A phylum of mostly single-celled protists included in the alveolates. They are abundant in the marine plankton; some are photoautotrophs, containing brown xanthophyll pigments in addition to chlorophyll, and are important primary producers in marine ecosystems; many of these are mixotrophs. Approximately half the known species are heterotrophic. Dinoflagellates characteristically have two flagella for locomotion and most have a rigid cell wall of cellulose encrusted with silica. Some species (e.g. Noctiluca miliaris) are bioluminescent. Vast numbers of dinoflagellates sometimes accumulate in surface waters as ‘algal blooms’, colouring the sea typically brown or red—hence the term red tide. Some species release powerful toxins into the water, which can kill fish, contaminate shellfish, and even affect the breathing of people near the shore. Photosynthesizing dinoflagellates form symbiotic associations with coral animals and are vital for the health of coral reefs.