Pfiesteria piscicida is a dinoflagellate responsible for many blooms in the 1980s and 1990s on the coast of North Carolina.
Its living cycle is extremely complex: scientists have found at least 24 different stages, spanning from cyst to several amoeba-like forms. The organism apparently moves through these different stages as environmental conditions require.
Pfiesteria produces a powerful toxin, used to kill fishes, even relatively large ones. The toxin can also be carried in the air, as experimenters learned when cultivating the organism in laboratory.