Bloom Dynamics of Cyanobacteria and Their Toxins: Environmental Health Impacts and Mitigation Strategies

1Cyanobacteria are considered the most primitive groups of photosynthetic prokaryotes and possibly appeared on the Earth about 3.5 billion years ago. They are ubiquitous in nature and thrive in a variety of ecological niches ranging from desert to hot springs and ice-cold water. Most cyanobacteria are an immense source of several secondary natural products with applications in the food, pharmaceuticals, cosmetics, agriculture, and energy sectors. Moreover, some species of cyanobacteria grow vigorously and form a dominant microflora in terms of their biomass and productivity in specific ecosystems. Bloom formations (Figure 1) due to excessive growth of certain cyanobacteria followed by the production of toxic compounds have been reported in many eutrophic to hypertrophic lakes, ponds, and rivers throughout the world. A range of toxic secondary compounds, called cyanotoxins, have been reported from cyanobacteria inhabiting freshwater and marine ecosystems. These toxic compounds are highly detrimental for survival of several aquatic organisms, wild and/or domestic animals, and humans. Aquatic organisms, including plants and animals, and phyto/zoo-planktons inhabiting under toxic bloom rich ecosystems, are directly exposed to the harmful effects of different cyanotoxins. The intoxication occurring in wild and/or domestic animals and humans is either due to direct ingestion of cells of toxin producing cyanobacteria or the consumption of drinking water contaminated with cyanotoxins. The toxicity of different cyanotoxins is directly proportional to the growth of cyanobacteria and the extent of their toxin production. It has been shown that the growth of different cyanobacteria and their toxin biosynthesis is greatly influenced by different abiotic factors such as light intensity, temperature, short wavelength radiations, pH, and nutrients. Global warming and temperature gradients can significantly change species composition and favor blooms of toxic phytoplanktons.

2It has been assumed that cyanotoxins play an important role in chemical defense mechanisms giving survival advantages to the cyanobacteria over other microbes or deterring predation by higher trophic levels. Cyanotoxins may also take part in chemical signaling. Overall, information regarding the specific role(s) of cyanotoxins in the life of individual cyanobacteria or their ecological and biotechnological operations is still very limited and needs extensive research. In the present review, we summarize the recent advances on bloom dynamics, cyanotoxin production, and mitigation strategies as well as their consequences on environmental health perspectives.

3Occurrence of toxic cyanobacterial blooms (cyanoblooms) is a serious global problem which affects the water quality due to the production and accumulation of different cyanotoxins and other malodorous compounds. These blooms may cause an increase of biological oxygen demand (BOD) and anoxia in the water bodies, and death of aquatic life. The factors contributing to the worldwide occurrence of cyanobacterial blooms are still debatable. Nevertheless, cultural eutrophication from domestic, industrial, and agricultural wastes as well as global climate change can play a major role in the global expansion of harmful algal blooms and toxin production (;;;;;;;;; Figure 2). Excessive loads of certain inorganic and/or organic nutrient concentrations are considered as strong risk factors for bloom promotion both in fresh and marine water habitats. The anthropogenically mediated change in the N/P ratio has frequently been interrelated to the appearance of cyanobacterial blooms. The phosphorus concentration was found as a primary regulating factor for increased cyanobacterial growth and changes of genotypes, both of which were found to be closely related to the water temperature, signifying the role of eutrophication in the occurrence of toxic blooms. Recently, presented a novel conceptual model linking anoxia, phosphorus (P), nitrogen (N), iron (Fe), and sulfate to the formation of harmful cyanobacterial blooms across three gradients, i.e., nutrients, salinity, and acidity. Continued transfer of sediments to a water body may block the natural flow of water and enrich the dissolved organic carbon and other compounds leading to potential risk of bloom formation.

