plankton

Small, often microscopic, forms of plant and animal life that live in the upper layers of fresh and salt water, and are an important source of food for larger animals. Marine plankton is concentrated in areas where rising currents bring mineral salts to the surface.

Pfiesteria piscicida is the only predatory phytoplankton. It stuns its prey by producing a powerful toxin that also causes the deaths of nearby fish and may be harmful to humans exposed to it.