Microcystins Cyanotoxins

Selected species of Cyanobacteria (blue-green algae) produce environmentally persisten secondary metabolites. These toxic metabolites are classified into four categories: hepatotoxins, dermatoxins, irritant toxins, and cytotoxins, depending on their mode of action. Of these, hepatotoxic microcystins are the most common and can usually be found in over half of the freshwater bodies where cyanobacteria are present. Toxin production by the cyanobacterium Microcystis aeruginosa was first reported in 1946 and additional toxic species have been described. Exposure to environmentally stable microcystins in food, drinking water, nutritional supplements and during medical dialysis can cause significant and sometimes fatal hepatoxicity and possible tumor induction in humans and animals.

Microcystins are a group of over 90 liver toxins (hepatotoxins) produced by Cyanobacteria, of which microcystin-LR (MC-LR) is the most common. Their toxicities result from the inhibition of protein functions and disruption of the cell structures. They also promote formation of free radicals in liver tissues. Microcystins can be introduced to tissues of organisms through the diet or by ingestion of contaminated water. Fish consumption is a potential route of human exposure to the hepatotoxic microcystins, especially in lakes and reservoirs that routinely experience significant toxic Microcystis blooms.

Hepatotoxic Seafood Poisoning (HSP) risk upon consumption of MC contaminated seafood is increasingly documented. Incidents of human sickening and death due to microcystin exposure have been documented. Chronic exposure to low levels of microcystins may promote liver tumors and has been linked to liver and other cancers.1

Microcystins can accumulate in the tissues of organisms. Bivalves, snails, shrimp, fish, and frogs have all tested positive for microcystins. Different species vary in their sensitivity to microcystins. Rainbow trout can withstand doses of microcystins which are lethal in mice without dying. Rainbow trout are more tolerant of microcystin exposure. Rapid removal of microcystins from tissues is observed in several fish species, including common carp and silver carp.

Deaths of marine mammals from microcystin intoxication provides evidence implicating land-sea flow with trophic transfer through marine invertebrates as the most likely route of exposure.


  1. The Fate of Microcystins in the Environment and Challenges for Monitoring Justine R. Schmidt,1,* Steven W. Wilhelm,2 and Gregory L. Boyer1 Toxins (Basel)v.6(12); 2014 Dec
  2. Hepatotoxic Seafood Poisoning (HSP) Due to Microcystins: A Threat from the Ocean? Katerina Vareli et al. Mar Drugs. 2013 Aug; 11(8): 2751–2768.
  3. Variations in the Microcystin Content of Different Fish Species Collected from a Eutrophic Lake Justine R. Schmidt,1 et al Toxins (Basel)v.5(5); 2013 May
  4. Evidence for a Novel Marine Harmful Algal Bloom: Cyanotoxin (Microcystin) Transfer from Land to Sea Otters Melissa A. Miller et al. PLoS One. 2010; 5(9): e12576.



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