4Global climate change followed by changes in air/water temperature gradients, as well as increased nutrient precipitation can affect the cyanobacterial bloom formation and production of different cyanotoxins. Several environmental factors related to the dynamics of the abundance of toxic cyanobacterial bloom formation have been verified. Warm and calm weather and low turbulence can enhance the formation of cyanobacterial blooms. Increased emission of ozone depleting substances (ODSs), due to huge burning of fossil-fuels and concomitant changes in air temperature, may promote the water cyanobacterial growth. As a result of climate change, the frequent droughts in summer as well as flash-flooding may lead to abandoned nutrient discharges from urban areas to unloading water bodies such as ponds, lakes, ditches, and estuaries with the consequence of the augmentation of toxic blooms and the increase of the BOD of a water reservoir. Nitrogen limitation under drought condition may cause a shift from non-N2-fixing to N2-fixing cyanobacteria leading to an increase in biologically available nitrogen and a subsequent production of cyanotoxins. The increased salination due to summer droughts, rising sea levels, wind flow, and common practices of the use of freshwater for agricultural irrigation, all have led to the origin and existence of several salt tolerant freshwater toxic cyanobacteria as evidenced by an increased number of blooms in brackish waters. Under increased temperature and low wind mixing, the water column becomes stagnant and a large number of buoyant cyanobacteria move upward at the water surface causing dense surface blooms to fulfill their photosynthetic needs (;,; Figure 2). It has been established that dense cyanobacterial blooms require excessive CO2 to support their photosynthetic growth. Furthermore, global climate change due to anthropogenically released ODS and increased atmospheric CO2 levels can minimize carbon limitation of photosynthetic growth leading to increased algal biomass productions in the water reservoirs. Moreover, increased CO2 levels may increase the problems associated with the harmful cyanobacteria in eutrophic lakes. Recently, reported that rising CO2 levels may result in a marked intensification of phytoplankton blooms in eutrophic and hypertrophic waters. Climate change, which is predicted to lead the changes in rainfall patterns along with an increase in temperature may also influence the occurrence and severity of toxic cyanobacterial blooms due to a significant impact on inland water resources. It has been suggested that UV-B radiation may significantly influence strain composition of cyanobacterial blooms in favor of microcystin (MC) producers. Several species/strains of bloom forming cyanobacteria produce different toxic peptides and alkaloids (Table 1), which are a major threat to the safe drinking water and pose a serious threat to the global environmental and human health. Until now, a number of views have been given for world-wide occurrence of cyanobacterial blooms; however, the exact mechanisms and the role of different environmental factors regulating the bloom dynamics are disputable and yet to be understood.

5Our understanding of the responses of various environmental factors associated with climate change and their impact on marine/freshwater ecosystems is based on several experimental and/or inferential data. From the above discussions, it is clear that the appearance of a cyanobacterial bloom is the consequence of several coherent signals. It is utmost important to unravel the specific effects of nutrient enrichment and other global climate change on our aquatic ecosystem, and to establish the facts on how the structure and function of an ecosystem can be maintained. Moreover, if the existing level of anthropogenically induced nutrient loading in the water bodies and environmental warming continues, multiple-fold increase in algal bloom followed by contamination of our aquatic ecosystem by several toxic substances is expected in future. Henceforth, most conceptual and empirical research on the triggers of cyanobacterial blooms is needed to understand the multifarious set of situations that influence the worldwide incidence of toxic cyanoblooms.

6Cyanobacteria produce a wide range of toxic secondary compounds causing human and domestic/wildlife intoxication. A number of bloom forming cyanobacteria from diverse habitats have been reported to produce different toxins. Chemically, the cyanotoxins are divided into three main groups, i.e., cyclic peptides (MCs and nodularins), alkaloids (anatoxin-a, anatoxin-a(s), saxitoxins, cylindrospermopsin, aplysiatoxin, lyngbiatoxin-a), and lipopolysaccharides (LPSs; ). However, based on biological effects, the cyanobacterial toxins can be classified into five functional groups such as hepatotoxins, neurotoxins, cytotoxins, dermatotoxins, and irritant toxins.

7Among the different cyanobacterial toxins, MCs are the most frequently occurring cyanotoxins in surface as well as drinking water and widely investigated hepatotoxins. MCs are cyclic heptapeptides (Figure 3) produced by several strains of cyanobacteria (;;;;; Table 1). Currently, more than 90 variants of MCs are known, all with the general structure cyclo-(D-Ala-X-D-MeAsp-Z-Adda- D-Glu- Mdha), X and Z being variable L-amino acids. On the basis of acute toxicity, microcystin-LR (MC-LR) is considered the most potent hepatotoxin.

8Black – tailoring enzymes, red – polyketide synthases, blue – non-ribosomal peptide synthetases, light black – non-microcystin synthetase, green – ABC transporter (adapted from;;;; Gene cluster not drawn to scale).

9Microcystin is synthesized non-ribosomally by large multi-enzyme complexes comprising different modules including non-ribosomal peptide synthetases (NRPSs) as well as polyketide synthases (PKSs), and several tailoring enzymes. The gene cluster responsible for MC biosynthesis has been identified in different cyanobacteria. In the cyanobacterium Microcystis aeruginosa PCC7806, the MC gene clusters spans 55 kb of DNA and is composed of 10 (mcyABCDEFGHIJ) bidirectionally transcribed open reading frames (ORFs) arranged in two divergently transcribed operons, mcyA-C and mcyD-J (; Figure 3). The assembly of MC begins with the activation of a phenylalanine-derived phenyl propionate starter unit at the NRPS/PKS hybrid enzyme McyG. The gene clusters encoding MC biosynthesis sequence from the Microcystis, Planktothrix, and Anabaena species revealed that the arrangements of ORFs in the mcy cluster vary among different genera. However, a high sequence similarity between the mcy gene clusters of different genera suggests a common ancestor for MC synthesis.

