Cyanobacteria are found almost everywhere, but particularly in lakes and in the ocean where, under high concentration of phosphorus conditions, they reproduce exponentially to form blooms.

They are important primary producers as well as an immense source of several secondary products, including an array of toxic compounds known as cyanotoxins.

Abundant growth of cyanobacteria in freshwater, estuarine, and coastal ecosystems due to increased anthropogenic eutrophication and global climate change has created serious concern toward harmful bloom formation and surface water contamination.

[8] Cyanobacteria are considered the oldest groups of photosynthetic prokaryotes [9] and possibly appeared on the Earth about 3.5 billion years ago.

[10] They are ubiquitous in nature and thrive in a variety of ecological niches ranging from desert to hot springs and ice-cold water.

Cyanobacteria are an immense source of several secondary natural products with applications in the food, pharmaceuticals, cosmetics, agriculture, and energy sectors.

[11] Moreover, some species of cyanobacteria grow vigorously and form a dominant microflora in terms of their biomass and productivity in specific ecosystems.

Bloom formations 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.

[12][8] 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, as well as phytoplankton and zooplankton 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.

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.

[13][14][12] Global warming and temperature gradients can significantly change species composition and favor blooms of toxic phytoplanktons.

[15][16][8] It 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.

The prefix cyan comes from the Greek κύανoς meaning "a dark blue substance",[19] and usually indicates any of a number of colours in the blue/green range of the spectrum.

However modern sources tend to regard this as outdated;[20] they are now considered to be more closely related to bacteria,[21] and the term for true algae is restricted to eukaryotic organisms.

The resulting dense swarm of phytoplankton is called an algal bloom; these can cover hundreds of square kilometres and can be easily seen in satellite images.

Water acidity also cycles daily during a bloom, with the pH reaching 9 or more during the day and dropping to low values at night, further stressing the ecosystem.

They present health risks for wild and domestic animals as well as humans, and in many areas pose major challenges for the provision of safe drinking water.

They are produced by a large variety of organisms, including cyanobacteria, and are part of the group of natural products, also called secondary metabolites.

The progressive symptoms of anatoxin-a exposure are loss of coordination, twitching, convulsions and rapid death by respiratory paralysis.

[48] There is continued interest in anatoxin-a because of the dangers it presents to recreational and drinking waters, and because it is a particularly useful molecule for investigating acetylcholine receptors in the nervous system.

[50] The outbreak was traced back to a bloom of Cylindrospermopsis raciborskii in the local drinking water supply, and the toxin was subsequently identified.

[51] Cylindrospermopsin is toxic to liver and kidney tissue and is thought to inhibit protein synthesis and to covalently modify DNA and/or RNA.

It acts on the voltage-gated sodium channels of nerve cells, preventing normal cellular function and leading to paralysis.

The blocking of neuronal sodium channels which occurs in paralytic shellfish poisoning produces a flaccid paralysis that leaves its victim calm and conscious through the progression of symptoms.

[60] The toxin cascades through the food-chain: Among other animals, it affects fish and waterfowl such as coots or ducks which feed on hydrilla colonized with the cyanobacterium.

[61] Vacuolar myelinopathy is characterized by widespread vacuolization of the myelinated axons (intramyelinic edema) in the white matter of the brain and spinal cord.

[62] The non-proteinogenic amino acid beta-Methylamino-L-alanine (BMAA) is ubiquitously produced by cyanobacteria in marine, freshwater, brackish, and terrestrial environments.