Are blue-green algae a risk to your life or at least to your liver?

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Eutrofication has increased the number of many micro-organisms in rivers, lakes and ponds. Cyanobacteria which are sometimes called blue-green algae after an earlier nomenclature are a typical consequence of eutrofication. Cyanobacteria normally grow in water in low numbers. Some species thrive if the nitrate concentrations are high, but some cyanobacteria are able to utilize atmospheric nitrogen and then their growth depends more on the phosphate concentration. Rising temperatures also favour their growth.

Cyanobacteria often develop gas bubbles, and if they are abundant, the cells float to the surface and cause algal blooms. They can easily be seen from an aeroplane during the summer. The floating mats of cyanobacteria drift with wind and water currents, and they can form a slimy, stinking porridge along the shore. They may deplete oxygen from the water, killing fish and shading underlying phytoplankton preventing their growth. Climate change is a potent catalyst for the further expansions of blooms. When it is windy, turbulence mixes the water and these microorganisms become less detectable.


More than half of algal blooms are very poisonous. The toxins[1] can be classified crudely into liver toxins and neurotoxins.[2] There are very many species of cyanobacteria, however, thus there are many different toxins. Only some of them are detectable and have been characterised, and today efforts are being made to study them in detail. The liver toxins are more common than the neurotoxins. Liver toxins can be divided into those made by fresh water cyanobacteria (microcystins) and those of their brackish water counterparts (nodularins). These molecules are cyclic peptides formed of amino acids. They cause swelling of the liver and shock in a few hours. Neurotoxins (e.g. anatoxins and saxitoxins) are small-molecular alkaloids[3] or organophosphate-containing cyclic hydrocarbons. Both anatoxins and saxitoxins cause muscle paralysis. In fact, saxitoxins are some of the most toxic neurotoxins that are known.


Both cattle and dogs have died from algal toxins after drinking water severely contaminated with cyanobacteria, but serious poisonings of people are rare. Bathers may experience many symptoms, if they swim in water populated by cyanobacteria, e.g. headache, skin problems, eye, throat and ear irritation, and sometimes fever, vomiting and diarrhoea. It is obvious that one should not use water close to algal blooms for drinking water, because even boiling will not destroy the toxins, and other use (e.g. for washing) should be avoided. The modest filtering systems typically found in small waterworks are not able to remove the toxins.

Algal toxins cannot usually be found in effectively processed tap water even if there are algal blooms in the raw water. The toxins are diluted when the cells break down, and the effective water processing steps destroy or remove the toxins. Algal toxins also do not accumulate in the edible parts of fish. However, ocean mussels may be a source of poisonings, because they filter plankton such as dinoflagellata which synthesise saxitoxin, possibly in synergy with microbes. These blooms are sometimes called "red tide."[4] The variation in toxin production is such that it is difficult to predict toxicity, since it varies both temporally and regionally.

Algal blooms are unpleasant phenomenon in lakes and along the seashore, and they are associated with eutrofication and climate change. They may cause symptoms in people swimming in these waters. Unless used as drinking water, the other risks associated with this water are not great, though due care should be exercised.

Notes and references

  1. Toxin is a poison formed by a living organism. Animals and plants as well as microbes can produce toxins.
  2. Neurotoxin is a toxin poisonous to nervous tissue.
  3. Alkaloids are nitrogen-containing hydrocarbons which are often biologically active.
  4. Cf. the Nile and other rivers turning to blood, Exodus 8:19-21

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