10Similar to MCs, cyclic pentapeptide toxic compounds, nodularins (NODs; Figure 3) represent the second group of hepatotoxins produced by the cyanobacteria Nodularia and Nostoc. At present, more than seven variants of NOD have been reported. Both hepatotoxins (MCs and NODs) contain a unique hydrophobic amino acid, Adda (2S,3S,8S,9S-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-deca-4,6-dienoic acid). Chemically, NODs differ from MCs in terms of the absence of two core amino acids and have N-methyldehydrobutyrine (Mdhb) instead of N-methyldehydroalanine (Mdha; ). Similar to MCs, NODs are also produced non-ribosomally from nda gene clusters by means of NRPS-PKS enzyme systems (; Figure 3). In the cyanobacterium Nodularia spumigena NSOR10, the locus of nda gene clusters (48 kb) consists of nine ORFs (ndaA–I) transcribed from a bidirectional regulatory promoter region. Moreover, MCs and NODs show similar biological activity in spite of their different chemical structures. These cyclic peptides inhibit the specific protein serine/threonine phosphatases-1 (PP1) and -2A (PP2A) which are important regulatory enzymes in eukaryotic cells.

11A number of toxic alkaloids have been found in different cyanobacteria. The alkaloids anatoxin-a (MW = 165 Da) and its homolog homoanatoxin-a (MW = 179 Da) are fast-acting neurotoxins, also known as fast death factors (FDFs). Anatoxin-a (Figure 4) was first isolated from Anabaena flos-aquae and so far has been found in several cyanobacteria such as A. circinalis, A. planctonica, A. spiroides, Aphanizomenon, Cylindrospermum, Planktothrix, and M. aeruginosa (;; Table 1). The alkaloid homoanatoxin-a has a methylene group at C-2 instead of the acetyl group (Figure 4) and structurally resembles anatoxin-a. Homoanatoxin-a has been isolated from the cyanobacteria Oscillatoria (Planktothrix) formosa, Phormidium formosum, Anabaena, and Raphidiopsis mediterranea. Another homolog of anatoxin, anatoxin-a(s) (MW 252 Da; Figure 4), isolated from A. flos-aquae and A. lemmermannii, is a potent acetylcholinesterase (AChE) inhibitor but more lethal than anatoxin-a. It is synthesized in the cell from ornithine via putrescine catalyzed by the enzyme ornithine decarboxylase. Moreover, the partial genome sequencing demonstrated the presence of putative gene cluster encoding the biosynthetic pathway of anatoxin-a and homoanatoxin-a in cyanobacteria such as Oscillatoria PCC 6506 and Anabaena strain 37 (; Figure 4).

12Saxitoxin and its analogs (e.g., neosaxitoxin; Figure 5) are a group of carbamate alkaloid toxins which all are highly potent neurotoxins. These are tricyclic compounds, consisting of a tetrahydropurine group and two guanidine subunits, commonly called paralytic shellfish poisons (PSPs). Currently, about 27 variants of saxitoxins have been found in different cyanobacteria such as Aphanizomenon, Anabaena flos-aquae, Anabaena circinalis, Lyngbya wollei, and Cylindrospermopsis raciborskii (Table 1). Regulation of saxitoxin biosynthetic pathway and characterization of some enzymes involved are not well-studied. However, it has been postulated that biosynthesis of saxitoxin depends on the multifunctional PKS enzyme, SxtA. The saxitoxin biosynthetic gene cluster (25.7–36 kb) includes 33 genes, reported in cyanobacteria such as Cylindrospermopsis raciborskii (strain T3), Anabaena circinalis (strain AWQC131C), Aphanizomenon strain NH-5, Lyngbya wollei, and Raphidiopsis brookii (strain D9;;,;;;; Figure 5). The positions of genes encoding biosynthetic enzymes, transporters, and regulatory proteins within the cluster differ among the different cyanobacterial strains dicsussed above. Moreover, the toxic profile expressed in different strains is determined by the position and presence, or absence, of specific genes in the respective clusters.

