraperitoneally injected with 5.3 and 7.61 μg STXeq/kg (low and high dose, respectively) of A. flos-aquae DC-1 aphantoxins. The swimming activity was investigated by observation combined with video at 6 timepoints from 1 to 24 h post-exposure. Both aphantoxin doses were associated with delayed touch responses, reduced head-tail locomotory abilities, inflexible turning of head, and a tailward-shifted center of gravity. The normal S-pattern (or undulating) locomotor trajectory was replaced by a mechanical motor pattern of swinging the head after wagging the tail. Finally, these fish principally distributed at the top and/or bottom water of the aquarium, and showed a clear polarized distribution pattern at 12 h post-exposure. Further analysis of neurological function demonstrated that both aphantoxin doses inhibited brain acetylcholinesterase activity. All these changes were dose- and time-dependent. These results demonstrate that aphantoxins can alter locomotor capacity, touch responses and distribution patterns by damaging the cholinergic system of zebrafish, and suggest that zebrafish locomotor behavior and acetylcholinesterase can be used as indicators for investigating aphantoxins and blooms in nature.

Neurotoxic emergencies

This article is intended for clinicians treating neurotoxic emergencies. Presented are causative agents of neurotoxic emergencies, many of which are easily mistaken for acute psychiatric disorders. Understanding the wide variety of agents responsible for neurotoxic emergencies and the neurotransmitter interactions involved will help the psychiatrist identify and treat this challenging population.

Zebrafish neurotoxicity from aphantoxins--cyanobacterial paralytic shellfish poisons (PSPs) from Aphanizomenon flos-aquae DC-1

Aphanizomenon flos-aquae (A. flos-aquae), a cyanobacterium frequently encountered in water blooms worldwide, is source of neurotoxins known as PSPs or aphantoxins that present a major threat to the environment and to human health. Although the molecular mechanism of PSP action is well known, many unresolved questions remain concerning its mechanisms of toxicity. Aphantoxins purified from a natural isolate of A. flos-aquae DC-1 were analyzed by high-performance liquid chromatography (HPLC), the major component toxins were the gonyautoxins1 and 5 (GTX1 and GTX5, 34.04% and 21.28%, respectively) and the neosaxitoxin (neoSTX, 12.77%). The LD50 of the aphantoxin preparation was determined to be 11.33 μg/kg (7.75 μg saxitoxin equivalents (STXeq) per kg) following intraperitoneal injection of zebrafish (Danio rerio). To address the neurotoxicology of the aphantoxin preparation, zebrafish were injected with low and high sublethal doses of A. flos-aquae DC-1 toxins 7.73 and 9.28 μg /kg (5.3 and 6.4 μg STXeq/kg, respectively) and brain tissues were analyzed by electron microscopy and RT-PCR at different timepoints postinjection. Low-dose aphantoxin exposure was associated with chromatin condensation, cell-membrane blebbing, and the appearance of apoptotic bodies. High-dose exposure was associated with cytoplasmic vacuolization, mitochondrial swelling, and expansion of the endoplasmic reticulum. At early timepoints (3 h) many cells exhibited characteristic features of both apoptosis and necrosis. At later timepoints apoptosis appeared to predominate in the low-dose group, whereas necrosis predominated in the high-dose group. RT-PCR revealed that mRNA levels of the apoptosis-related genes encoding p53, Bax, caspase-3, and c-Jun were upregulated after aphantoxin exposure, but there was no evidence of DNA laddering; apoptosis could take place by pathways independent of DNA fragmentation. These results demonstrate that aphantoxin exposure can cause cell death in zebrafish brain tissue, with low doses inducing apoptosis and higher doses inducing necrosis.

An overview on the marine neurotoxin, saxitoxin: genetics, molecular targets, methods of detection and ecological functions

Marine neurotoxins are natural products produced by phytoplankton and select species of invertebrates and fish. These compounds interact with voltage-gated sodium, potassium and calcium channels and modulate the flux of these ions into various cell types. This review provides a summary of marine neurotoxins, including their structures, molecular targets and pharmacologies. Saxitoxin and its derivatives, collectively referred to as paralytic shellfish toxins (PSTs), are unique among neurotoxins in that they are found in both marine and freshwater environments by organisms inhabiting two kingdoms of life. Prokaryotic cyanobacteria are responsible for PST production in freshwater systems, while eukaryotic dinoflagellates are the main producers in marine waters. Bioaccumulation by filter-feeding bivalves and fish and subsequent transfer through the food web results in the potentially fatal human illnesses, paralytic shellfish poisoning and saxitoxin pufferfish poisoning. These illnesses are a result of saxitoxin’s ability to bind to the voltage-gated sodium channel, blocking the passage of nerve impulses and leading to death via respiratory paralysis. Recent advances in saxitoxin research are discussed, including the molecular biology of toxin synthesis, new protein targets, association with metal-binding motifs and methods of detection. The eco-evolutionary role(s) PSTs may serve for phytoplankton species that produce them are also discussed.

