Dinoflagellates are a group of flagellate unicellular organisms which are considered a type of algae. Their name is from the Greek dinos meaning "whirling" and flagella which means whip. This is a reference to their characteristic whirling motion as they move through the water using two flagella: a longitudinal flagellum and a transverse flagellum. Dinoflagellates make up a major portion of eukaryotic (complex unicellular) oceanic plankton, second in abundance only to the diatoms.
Dinoflagellates are known as the source of red tides and one of the sources of oceanic bioluminescence. "Red tides," also known as harmful algal blooms, occur when phytoplankton (photosynthesizing protists) release harmful chemicals into the water to kill fish and other organisms that threaten to eat them. These chemicals can get into airborne moisture and irritate the lungs and mucus membranes of people on the beach. The phenomenon is most common on the Gulf Coast of Florida. Sometimes red tides are cased by human activity, such as agricultural runoff, which provides an abundant nutrient source to the dinoflagellates. Harmful algal blooms are also associated with oxygen depletion, as the organisms in the red tide take up a lot of oxygen and then perish, the oxygenated molecules in their bodies sinking to the bottom of the sea.
Dinoflagellates have characteristics of both plants and animals. They can photosynthesize, but they are also mobile. Dinoflagellates have various adaptations to survive microscopic predators. Some species have cellulose armor called a theca, while others have body spikes called extrusomes, which can also release substances such as harmful chemicals outside the cell. Dinoflagellates reproduce both asexually, by dividing, and sexually, by combining with another member of its species and forming a zygote. The zygote releases substances that cause it to be enclosed within a cyst, in a process called encystment. After a certain period of time, the cyst breaks, and the cell divides, fresh with new genetic material. The cysts, called dinocysts, are the only representatives of dinoflagellates in the fossil record.
Dinoflagellates can be summarized as small (though some species are as large as 2 mm), planktonic (90% are marine plankton), motile, and often armored. Some dinoflagellates are actually not photosynthetic, but most are. Dinoflagellates are also characterized by their compact genomes, some of which have been fully sequenced.