13The cyanotoxin cylindrospermopsin (CYN: MW 415 Da) is a polyketide-alkaloid having a tricyclic guanidine moiety and sulfate groups (Figure 6). Presently, some analogs of CYN such as deoxy-cylindrospermopsin, demethoxy-cylindrospermopsin and 7-epicylindrospermopsin have been identified in the cyanobacteria C. raciborskii and Aphanizomenon ovalisporum. The CYN variant 7-epicylindrospermopsin differs due to the orientation of the hydroxyl group close to the uracil moiety, and the other variant deoxy-cylindrospermopsin is characterized by a missing oxygen atom related to the initial hydroxyl group close to uracil moiety. Moreover, a number of cyanobacteria such as Cylindrospermopsis raciborskii, Aphanizomenon ovalisporum, Aphanizomenon flos-aquae, Anabaena lapponica, Anabaena bergii, Lyngbya wollei, Umezakia natans, Raphidiopsis curvata, and Oscillatoria (Planktothrix) have been reported to produce CYN and its analogs. reported the presence of the cyanotoxins CYN and deoxy-CYN from the cyanobacterium Raphidiopsis mediterranea FSS1-150/1 of a eutrophic reservoir in Queensland, Australia. CYN shows hepatotoxic, nephrotoxic, and cytotoxic effects and is a potential carcinogen owing to the inhibition of glutathione, cytochrome P450 and protein synthesis. The gene cluster (cyr) encoding the enzymes of the CYN biosynthesis (Figure 6) has been reported to be present in several cyanobacteria such as C. raciborskii, Aphanizomenon strain 10E6, and Oscillatoria PCC 6506. The arrangements of genes and flanking regions differ across genera; however, all the gene clusters are highly conserved with respect to the nucleotide sequence of orthologous genes. In case of the cyanobacterium C. raciborskii AWT205, the cyr gene cluster (42 kb) encodes 15 ORFs (cyrA-O). The biosynthesis of CYN is initiated by an amidinotransferase and completed by NRPS-PKS-type enzymes in combination with tailoring enzymes. As stated above, the gene cluster for CYN biosynthesis has been sequenced from several cyanobacteria; however, few studies have been conducted on its transcriptional organization and promoter structure.

14The endotoxins LPSs consist of an internal acylated glycolipid (lipid-A), core domain (an oligosaccharide) and an outer polysaccharide (O-antigen) chain. In general, the fatty acid component (lipid-A) of LPS is responsible for the toxic actions such as irritant and allergenic responses in human and animal tissues. The LPSs present in cyanobacteria differ from those in enteric bacteria by having a larger variety of long chain unsaturated fatty acids and hydroxy fatty acids and the lack of phosphate. Moreover, there is substantial diversity of LPSs composition among the cyanobacteria, although variations are basically related to phylogeny. Different genera of cyanobacteria have distinct LPSs compositions conserved within the particular genus. Several cyanobacteria such as Anacystis nidulans, Microcystis, Anabaena, Spirulina, and Oscillatoria all have been reported to produce LPS toxin. The structure of the lipid-A subunit in the cyanobacterial LPS molecule has not been clearly identified, and furthermore, the exact mechanism of LPS toxicity produced by cyanobacteria is still unknown.

15Besides the above mentioned cyanotoxins, a number of toxins such as aplysiatoxin, kalkitoxin, antillatoxin, lyngbyatoxins, cyanopeptolin, aurilides, and jamaicamides have been reported to be present in different cyanobacteria in fresh and/or marine water habitats (Figure 7). A phenolic bislactone alkaloid aplysiatoxin has been reported from several cyanobacteria such as Lyngbia majuscula, Schizothrix calcicola, Trichodesmium erythraeum, and Oscillatoria nigroviridis. Aplysiatoxin and debromoaplysiatoxin (Figure 7) are potent tumor promoters and protein kinase C activators and show signs of several lethal effects. Moreover, an analog of the tumor-promoting aplysiatoxin has been reported as an antineoplastic agent rather than a tumor-promoting substance. Recently, the analogs of aplysiatoxin debromoaplysiatoxin and anhydrodebromoaplysiatoxin, as well as two new analogs, 3-methoxyaplysiatoxin and 3-methoxydebromoaplysiatoxin have been reported from the marine cyanobacterium Trichodesmium erythraeum. The alkaloid lyngbyatoxin, a prenylated cyclic dipeptide compound, was isolated from Lyngbya majuscula and has several similarities with aplysiatoxin in its mechanism of toxicity and both are potent tumor promoters. Kalkitoxin (Figure 7) is a lipopeptide neurotoxin produced by some species of cyanobacteria such as L. majuscula. The antillatoxin is an ichthyotoxic cyclic depsipeptide isolated from L. majuscula. A number of bioactive peptides such as microviridins, microginins, cyanopeptolides, and β-N-methylamino-L-alanine (BMAA; Figure 7) have also been reported from diverse cyanobacteria, but their toxicological profiles and impacts on the environment as well as human health are not known. Moreover, the cyanobacterial neurotoxin, BMAA has been suggested to function as a causative agent for certain neurodegenerative diseases. The compound curacin-A, isolated from L. majuscula, exhibited potent anti-proliferative and cytotoxic activity against colon, renal, and breast cancer derived cell lines. A cyanobacterial toxin cyanopeptolin (CP1020) produced by Microcystis and Planktothrix strains was found to cause transcriptional alterations of genes involved in DNA damage and repair. Recently, two new cyanobacterial peptides named micropeptins 1106 and 1120 were reported from cyanobacterial blooms in North Carolina’s Cape Fear River. However, their biological activities have not yet been determined. Moreover, several studies indicate the presence of several additional, still unidentified and not characterized biotoxins in cyanobacterial blooms.

