Dinoflagellate protozoa Pyrocystis fusiformis floating in seawater, seen through an endoscope. Dinoflagellates are unicellular protozoans. About 90 are found in marine environments as plankton. These cells are bioluminescent at night, when the movement of tides, storms and passing ships cause them to emit short flashes of light. In favourable conditions large blooms of dinoflagellates form, known as red tides. These can contain toxins that are harmful to marine life and people who consume seafood.