The chemistry and biology of organic guanidine derivatives

The chemistry and biology of organic natural guanidines are reviewed, including the isolation, structure determination, synthesis, biosynthesis and biological activities of alkaloids, non-ribosomal peptides, guanidine-bearing terpenes, polyketides and shikimic acid derivatives from natural sources.

Neurotoxic effects evoked by cyanobacterial extracts suggest multiple receptors involved in electrophysiological responses of molluscan (CNS, heart) models

The responses of the snail central neurons (Helix pomatia, Lymnaea stagnalis) and the isolated Helix heart were characterized evoked by cyanobacterial extracts (Cylindrospermopsis raciborskii ACT strains) isolated from Lake Balaton (Hungary). The nicotinergic acetylcholine (ACh) receptors in the CNS (both excitatory and inhibitory) were blocked by the extracts of ACT 9502 and ACT 9505 strains and the anatoxin- a (homoanatoxin-a) producing reference strain of Oscillatoria sp. (PCC 6506), similar to the inhibitory effects of the pure anatoxin-a. The enhancement of the ACh responses by the ACT 9504 extract suggests additional, probably acetylcholine esterase inhibitory mechanisms. On the isolated Helix heart the crude ACT 9505 and PCC 6506 extracts evoked frequency increase and transient twitch contraction, opposite to the ACh evoked heart relaxation. Anatoxin-a similarly contracted the heart but did not increase its contration frequency. These data suggest the involvement of some non-cholinergic mechanisms, acting very likely by direct modulation of the electrical or contractile system of the isolated heart. Diversity of the effects evoked by the cyanobacterial extracts in the CNS and heart suggest pharmacologically different neuroactive components among the secondary metabolites of the cyanobacteria acting on both (anatoxin-a like) cholinergic and (unidentified) non-cholinergic receptors.

Biosynthetically intriguing chlorinated lipophilic metabolites from geographically distant tropical marine cyanobacteria

Five new vinylchlorine-containing metabolites, the lipoamides janthielamide A and kimbeamides A-C and the ketide-extended pyranone kimbelactone A, have been isolated from collections of marine cyanobacteria made in Curaçao and Papua New Guinea. Both janthielamide A and kimbeamide A exhibited moderate sodium channel blocking activity in murine Neuro-2a cells. Consistent with this activity, janthielamide A was also found to antagonize veratridine-induced sodium influx in murine cerebrocortical neurons. These lipoamides represent the newest additions to a relatively rare family of marine cyanobacterial-derived lipoamides and a new structural class of compounds exhibiting neuromodulatory activities from marine cyanobacteria.

A review of pharmacological and toxicological potentials of marine cyanobacterial metabolites

Novel toxic metabolites from marine cyanobacteria have been thoroughly explored. Biologically active and chemically diverse compounds that could be hepatotoxic, neurotoxic or cytotoxic, such as cyclic peptides, lipopeptides, fatty acid amides, alkaloids and saccharides, have been produced from marine cyanobacteria. Many reports have revealed that biosynthesis of active metabolites is predominant during cyanobacterial bloom formation. Marine cyanobacterial toxic metabolites exhibit important biological properties, such as interfering in signal transduction either by activation or blockage of sodium channels or by targeting signaling proteins; inducing apoptosis by disrupting cytoskeletal proteins; and inhibiting membrane transporters, receptors, serine proteases and topoisomerases. The pharmacological importance of these metabolites resides in their proliferation and growth-controlling abilities towards cancer cell lines and disease-causing potent microbial agents (bacteria, virus, fungi and protozoa). Besides their toxic and pharmacological potentials, the present review discusses structural and functional resemblance of marine cyanobacterial metabolites to marine algae, sponges and mollusks.