16The increased incidence of toxic cyanobacterial blooms is posing potential risks to aquatic ecosystem as well as human and animal health. Cyanotoxins may cause several harmful effects on humans or animals either through direct contact or by means of intake of untreated contaminated water and food. Aquatic organisms may be affected either through direct ingestion of toxic cyanobacterial cells or through contact with cyanotoxins. It has been established that intake of contaminated water or food is a key route for cyanotoxin intoxication. Several secondary compounds have been reported to have their toxic effects on different organisms ranging from plant to animals. In the subsequent section we have focused on the adverse toxic effects of some common cyanotoxins on aquatic/wild animals and humans.

17The cyanotoxin MCs are well-known for their toxic effects. MCs can affect the cellular system through disorganization of cytoskeleton, cell proliferation, genome damage, inhibition of enzyme activity, imprecise mitotic cell division, loss of membrane integrity, oxidative stress, and lipid peroxidation. To know the detailed mechanisms or mode of action of MCs, readers are referred to the recent review by. MCs act by blocking protein PP1 and -2A, causing toxicity at the hepatic level. It has been demonstrated that MC-LR can induce reproductive as well as cardio-toxicity in animals. MC-LR was found to cause normocyte anemia and the bone marrow injury, and also affected the immune system of rabbits. Moreover, a number of fatal poisonings of MCs regarding the health risk of domestic and wild animals, birds, fish, and several other aquatic as well as terrestrial organisms have been reported worldwide. The mass mortalities of Lesser Flamingos were reported at Lake Bogoria, Kenya due to MC intoxication. A new episode of cyanotoxin (MC-LR, -YR, and -RR) intoxication and mass mortalities of Lesser Flamingos (Phoeniconaias minor Geoffroy) have also been reported at Lake Manyara in Tanzania. At least 6,000 birds belonging to 47 species, including endangered species such as the marbled teal (Marmaronetta angustirostris) and white-headed duck (Oxyura leucocephala), died due to MC-LR intoxication at the Doñana National Park, Spain.

18Despite numerous reports of cyanotoxins impact on the aquatic organisms and wild or domestic animals, the epidemiological facts for cyanotoxins intoxication in humans are very limited. Recent studies have established the cytotoxic and genotoxic potentials of various cyanotoxins including MCs. The use of untreated water contaminated with cyanobacterial blooms and MCs resulted in normocytic anemia, liver failure and several other symptoms such as nausea, vomiting, and acute liver damage leading to human death in a hemodialysis center in Caruaru, Brazil. The use of MC-contaminated water can be a potential risk factor for liver and colorectal cancer among humans. Moreover, MCs may cause hepatotoxicity and neurotoxicity, kidney impairment, allergies and eye, ear and skin irritation, and certain gastrointestinal disorders such as nausea/vomiting and diarrhea in humans.

19As stated above, the cyanotoxin NODs have chemical structure as well as mechanisms of action similar to those of MCs; however, NODs have not been studied as extensively as MCs. NODs are a potent inhibitor of protein phosphatase 1 and 2A and show accumulative toxicity and tumor formation. The toxic effects of NODs have also been investigated in fish. In the flatfish Platichthys flesus, NODs induced oxidative stress as indicated by a decrease of GST and CAT activity resulting in increased vulnerability of the cells to reactive oxygen species (ROS; ). NOD can also induce apoptosis and hyperphosphorylation of signaling proteins in cultured rat hepatocytes. Nevertheless, not much toxicological data are available for NODs carcinogenicity in humans.

20A cytotoxic alkaloid CYN can irreversibly inhibit the biosynthesis of protein and glutathione leading to cell death. A Cylindrospermopsis bloom episode was found to cause cattle mortalities and human poisonings in north Queensland. Moreover, a number of disorders such as damage to liver, kidney, thymus, and heart, as well as hepatic and renal toxicity were observed in mice. CYN may induce DNA strand breaks and possibly disrupt the kinetochore spindle, leading to chromosome loss, specifying its clastogenic and aneugenic action. In primary rat hepatocytes, CYN has been shown to inhibit protein and glutathione synthesis and induce apoptosis. Recently, studied the effects of CYN on human intestinal Caco-2 cells and reported that CYN can modulate different biological functions by overexpressing the genes encoding proteins involved in DNA damage repair and transcription including modifications of nucleosomal histones. It has been shown that CYN may cause a decrease in glutathione synthesis and induce oxidative stress in fish. Indeed, CYN can accumulate in various organs of fish, leading to deleterious effects on their normal physiology and biochemistry. CYN may interfere with the basic functions of fish phagocytic cells and as a consequence, influence the fish immunity.

