Definition of microcystin
: any of various toxins that are produced by freshwater cyanobacteria (genus Microcystis, especially M. aeruginosa), have hepatotoxic effects in animals including humans, and may occur in large quantities and contaminate water supplies during algal blooms (see 2bloom 1d) … two water samples from a Toledo treatment plant tested positive for microcystin, a toxin possibly caused by an algae bloom in Lake Erie. Residents were told not to drink, or even boil, the water tainted with microcystin, which can cause nausea and impair liver function. — Lauren Raab When cyanobacteria die, they release microcystin, and scientists have found that remaining cyanobacteria respond to the molecule by making more of their own. — Carl Zimmer
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