Learn about Harmful Algae, Cyanobacteria and Cyanotoxins

Overview of Harmful Algae

Algae are natural and important primary producers in aquatic ecosystems. Some species, under certain conditions can produce toxins that affect human health and the environment. In addition, some algae can produce excessive biomass (blooms) that can lead to harmful impacts on the environment, such as low/no oxygen (hypoxia/anoxia) or acidification, or on human communities, such as unsightly conditions or taste and odor issues that affect recreation, tourism, and drinking water. Algal growths that lead to impacts on human health and the environment are referred to as Harmful Algal Blooms (HABs) and these web pages provide an overview of these phenomena.

Several different types of algae can produce toxic HABs. The most common algae associated with marine toxic HABs include the dinoflagellates (e.g., Florida red tide Karenia brevis) and diatoms (Pseudo-nitzschia) while those most associated with freshwaters are dominated by cyanobacteria (e.g., Microcystis). Many more types of algae can produce the non-toxic excessive biomass HAB conditions that cause hypoxia and odor/aesthetic issues including the diatoms, dinoflagellates, cyanobacteria, green algae (e.g., Cladophora) and brown algae (e.g., Sargassum).  

Under the Harmful Algal Bloom and Hypoxia Research and Control Act (HABHRCA), the National Oceanic and Atmospheric Administration (NOAA) is primarily responsible for estuarine, marine, and Great Lake waterbodies, whereas the EPA bears primary responsibility for all other freshwaters. So, these pages will focus on freshwater cyanobacteria, the primary HAB causing taxa in freshwaters. However, the EPA has regulatory responsibilities for marine waters affected by HABs under the Clean Water Act and there are a few non-cyanobacterial taxa that produce toxins in freshwater. Therefore, these webpages will also provide some coverage of marine and other freshwater HABs. Interested readers are encouraged to refer to the more detailed resources on coastal and marine HABs provided by NOAA and by the Woods Hole Harmful Algae webpage funded by NOAA.

• Harmful Algal Blooms - NCCOS Coastal Science
• Harmful Algal Blooms

Overview of Cyanobacteria

Cyanobacteria, also known as blue-green algae, are frequently found in freshwater, estuarine, and marine waters. These tiny, or microscopic, organisms use direct sunlight to produce their own food (i.e., are photosynthetic) and are very important to aquatic ecosystems as many species can fix nitrogen from the atmosphere and make it available to the aquatic food web. Although important to a variety of organisms and to the food webs they support, excessive growth of cyanobacteria can lead to “blooms” that can cause ecological and human health concerns. Cyanobacteria and green algae blooms are often confused, since both can produce dense surface and water column growths that can impede recreation (swimming, boating and fishing), cause unwanted odor issues (especially in drinking water and fish tissue), and contribute to oxygen depletion. However, unlike cyanobacteria, green algae are not generally known to produce toxins. In contrast, some cyanobacteria are able to produce highly potent toxins, known as cyanotoxins, that can lead to a variety of health problems for both human and aquatic species (e.g., abdominal, neurological, and skin issues).

Species of Cyanobacteria that Produce Cyanotoxins

Cyanotoxins can be produced by a wide variety of cyanobacteria, with some genera producing multiple toxins in a single bloom.  Some of the most commonly occurring toxin producing genera include Microcystis, Dolichospermum (previously Anabaena), Planktothrix, Microseira, Nostoc and Microcoleus. But these are only a small sample of toxin producing cyanobacteria taxa.

Microcystis is the most common bloom-forming genus. Its 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 Dolichospermum form slimy blooms on the surface of eutrophic lakes and reservoirs. Dolichospermum blooms may develop quickly and resemble green paint. In less eutrophic waters, some species also form colonies, which are large dark dots in water samples and on filters after filtration.

Planktothrix 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. In contrast to the first 3 taxa which are most commonly water column or planktonic.

Microseira, Microcoleus, and Nostoc are primarily benthic (bottom living) cyanobacteria that can produce toxins in lakes, streams and rivers. Microseira forms greenish-black, dark blue or black woolly-looking mats on the bottom of streams and ponds which may float to the surface due to trapped gas, trailing filament “streamers” behind them. Microcoleus forms dark blue-green, red-brown, yellow-brown or brown films on bottom substrates including rocks, plants, and other algae and can also float to the surface. Nostoc, a common cyanobacteria in aquatic and terrestrial environments forms dark green or brown gelatinous bulbous or ear-shaped colonies on hard substrates in streams and rivers or in soft sediments among plants in the margins of lake and ponds.