21A number of neurotoxic alkaloids from cyanobacteria have been reported, exerting their action on the neuromuscular system by blocking skeletal and respiratory muscles leading to respiratory failure. The cyanotoxins such as anatoxins, saxitoxins, antillatoxin, kalkitoxin, and jamaicamide are major groups of neurotoxic compounds. It has been established that anatoxin-a is a potent depolarizing neuromuscular blocking agent which acts by binding to nicotinic receptors for acetylcholine in the central nervous system (CNS), peripheral nervous system (PNS) and in neuromuscular junctions. Several studies regarding the mechanisms of anatoxins toxicity were performed in mice using the sub-lethal or lethal doses of anatoxin-a. Anatoxin-a, well-known as a “Very Fast Death Factor,” can cause contraction, muscular paralysis, and respiratory arrest leading to death of mice in a very short time after intraperitoneal injection (i.p. mouse LD50: 250 to 375 μg/kg; ). Anatoxin-a can impair blood pressure, heart rate and gas exchange triggering hypoxia, respiratory arrest and severe acidosis leading to death of the animals. The toxicological properties of homoanatoxin-a are more or less identical to those of anatoxin-a. The neurotoxic alkaloid saxitoxins are considered the most toxic compounds. The mode of action of all analogs of saxitoxins is more or less similar; however, they differ in toxicity. Saxitoxins may block voltage-gated sodium channels in nerve cells and discontinue the entry of sodium flow, preventing the generation of a proper action potential or electrical transmission in nerves and muscle fibers leading to paralysis of muscles and death by respiratory arrest in mammals. Another neurotoxic cyanotoxin antillatoxin is a novel ichthyotoxic (LC50 = 0.1 μM) cyclic lipopeptide isolated from the marine cyanobacterium Lyngbya majuscula. Antillatoxin-A prompted a rapid neuronal death in cerebellar granule cell cultures (LC50 = 0.18 μM; ). Voltage-gated sodium channels were shown as the main molecular target of antillatoxin. The neurotoxic compound kalkitoxin isolated from L. majuscula is a thiazoline-containing lipopeptide compound. Lyngbyatoxin-A, a cyclic dipeptide found in L. majuscula, appears to have been responsible for a severe oral and gastrointestinal inflammation suffered by a person who accidentally ingested this cyanobacterium. Kalkitoxin was shown ichthyotoxic to the goldfish Carassius auratus and toxic to the aquatic crustacean brine shrimp (Artemia salina) with an LC50 700 and 170 nM, respectively. Kalkitoxin may also block voltage-gated sodium channels. The neurotoxic amino acid BMAA acts in mammals as a glutamate agonist. BMAA increases the intracellular concentration of calcium in neurons and induces neuronal activity by hyperexcitation.

22The endotoxic LPSs are known to cause fever in mammals and are involved in septic shock syndrome and liver injury. LPS can impair the immune system and also affect the detoxification system of diverse organisms. Until now, very little is known about the LPS intoxication and its toxicity is assumed to be associated with the host-mediated factors. More extensive research is needed to clarify a definite toxicity mechanism of LPS. Overall, it is no doubt that the acute effects of several cyanotoxins represent the major concern for ecological health impacts.

23The increased incidence of toxic cyanobacterial blooms (cyanoblooms) worldwide and their potential health risks have generated tremendous concern for dynamic management of toxic cyanoblooms. The economic cost of freshwater blooms in the United States was estimated to be about 2.2–4.6 billion dollars/annum. Henceforth, advanced approaches or development of a new technology is needed to terminate or prevent/suppress the harmful cyanobacterial blooms for environmental sustainability and economic vitality. Several factors boosting the incidence of harmful cyanobacterial blooms, such as nutrient input, wind velocity, sediment deposition, reduced water flow, increased salinity and temperature gradients, global warming and drought can be regulated to a certain extent to eliminate or minimize the bloom incidence. The approaches implemented for bloom suppression should be environmentally sustainable without adversely influencing the aquatic ecosystems. A number of strategies or approaches such as chemical, physical, biological, and other cognizance approaches came into consideration for mitigating the harmful cyanobacterial bloom incidences.

