microcystines

Microcystins are a family of potent hepatotoxins produced by freshwater cyanobacteria, including Microcystis, Anabaena, Oscillatoria, and Nostoc. They can accumulate in water during algal blooms and pose risks to drinking water supplies, recreational waters, and aquatic life. Microcystins are most commonly encountered as microcystin-LR, but hundreds of congeners exist, differing at two amino-acid positions in the cyclic heptapeptide structure.

Chemically, microcystins are cyclic heptapeptides that contain unusual amino acids, notably the Adda moiety and Mdha. The Adda group is central to their biological activity. The toxins are relatively stable in water and resistant to some environmental conditions, though they can degrade under prolonged exposure to sunlight and certain chemical treatments. The diversity of congeners means that toxicity can vary among variants, with microcystin-LR often used as a reference due to its well-characterized potency.

The primary mechanism of action is inhibition of serine/threonine protein phosphatases 1 and 2A, leading to dysregulated phosphorylation and hepatocyte damage. Acute exposure can cause gastrointestinal symptoms, liver injury, and, in severe cases, liver failure; chronic exposure has been linked to longer-term liver effects in animal studies and is a concern for human health. The IARC has classified microcystin-LR as possibly carcinogenic to humans (Group 2B). Regulatory guidelines vary by country, but the World Health Organization has issued drinking-water guidance values to limit risk, often cited around 1 microgram per liter for drinking water.

Occurrence is linked to nutrient-rich, warm freshwater environments that promote cyanobacterial blooms. Monitoring employs immunoassays (ELISA) and chromatographic methods (LC-MS/MS) to detect total microcystins and individual congeners. Mitigation strategies include source water protection, advanced water treatment (activated carbon, ozonation, UV-based processes, membrane filtration), and, where feasible, remediation of bloom conditions through nutrient management and ecological approaches. Biodegradation by specialized bacteria can also reduce microcystin concentrations in aquatic systems.