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The dinoflagellates (Pyrrhophyta, Dinophyta, Dinoflagellata) are a large group of flagellate protozoa. Most forms are marine and make up an important class of plankton, with photosynthetic forms more common than any other algae save the diatoms. A few are found in freshwater or as endosymbionts (zooxanthellae) of other creatures, which may even by dependent on their presence.

About half of all dinoflagellates contain chloroplasts. These are pigmented similar to those of the stramenopiles, containing chlorophylls a and c, and are surrounded by three membranes so must have been taken from an ingested cell. Colorless forms feed on other protozoa; the largest dinoflagellate, Noctiluca, is 2 mm across and is big enough to eat fish eggs. Noctiluca is also among a few species which exhibit bioluminescence - they produce light through the oxidation of luciferin. From this comes the name Pyrrhophyta: fire plants.

The name dinoflagellate refers to the whirling motion the cells sometimes perform. Most cells have two unequal flagella, one of which extends posteriorly and propels the cell, and the other circling the cell transversely, used for turning and maneuvering. Both lie in grooves, called the sulcus and the cingulum, respectively. In most forms the remainder of the cell is covered by plates of cellulose armor, supported by vesicles called alveoli - these unite dinoflagellates with other alveolates.

Normal dinoflagellates are haploid and usually reproduce by fission, although sexual reproduction does occur. The zygotes are motile and under adverse conditions form cysts, which are found as common microfossils since the Triassic and reach back at least to the Silurian. Some of the older acritarchs may also represent dinoflagellates, and other evidence suggests the group reaches back to the precambrian.

The organisms that cause red tides are dinoflagellates. Other well-known forms include Peridinium, Ceratium, and Gonyaulax.