24Cyanoblooms can be controlled to a certain extent using some chemicals such as algicides, inhibitors or flocculants; however, use of these chemicals can inevitably recontaminate water bodies. The use of certain pigments (aquashade) can reduce the amount of light availability, and inhibit the growth of harmful algae; however, this approach may not be effective due to growth inhibition of other beneficial microalgae, thereby undesirably influencing the aquatic ecosystems. The use of some algicides has been reported to decline the bloom formation. The natural product cyanobacterin has been shown to be toxic to most cyanobacteria at a concentration of approximately 5 μM. Many biologically derived (but non-antibiotic) bioactive substances are known to inhibit the growth of aquatic bloom-forming cyanobacteria. Recently, have shown the fast removal (up to 98.99%) of MC-LR by a low-cytotoxic microgel- Fe(III) complex. Preoxidation with chlorine dioxide followed by flocculation and settling was found effective in removing cyanobacterial blooms and MCs. The use of aluminum salts can be used as algicides for nuisance algae and cyanobacteria control. The use of slaked lime [Ca(OH)2] or calcite (CaCO3) has also been reported to remove the algal communities, including cyanobacteria. Aluminum compounds can be used to remove the nutrients from industrial and domestic wastewaters. Besides aluminum, several other metals such as iron and copper are used to remove the algal blooms. The salt of copper (CuSO4.5H2O) is widely used as an algicide. The herbicide diuron together with copper sulfate as well as other copper-based compounds have been approved by the United States Environmental Protection Agency (USEPA) for use as algicides in fish production ponds. Moreover, the use of synthetic compounds for bloom control has their own limitations, and therefore, a range of natural chemicals (e.g., anthraquinone, nostocarboline, and stilbenes) from diverse organisms have been derived as potent substituents of synthetic algicides (,;;; Table 2). Recently, reviewed the availability of different chemical compounds for prevention and management of cyanobacterial blooms.

25Bloom control by physical methods generally involves mechanical removal techniques or short wavelength radiation treatment to control the incidence of cyanobacteria. The use of new and improved technologies can eliminate industrial/agricultural/ household pollutants to a certain extent to minimize the environmental pollution, including the water pollution by the incidence of harmful algal blooms. Global climate change and rising fresh water demand for multipurpose usage caused a remarkable increase in drought frequency and decreased freshwater flow rates. However, increasing flow rates and decreasing water residence time can remove fresh water algal blooms of a reservoir even in nutrient-rich conditions. The artificial circulation for increased water flow is reported to suppress the blooms, but it may also cause habitat disturbance. Moreover, a solar powered circulation (SPC) has been designed to create long-distances circulation of the epilimnion (>200 m) to suppress freshwater harmful algal blooms. Data obtained from a case study of nutrient-enriched, source-water reservoirs, revealed the role of SPC in reduction of cyanobacterial peak density by about 82 and 95% during the first and second year of SPC deployment, respectively. Intensity of light and temperature play a significant role in bloom incidence as mentioned above. However, the increase in incidence of light and temperature can hardly be controlled in a large water reservoir, where as it is energy intensive in smaller water bodies. Short wavelength ultraviolet radiation can bring about a rapid degradation of the cyanotoxins MCs. Moreover, it has been concluded that photosensitized processes may play an important role in the photochemical transformation of cyanotoxins (e.g., MC-LR; cylindrospermopsin) in the natural water. Simulated waterfalls or fountains may also be effective to control the cyanobacterial blooms in smaller water bodies; however, it requires electric-grid power constantly. The use of hydraulic jet cavitation may be a good approach to cyanobacterial water-bloom management. Moreover, cavitation treatment can disintegrate gas vesicles of cyanobacterial cells, and can remove up to 99% cyanobacteria growing in a lake, ponds or reservoirs.

26Control of cyanoblooms through biological mechanisms such as regulation of nutrient uptake or availability, alteration of normal physiology (such as a decrease in photosynthetic pigment), and/or direct feeding of cyanobacterial biomass by some aquatic organisms may be promising ways of ecological restoration. The gastropod Radix swinhoei can ingest cyanobacteria and survive well without loss in fecundity in the water reservoirs with cyanobacterial blooms. The combined use of snails (R. swinhoei) and a submerged plant (Potamogeton lucens) in eutrophic waters can eliminate cyanobacterial bloom by minimizing the eutrophication; however, this method is under the preliminary stage due to the lack of the field study. Occurrence and growth of aquatic plants are considered good candidates for limiting algal growth as the aquatic plants directly compete with algae for nutrients, light and space. Some aquatic plants release different allelochemicals that can inhibit the growth of cyanobacteria and other phytoplanktons. Biodegradation using different species/strains of bacteria (Table 3) and other organisms may be the most efficient process to control the fate of some cyanotoxins in natural waters.

