Algae are a natural component of aquatic ecosystems; however, when present in large quantities as “blooms”, they can pose a significant potential threat to human and ecological health. These harmful algal blooms (HABs) are often composed of microorganisms known as cyanobacteria, which have the potential to produce toxins that can cause adverse health effects in humans and animals through the contamination of waterways used for recreational purposes and as drinking water supplies.

Photo of Microcystis bloom at Ohio River, 2008. Photo by Jim Crawford, Ohio EPA
Microcystis bloom at Ohio River, 2008. Photo by Jim Crawford, Ohio EPA

What are cyanobacteria?

The most commonly occurring groups of freshwater algae are diatoms, green algae, and blue-green algae, which are more correctly known as cyanobacteria. Cyanobacteria refer to a group of microorganisms that possess characteristics of algae (chlorophyll-a and oxygenic photosynthesis). They are found in fresh, estuarine, and marine waters in the U.S. Cyanobacteria are often confused with filamentous green algae, because both can produce dense mats that can impede activities like swimming and fishing, and may cause odor problems and oxygen depletion; however, unlike cyanobacteria, filamentous algae are not generally thought to produce toxins. Freshwater cyanobacterial blooms that produce highly potent cyanotoxins are known as cyanobacterial HABs (cyanoHABs).

What are cyanotoxins?

Freshwater cyanobacterial blooms may be dominated by a single-species or composed of a variety of toxic and non-toxic strains (i.e., a specific genetic subgroup within a particular species). Cyanotoxins are produced and contained within the actively growing cyanobacterial cells (i.e., intracellular toxins). The release of these toxins in an algal bloom into the surrounding water as dissolved (extracellular) toxins occurs mostly during cell death and lysis (i.e., cell rupture) as opposed to the continuous excretion from the cells.

Cyanotoxins can affect the liver (hepatotoxic), the nervous system (neurotoxic) and the skin (acutely dermatotoxic); however, hepatotoxic freshwater blooms of cyanobacteria are more commonly found than neurotoxic blooms throughout the world.

Group of Colonies
Microcystis colonies seen through a microscope. Source: Dr. Hans W. Paerl, UNC-CH Institute of Marine Sciences.

What species of cyanobacteria produces toxins?

Cyanotoxins can be produced by a wide variety of planktonic (i.e., free living in the water column) cyanobacteria. Some of the most commonly occurring genera are Microcystis, Anabaena, and Planktothrix (Oscillatoria).

Microcystis is the most common bloom-forming genus, and is almost always toxic. Microcystis blooms resemble a greenish, thick, paint-like (sometimes granular) material that accumulates along shores. Scums that dry on the shores of lakes may contain high concentrations of microcystin for several months, allowing toxins to dissolve in the water even when the cells are no longer alive or after a recently collapsed bloom.

Species of the filamentous genus Anabaena form slimy summer blooms on the surface of eutrophic lakes and reservoirs. Anabaena blooms may develop quickly and also resemble green paint. In less eutrophic waters, some species also form colonies, which are seen as large dark dots in water samples and on filters after filtration.

Planktothrix agardhii (previously named Oscillatoria agardhii) forms long, slender, straight filaments that usually remain separate but form dense surface scums. Its presence may be revealed by a strong earthy odor and the filaments are easily detected visually in a water sample.

What are the most commonly found cyanotoxins in the U.S.?

Based on the surveys that have been carried out to date in U.S. waters, the most commonly identified cyanotoxins are microcystins, cylindrospermopsins, anatoxins and saxitoxins.


Microcystins are a group of at least 80 toxin variants which share a cyclic heptapeptide structure and primarily affect the liver (hepatotoxin). Microcystins are the most widespread cyanobacterial toxins and can bioaccumulate in common aquatic vertebrates and invertebrates such as fish, mussels, and zooplankton. Microcystins are produced by Microcystis, Anabaena, Planktothrix (Oscillatoria), Nostoc, Hapalosiphon, Anabaenopsis and Snowella lacustris. Nodularin, which is structurally related to microcystin and has a similar mode of toxicity, has been isolated from only one species of cyanobacteria, Nodularia spumigena. Recent evaluation of carcinogenesis from microcystin exposure by the International Agency for Research in Cancer has determined that microcystin- LR is possibly carcinogenic to humans (Group 2B), and has been linked to incidences of human liver and colon cancer.


Cylindrospermopsin is usually produced by Cylindrospermopsis raciborski, Aphanizomenon ovalisporum, Anabaena bergii, Umezakia natans, and Raphidiopsis curvata. The primary toxic effect of this toxin is irreversible damage to the liver. It also appears to have a progressive effect on several other vital organs. Effects of poisoning in humans included hepatoenteritis and renal insufficiency. Although the evidence of carcinogenicity in humans and experimental animals is inadequate, there is strong evidence of the tumour-promotion capacity of microcystin-LR to place them in Group 2B as possibly carcinogenic to humans.

Oscillatoria seen through a microscope. Source: Dr. Hans W. Paerl, UNC-CH Institute of Marine Sciences.


Anatoxins binds to neuronal nicotinic acetylcholine receptors affecting the central nervous system (neurotoxins). There are multiple variants, including anatoxin-a, homoanatoxin-a, and anatoxin-a(s). These toxins are mainly associated with the cyanobacterial genera Oscillatoria species, Cylindrosperum, Planktothrix spp., Aphanizomenon spp., Lyngbya and species such as Anabaena flos–aquae and A. planktonica.


Saxitoxins are representative of a large toxin family referred to as the Paralytic Shellfish Poisoning (PSP) toxins. When toxigenic marine dinoflagellates are consumed by shellfish, toxins concentrate and toxic quantities are delivered to consumers of the shellfish. These toxins have been reported also in freshwater cyanobacteria including Aphanizomenon flos–aquae, Anabaena circinalis, Lyngbya wollei, Planktothrix spp. and a Brazilian isolate of C. raciborskii.


Interagency, International Symposium on Cyanobacterial Harmful Algal Blooms
US EPA IRIS Toxicological Reviews for Microcystins, Anatoxin-a, and Cylindrospermopsin
WHO Water Related Diseases: Cyanobacterial Toxins
WHO Cyanobacteria and Cyanotoxins in Drinking Water
WHO IARC Monographs on the Evaluation of Carcinogenic Risks to Humans; Ingested Nitrate and Nitrite, and Cyanobacterial Peptide Toxins, VOLUME 94
CDC Harmful Algal Blooms (HABs)
CDC Recent Water-related Response Activities

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