Dinoflagellate

Dinoflagellates are a diverse group of unicellular protists that populate marine and freshwater systems. They are part of the phylum Dinoflagellata within the Alveolata, and most species are planktonic. A hallmark feature is their cell covering of cellulose plates forming the theca, along with two flagella—one encircling the cell in a groove and one trailing for steering—giving many species a distinctive spinning movement.

Many dinoflagellates are photosynthetic, possessing plastids acquired by secondary endosymbiosis, but a substantial number are nonphotosynthetic or mixotrophic, feeding on particles or other organisms. They occur in a range of aquatic habitats, from open oceans to estuaries, and are especially abundant in the photic zone of tropical and temperate seas.

Ecologically, dinoflagellates contribute to primary production and form a key link in marine food webs. Some species live in mutualistic relationships with corals, forming the zooxanthellae that supply corals with energy. Others are bioluminescent and emit light when disturbed. A subset can form harmful algal blooms, commonly called red tides, and release potent toxins that accumulate in shellfish and can affect human and animal health.

Reproduction is typically asexual through mitotic division, but sexual reproduction can occur under stress, leading to the formation of resting cysts that persist in sediments and help populations endure unfavorable conditions.