27Development of wastewater research and management program is highly amenable to prevent or control the worldwide incidence of algal blooms and maintaining the ecological integrity and sustainability. Documentation of different environmental factors responsible for increased incidence of harmful cyanoblooms is crucial toward the development of demarcated management strategies. Moreover, interactive management of anthropogenic over nutrient-enrichment and global climate change is a major task for ensuring the protection and sustainability of aquatic ecosystems. The availability of phosphorus plays an important role in the growth of cyanobacteria and other microalgae or phytoplanktons; henceforth, controlled input of phosphorus to the water reservoir may be an effective management strategy for restoration of aquatic ecosystems. In order to minimize the bloom boosting organic or inorganic nutrients coming from common practices such as excessive use of fertilizers (e.g., NPK) and detergents, prior wastewater treatment may be needed to reduce the incidence of cyanobacterial blooms. The modeling of different water bodies at risk of toxic blooms may be a good approach to develop a proactive algal bloom monitoring and management strategies. Moreover, the fundamental research and quantitative ecological awareness toward the bloom incidence can be a supportive tool guiding large-scale water management against harmful bloom incidence.

28Public environmental awareness (PEA) is a fundamental approach for the attainment of sustainable environment. PEA about the incidence and harmful effects of toxic cyanoblooms may be a dynamic approach to eradicate and avoid the blooms and their toxic effects. The edifying approaches will allow people to think about their practices in their day-to-day life, such as unregulated disposal of organic/inorganic domestic wastes in the water reservoir, thereby enhancing the risks of bloom formation. As discussed elsewhere, global climate change may potentially impact the success of cyanobloom incidence. An emphasis on social practice to minimize the bloom formation and intoxication can allow intellectuals actualizing the significant development to control the environmental pollution. A change in the PEA levels in response to the increased incidence of environmental pollution is indispensable for ensuring the effective environmental protection and restoration. An increase in public awareness regarding the environmental sustainability and ecosystem health can inform the policy or decision makers to develop the strategies or to set-up the environmental protection laws against anthropogenic environmental pollution (such as direct disposal of domestic or industrial waste in open water reservoir such as rivers, ponds, lakes, and catchments). Moreover, various means of environmental protection program should be launched worldwide by the concerned government or non-government organization (NGO) to spread the knowledge about different environmental issues such as harmful cyanobloom incidence.

29Overall, little is known concerning the formation of cyanoblooms and production of different variants of cyanotoxin in diverse water bodies. Furthermore, each of these strategies mentioned above has their own advantages and limitations, and more extensive collaborative work is needed to control or manage the occurrence of algal blooms worldwide. Since eutrophication is considered as the most immediate environmental consequence of cyanoblooms, the uncontrolled disposal of organic/inorganic nutrients in the water reservoir through agricultural runoff or through industrial and household sewage water must be diminished or even forbidden. The establishment of several dyes or chemical based industries are the source of several blooms forming substances and therefore the government law should strictly be implemented to sanitize unwanted industrial effluents before reaching the water bodies. Furthermore, the eutrophication of water reservoirs must be regularly checked for an increased prevalence of toxin producers mainly in the bloom sensitive areas of subtropical and temperate climates. Severity on global warming is also an important trigger that is likely to create toxic cyanoblooms, therefore a proper environmental management toward increasing global climate change is necessary for sustainability of the pollution-free aquatic ecosystems.

30Cyanobacterial blooms are an increasing issue in both the wastewater-treatment and drinking water systems. Eutrophication and global climate change is the key factors for the occurrence of cyanoblooms all over the world. Cyanoblooms and production of several cyanotoxins in water bodies may reduce the surface/drinking water quality leading to high health risk to the organisms in aquatic ecosystems as well as wild/domestic animals and humans. A number of cyanotoxins such as MCs, nodularins, cylindrospermopsins, anatoxins, saxitoxins, and LPSs have been recognized as the major environmental contaminants in the immediate aquatic ecosystems. Control of cyanobloom using the chemical approaches can be effective; however, some algicidal/herbicides chemicals can cause secondary pollution of aquatic ecosystems. Several mitigation strategies have been tested and employed at laboratory levels; however, their efficacy to remove the blooms has not been confirmed under field environments. Establishment of effective mitigation strategies such as chemical, biological as well as public cognizance approach toward environmental awareness may be the most realistic measure to overcome the worldwide incidence of algal blooms and the attainment of a sustainable environment. Some natural algicidal compounds are really very effective to control the cyanoblooms; however, their production and availability is still very limited. The cost-effective synthesis of these biochemicals would be highly valuable to control the cyanoblooms. Furthermore, several cyanobacteria may become resistant toward certain chemicals. The use of biocides or several different biological processes against cyanoblooms may also affect other non-target aquatic organisms. Hence, common ecotoxicological impacts should be sensibly evaluated in the milieu of the lack of ecological health risk assessment. Moreover, a combined policy should strictly be regulated to diminish the bloom-boosting cause such as massive eutrophication of aquatic ecosystems by anthropogenic sources.

31The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.