Overview of Cyanotoxins

Under the right conditions (i.e., light intensity, nutrients, temperature, salinity, etc.), cyanobacterial blooms can produce toxic byproducts known as cyanotoxins. These can occur in most freshwaters including wetlands, lakes, reservoirs, streams and rivers. Cyanotoxins are produced and contained within the cyanobacterial cells (intracellular). The release of these toxins in an algal bloom into the surrounding water occurs mostly during cell death and lysis (i.e., cell rupture) as opposed to continuous excretion from the cyanobacterial cells. However, some cyanobacteria species are capable of releasing toxins (extracellular) into the water without cell rupture or death. Once these toxins are released, they can cause numerous health effects ranging from minimal (i.e., diarrhea, headaches, and skin irritation) to life threatening (i.e., death). 

Most Commonly Measured Cyanotoxins in the U.S.

The most commonly measured cyanotoxins in the U.S. are microcystins, cylindrospermopsins, anatoxins/guanitoxins, saxitoxins and nodularins.

Microcystins

Microcystins are produced by a variety of both planktonic and benthic cyanobacteria, including members of the genera Dolichospermum (previously Anabaena), Fischerella, Gloeotrichia, Microcoleus, Microcystis, Nodularia, Nostoc, Oscillatoria, Phormidium, and Planktothrix. Microcystins are the most common cyanobacterial toxins and there are more than 200 forms or congeners with microcystin-LR being the most studied. They can be inside algal cells but are also commonly found in dissolved form, having been released from living or more commonly dead cells.  Microcystins can bioaccumulate in common aquatic invertebrates and vertebrates such as zooplankton, mussels, and fish. Microcystins are primarily hepatotoxins (affect the liver); however, there is evidence indicating they can impact the kidney and reproductive systems as well. Although there is evidence to link microcystin-LR with both liver and colorectal cancers, the EPA has determined that there is inadequate information to assess carcinogenic potential of microcystins in humans. This is primarily due to the limitations in available human studies (i.e., potential co-exposure to other contaminants) and lack of long-term animal studies evaluating cancer following oral exposure.

The EPA provides detailed discussion on the human health effects of microcystins in its health effects support and drinking water health advisory documents for cyanotoxins.

• US EPA Drinking Water Health Advisory for the Cyanobacterial Toxin Cylindrospermopsin

Anatoxins/Guanitoxins

Anatoxins are neurotoxins that bind to neuronal nicotinic acetylcholine receptors affecting the central nervous system. These toxins are mainly associated with the cyanobacterial genera Chrysosporum, Cuspidothrix, Cylindrospermum, Dolichospermum (previously Anabaena), Microcystis, Oscillatoria, Planktothrix, Phormidium, Raphidiopsis, Tychonema, and Woronichinia.  There are multiple congeners, including anatoxin-a and homoanatoxin-a with anatoxin-a being the most common.  Anatoxins primarily cause serious neurologic distress with signs ranging from loss of coordination to death due to respiratory failure. There is information to suggest that anatoxins also bioaccumulate in various aquatic invertebrates and vertebrates.

There is insufficient information available on the carcinogenicity of anatoxin-a in humans or animals or on potential carcinogenic precursor effects.

The EPA provides detailed discussion on the human health effects of anatoxins/guanitoxins in its health effects support document for cyanotoxins.

• US EPA Health Effects Support Document for the Cyanobacterial Toxin Anatoxin-a

Saxitoxins

Saxitoxins are representative of a large toxin family referred to as the Paralytic Shellfish Poisoning (PSP) toxins. These toxins are produced from marine dinoflagellates and by freshwater cyanobacteria including species of Aphanizomenon, Dolichospermum, Microseira, Planktothrix and Raphidiopsis. Saxitoxins are primarily neurotoxic and cause effects ranging from minor tingling and numbness to death from respiratory failure. Exposure from saxitoxins typically occurs from ingestion of contaminated shellfish. No information is available on the carcinogenicity of saxitoxins.

Nodularins

Nodularin is produced from species of the genus Nodularia that primarily thrive in salty or brackish coastal or inland waters. Several congeners for nodularin exist, with nodularin-R being the most common. Similar to microcystin, nodularins impact the liver and are characterized as hepatotoxins; however, they also cause overall oxidative stress. Exposure primarily occurs from contaminated water; however, bioaccumulation of nodularin in aquatic vertebrates and invertebrates can occur and lead to additional risk.

For detailed information on toxins, visit the EPA’s compilation on common toxins found in fresh and marine waters:

• Common Toxins Produced by Cyanobacteria, Dinoflagellates, and Diatoms

For additional information on cyanotoxins:

• Harmful Algal Blooms - NOAA/NCCOS Coastal Science website   
• Woods Hole - Harmful Algal Blooms