General Information: Toxic bloom-forming cyanobacterium. Microcystis aeruginosa is a common cyanobacterium which can form blooms from late summer to early autumn in nutrient-rich lakes, ponds, and reservoirs. This organism produces a hepatotoxin, microcystin, which can kill fish, birds, and farm animals. This toxin also causes skin irritation and gastrointestinal symptoms in humans.