Toxins Found in Algal Blooms Can Be Life Threatening

How Dangerous Are Cyanotoxins

An algal bloom is not truly algae but rather blue-green algae or cyanobacteria. Cyanobacteria is a native constituent of freshwater and marine environments and has the potential to release toxins – cyanotoxins.


Depending on the specific chemical structure, cyanotoxins can be:

  1. Neurotoxins that affect the nervous system
  2. Hepatotoxins that affect the liver
  3. Dermatoxins that affect the skin
  4. Other toxins that affect the stomach or intestines.

Worldwide, about 60% of samples collected from cyanobacterial blooms contain toxins. The danger to human and animal populations depends on the density of these toxins. If they are thinly dispersed, they are less dangerous. Surface scums, where cyanobacteria are concentrated, pose the major risk to public health.

Copper blue shoreline cyanobacteria scum that tested positive for toxicity

Copper blue shoreline cyanobacteria scum that tested positive for toxicity

Scums can be quickly broken by wave action and re-dispersed by renewed wind mixing. However, especially in shallow bays, scum material may take a long time to disperse, as a result of either wave wash or, ultimately, disintegration of the cells. Dying and lysing cells release their contents into the water, where pigments may adopt a copper-blue colour.

Generally, human adults don’t play in cyanobacteria scum. In contrast, dogs would not hesitate to jump into a big batch of scum and in some cases the bright blue-green color may attract children.


Chemical Structure of Microcystin-LR

Chemical Structure of Microcystin-LR

The most frequently occurring and widespread cyanotoxin in United States freshwater environments is microcystin. Microcystin is a hepatotoxin that can potentially damage the liver and in high doses death. For humans and animals, a lethal dose of microcystin causes death by liver necrosis within hours up to a few days. The main source of exposure is oral or intranasal. Imagine a water skier falling into freshwater concentrated with microcystin. Water will likely enter the mouth as well as the nose. Wearing a wetsuit also exposes the waterskier to dermatoxins. The wet suit keeps the toxins near the skin for extended periods of time.

The important thing to remember about microcystin is that the toxicity is cumulative. For example, one single dose is not going to likely cause damage, whereas many doses over a longer period of time may cause liver damage. For example, waterskiing once in a lake contaminated with microcystin will likely not cause liver damage.

However, if you rented a home on a lake contaminated with microcystin and recreated in the water every day, this may have the potential to cause liver damage. But even that would be difficult, because cyanobacteria blooms generally do not cover an entire lake. The blooms will move with wave and wind action. The most concentrated areas tend to be stagnant areas or near shore.

There are no human deaths from the recreational use of microcystin contaminated water. Although, there have been deaths associated with the use of microcystin contaminated water for dialysis. In 1996, 56 dialysis patients accidentally exposed to microcystins from the water used for dialysis died in Brazil. There are many reported deaths of pets, wildlife, and livestock from microcystin exposure.


A less common and less studied cyanotoxin is anatoxin. The toxin can cause strong salivation, cramps, tremors, diarrhea, vomiting, and rapid death. Anatoxin is among the most neurotoxic substances in the world. Fortunately, concentrations of anatoxins scarcely reach levels to cause harm in humans through oral ingestion. Although, fatalities of livestock and pets have occurred through the ingestion of highly concentrated toxic cyanobacterial scum. Dogs playing in the scum retain the toxicity in their fur and ingest it through grooming with the tongue.

Other Toxins

There are many other cyanotoxins not covered in this article. Cylindrospermopsin is a cytotoxin that blocks protein synthesis that can lead to kidney and liver failure. Generally this toxin is only found in tropical and subtropical regions.

Cyanotoxins Near You

Microcystins are the most common in freshwater environments in North America. Additionally, it is well documented that some cyanobacterial blooms can produce more than one type of toxin, both microsystin and anatoxin. Symptoms of cyanotoxin poisoning include abdominal pain, nausea, vomiting, diarrhea, sore throat, dry cough, headache, blistering of the mouth, dizziness, fatigue, and skin and eye irritations.

It is important to the remember that the cyanotoxins are still present in freshwater after visible signs of the cyanobacterial bloom have diminished. The natural degradation of cyanotoxins can take a few days to up to a month depending on the habitat and initial concentration of toxins in the cyanobacterial bloom.

For more information about HABS, check out the EPA website. Remember, the best way to avoid toxins is to not recreate in cyanobacteria infected waters.


About Melissa Schaar

I am a scientist, teacher, student, friend, and mom. I am on a journey to my ultimate vitality. That journey includes living to the fullest, loving everyday, laughing whenever possible, and a constant state of learning.