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Microcystins: The Ring-Shaped Liver Toxins Made by Blue-Green Algae

Microcystins are seven-residue ring peptides made by cyanobacteria in lakes and reservoirs. They damage the liver. Their structure, the EPA drinking-water advisory levels, the 1996 dialysis deaths in Brazil and who to call.

A natural toxinNot a medicineWater advisories come from local authorities

Caroline S · Published 2026-11-11

Length

Microcystin-LR is built from seven residues joined into a single ring of 25 atoms (ChEBI via PubChem CID 445434). Formula C49H74N10O12, molecular weight 995.2.

Sequence

A cyclic heptapeptide. Several of its seven residues are unusual amino acids not found in proteins, so a plain letter sequence is not given here; the L and R in the name stand for the leucine and arginine it contains.

Origin

Made by cyanobacteria (blue-green algae). ChEBI names Microcystis aeruginosa as the producer of microcystin-LR.

Last reviewed

2026-11-11

What is it good for?

Microcystins are poisons, not remedies. They block enzymes called protein phosphatases and chiefly harm the liver. People meet them through water: drinking water drawn from a lake or reservoir during an algal bloom, or swimming in one. Whether local water is safe is a question for the water utility or the state or local health department, not for this entry.

Illustration: A deep green ring-shaped molecular model on a white lab bench with copper reflections.
Illustration

What it is

Microcystins are a family of toxins made by cyanobacteria, the organisms often called blue-green algae. When these grow thickly in a lake or reservoir (a "bloom"), the toxins can end up in the water.

Each microcystin is a small ring-shaped peptide. The most studied one is microcystin-LR. Its ChEBI record (via PubChem CID 445434) describes seven residues joined into a single ring of 25 atoms, with the formula C49H74N10O12 and a molecular weight of 995.2. The record names the cyanobacterium Microcystis aeruginosa as its producer and calls it "the most studied of the microcystins".

Several of the seven residues listed in that record are unusual amino acids that are not among the twenty used in proteins. The library's peptide vs. protein entry explains why a seven-residue ring still counts as a peptide.

How it harms the body

ChEBI lists microcystin-LR as an inhibitor of phosphoprotein phosphatases. These are enzymes that switch other proteins off by removing phosphate groups. A 2012 study describes it as "a potent hepatotoxin" that inhibits phosphatases 1 and 2A (PMID 22414781). Hepatotoxin means a poison of the liver.

The clearest human record: Caruaru, 1996

The best-documented poisoning did not come from drinking. After a drought in February 1996, a dialysis unit in Caruaru, in north-east Brazil, used water from a lake with massive cyanobacterial growth. Dialysis puts treated water in close contact with the blood.

A 1998 report in The Lancet recorded what followed:

  • all 126 patients developed signs of nerve and liver poisoning;
  • 60 died;
  • liver injury ranged from abnormal blood tests to fatal liver failure;
  • microcystins were found in the patients' serum, in the dialysis filters and in the water-treatment columns.

The authors concluded that dialysis is "an additional high-risk exposure route", because the toxin enters the circulation directly.

What the EPA advisory levels are

In 2015 the US Environmental Protection Agency published drinking-water health advisories for microcystins. A health advisory describes a concentration at which harm is not anticipated over a stated period, here ten days.

Group EPA ten-day health advisory for microcystins (2015)
Bottle-fed infants and pre-school children 0.3 micrograms per litre
School-age children and adults 1.6 micrograms per litre

The EPA is explicit about what these numbers are not. They "are not regulations and should not be construed as legally enforceable federal standards", and they "may change as new information becomes available". The same page says several states have set their own standards or guidelines and directs readers to their state environmental or health department.

In 2019 the EPA also issued recommended criteria for swimming and other recreation in waters containing microcystins. Those values were not opened for this entry, so none is quoted.

What is not established

  • No human trial was found. Of 6,558 PubMed records naming microcystin, two are tagged as clinical trials. Both were opened: one is a study in carp, the other in medaka fish. ClinicalTrials.gov returns 2 studies for the word microcystin; by their titles both concern the health effects of exposure to harmful algal blooms, and neither record was opened for this entry.
  • The advisory levels include a margin of safety for gaps in the data, by the EPA's own description. They are guidance values.
  • This entry does not say how long-term, low-level exposure affects people. The sources opened for it do not settle that.
  • Nothing here says whether any particular water is safe. Only testing and the responsible authority can.

Where it stands

Question Answer as of 2026-11-11
What is it? A family of seven-residue ring peptides made by cyanobacteria
Main target The liver
US drinking-water limit No enforceable federal standard; EPA ten-day advisories of 0.3 and 1.6 micrograms per litre (2015)
Human trials None; 2 PubMed records tagged as trials are fish studies
Who to ask The water utility, the state or local health department; for symptoms, a clinician or US Poison Help, 1-800-222-1222

Other ring-shaped peptide toxins in the library are the mushroom poisons amatoxins and phalloidin.

What the research shows

Microcystins can kill people when they reach the blood directly

Caruaru, Brazil, 1996 (Lancet 1998, PMID 9800741): all 126 patients of a dialysis unit fell ill after lake water was used; 60 died; microcystins were found in serum and filters

Microcystin-LR inhibits protein phosphatases

ChEBI annotation (phosphoprotein phosphatase inhibitor); a 2012 fish study describes it as a potent liver toxin inhibiting phosphatases 1 and 2A (PMID 22414781)

A level in drinking water below which harm is not expected over ten days

EPA 2015 health advisories: 0.3 micrograms per litre for bottle-fed infants and pre-school children, 1.6 for school-age children and adults; guidance, not enforceable standards

Bars show how much of the evidence is in humans, not how well anything works.

Where it stands

Status

A natural toxin and water contaminant. No product with microcystin as an active ingredient was found in Drugs@FDA (openFDA search, 2026-11-11).

If exposure is suspected

Local advisories come from the water utility or the state or local health department. For symptoms after exposure: a clinician, or US Poison Help, 1-800-222-1222. This entry is not a guide to treatment.

Published record

PubMed: 6,558 records name microcystin; 2 carry a clinical-trial publication type, and both are fish studies (2026-11-11).

Frequently asked questions

What is microcystin?

Microcystins are a family of ring-shaped peptide toxins made by cyanobacteria, also called blue-green algae. The most studied, microcystin-LR, is a ring of seven residues. They mainly harm the liver.

How much microcystin is allowed in drinking water?

The United States has no enforceable federal limit. In 2015 the EPA published ten-day health advisory levels of 0.3 micrograms per litre for bottle-fed infants and pre-school children and 1.6 for older children and adults. Some states set their own standards.

Is microcystin a peptide?

Yes. Microcystin-LR is a cyclic heptapeptide: seven residues joined in one ring. Several of them are unusual amino acids that are not among the twenty used in proteins.

What happened in Caruaru?

In February 1996 a dialysis unit in Caruaru, Brazil, used water from a lake with heavy cyanobacterial growth. All 126 patients became ill and 60 died. Investigators found microcystins in the patients' blood and in the unit's water filters.

Who should I contact about an algal bloom or a water advisory?

The water utility or the state or local health department issues and lifts advisories; the EPA points to state agencies for state-specific rules. For symptoms after contact with a bloom, a clinician or a poison centre is the place to ask.