Dinoflagellates — Phylum Dinophyta
Dinoflagellates are, for the most part, unicellular photosynthetic protists. They are an important component of the oceanic planktonic community where they provide a food source for zooplankton. Luminescent species (i.e., Noctiluca, Pyrodinum, and Gonyaulax) are responsible for the glow sometimes seen in the Arctic Ocean at night.
Many dinoflagellate cells are armoured with cellulose plates which become encrusted with silica. Most have two flagella, which jointly cause the dinoflagellate to spin. In fact, this spinning motion is where the dinoflagellates get their name – the Greek word dinein means "to whirl". Most dinoflagellate chloroplasts have chlorophylls a and c, in addition to carotenoids and xanthophylls, which give them either a yellowish-green or brown colour.
Dinoflagellates can be free-living or symbiotic with jellyfish, anemones, molluscs, or corals. Symbiotic dinoflagellates lack armoured plates and flagella and are a simple, spherical golden brown globule that resides in their host cell. In this state, the dinoflagellate is referred to as a zooxanthella.