Molecular model of microcystin. Microcystin is a toxin produced by microcystis, a blue-green algae which thrives on phosphorus contamination from leaking septic systems, sewer drainage and runoff from fertilized lawns, farms and cattle feedlots. Contamination from algal blooms in Lake Erie caused a two-day ban on the use of drinking water in Toledo, Ohio, in August, 2014. Black=carbon, white=hydrogen, red=oxygen, blue=nitrogen.