Anatoxin-a synthetase gene cluster of the cyanobacterium Anabaena sp. strain 37 and molecular methods to detect potential producers

Cyanobacterial mass occurrences are common in fresh and brackish waters. They pose a threat to water users due to toxins frequently produced by the cyanobacterial species present. Anatoxin-a and homoanatoxin-a are neurotoxins synthesized by various cyanobacteria, e.g., Anabaena, Oscillatoria, and Aphanizomenon. The biosynthesis of these toxins and the genes involved in anatoxin production were recently described for Oscillatoria sp. strain PCC 6506 (A. Méjean et al., J. Am. Chem. Soc. 131:7512-7513, 2009). In this study, we identified the anatoxin synthetase gene cluster (anaA to anaG and orf1; 29 kb) in Anabaena sp. strain 37. The gene (81.6% to 89.2%) and amino acid (78.8% to 86.9%) sequences were highly similar to those of Oscillatoria sp. PCC 6506, while the organization of the genes differed. Molecular detection methods for potential anatoxin-a and homoanatoxin-a producers of the genera Anabaena, Aphanizomenon, and Oscillatoria were developed by designing primers to recognize the anaC gene. Anabaena and Oscillatoria anaC genes were specifically identified in several cyanobacterial strains by PCR. Restriction fragment length polymorphism (RFLP) analysis of the anaC amplicons enabled simultaneous identification of three producer genera: Anabaena, Oscillatoria, and Aphanizomenon. The molecular methods developed in this study revealed the presence of both Anabaena and Oscillatoria as potential anatoxin producers in Finnish fresh waters and the Baltic Sea; they could be applied for surveys of these neurotoxin producers in other aquatic environments.

Phosphate uptake performance of bacteria isolated from a full-scale Izmir municipal wastewater treatment plant

This study investigated the phosphate uptake capacities of bacteria isolated from aerobic and anaerobic phosphate removal tanks at a municipal wastewater treatment plant in Izmir, Turkey, removing chemical oxygen demand to nitrogen (COD-N) and phosphorus (P) on a full-scale basis. Conventional plating techniques and an enrichment culture method were used to isolate the colonies, with a total of 91 monoculture isolates from the sludge samples being subjected to phosphate uptake studies. A total of 64 of these isolates had high phosphate uptake capacities ranging from 3.7 x 10(10) to 1.0 x 10(-12) mg PO4(3-) cell(-1), and only 11 of the strains with high phosphate uptake were Gram-negative. The highest phosphorus uptake value was 3.7 x 10(-10) mg PO4(-3) cell(-1), which was achieved by Gram-positive bacteria. Gram-negative strains were identified as Acinetobacter baumannii with a 99% probability and as Pseudomonas aeruginosa with a 96-98% probability (API 20 NE).

Diversity and impact of prokaryotic toxins on aquatic environments: a review

Microorganisms are ubiquitous in all habitats and are recognized by their metabolic versatility and ability to produce many bioactive compounds, including toxins. Some of the most common toxins present in water are produced by several cyanobacterial species. As a result, their blooms create major threats to animal and human health, tourism, recreation and aquaculture. Quite a few cyanobacterial toxins have been described, including hepatotoxins, neurotoxins, cytotoxins and dermatotoxins. These toxins are secondary metabolites, presenting a vast diversity of structures and variants. Most of cyanobacterial secondary metabolites are peptides or have peptidic substructures and are assumed to be synthesized by non-ribosomal peptide synthesis (NRPS), involving peptide synthetases, or NRPS/PKS, involving peptide synthetases and polyketide synthases hybrid pathways. Besides cyanobacteria, other bacteria associated with aquatic environments are recognized as significant toxin producers, representing important issues in food safety, public health, and human and animal well being. Vibrio species are one of the most representative groups of aquatic toxin producers, commonly associated with seafood-born infections. Some enterotoxins and hemolysins have been identified as fundamental for V. cholerae and V. vulnificus pathogenesis, but there is evidence for the existence of other potential toxins. Campylobacter spp. and Escherichia coli are also water contaminants and are able to produce important toxins after infecting their hosts. Other bacteria associated with aquatic environments are emerging as toxin producers, namely Legionella pneumophila and Aeromonas hydrophila, described as responsible for the synthesis of several exotoxins, enterotoxins and cytotoxins. Furthermore, several Clostridium species can produce potent neurotoxins. Although not considered aquatic microorganisms, they are ubiquitous in the environment and can easily contaminate drinking and irrigation water. Clostridium members are also spore-forming bacteria and can persist in hostile environmental conditions for long periods of time, contributing to their hazard grade. Similarly, Pseudomonas species are widespread in the environment. Since P. aeruginosa is an emergent opportunistic pathogen, its toxins may represent new hazards for humans and animals. This review presents an overview of the diversity of toxins produced by prokaryotic microorganisms associated with aquatic habitats and their impact on environment, life and health of humans and other animals. Moreover, important issues like the availability of these toxins in the environment, contamination sources and pathways, genes involved in their biosynthesis and molecular mechanisms of some representative toxins are also discussed.