Dinoflagellate

The dinoflagellates (Greek δῖνος dinos 'whirling' and Latin flagellum 'whip, scourge') are a classification subgroup of protista.They are a large group of flagellate eukaryotes that constitute the phylum Dinoflagellata. Most are marine plankton, but they also are common in freshwater habitats. Their populations are distributed depending on sea surface temperature, salinity, or depth. Many dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy). In terms of number of species, dinoflagellates are one of the largest groups of marine eukaryotes, although this group is substantially smaller than diatoms. Some species are endosymbionts of marine animals and play an important part in the biology of coral reefs. Other dinoflagellates are unpigmented predators on other protozoa, and a few forms are parasitic (for example, Oodinium and Pfiesteria). Some dinoflagellates produce resting stages, called dinoflagellate cysts or dinocysts, as part of their lifecycles. Dinoflagellates are considered to be protists, with their own division, Dinoflagellata. About 1,555 species of free-living marine dinoflagellates are currently described. Another estimate suggests about 2,000 living species, of which more than 1,700 are marine (free-living, as well as benthic) and about 220 are from fresh water. The latest estimates suggest a total of 2,294 living dinoflagellate species, which includes marine, freshwater, and parasitic dinoflagellates. A bloom of certain dinoflagellates can result in a visible coloration of the water, colloquially known as red tide, which can cause shellfish poisoning if humans eat contaminated shellfish. Some dinoflagellates also exhibit bioluminescence—primarily emitting blue-green light. In 1753, the first modern dinoflagellates were described by Henry Baker as 'Animalcules which cause the Sparkling Light in Sea Water', and named by Otto Friedrich Müller in 1773. The term derives from the Greek word δῖνος (dinos), meaning whirling, and Latin flagellum, a diminutive term for a whip or scourge. In the 1830s, the German microscopist Christian Gottfried Ehrenberg examined many water and plankton samples and proposed several dinoflagellate genera that are still used today including Peridinium, Prorocentrum, and Dinophysis. These same dinoflagellates were first defined by Otto Bütschli in 1885 as the flagellate order Dinoflagellida. Botanists treated them as a division of algae, named Pyrrophyta or Pyrrhophyta ('fire algae'; Greek pyrr(h)os, fire) after the bioluminescent forms, or Dinophyta. At various times, the cryptomonads, ebriids, and ellobiopsids have been included here, but only the last are now considered close relatives. Dinoflagellates have a known ability to transform from noncyst to cyst-forming strategies, which makes recreating their evolutionary history extremely difficult. Dinoflagellates are unicellular and possess two dissimilar flagella arising from the ventral cell side (dinokont flagellation). They have a ribbon-like transverse flagellum with multiple waves that beats to the cell's left, and a more conventional one, the longitudinal flagellum, that beats posteriorly. The transverse flagellum is a wavy ribbon in which only the outer edge undulates from base to tip, due to the action of the axoneme which runs along it. The axonemal edge has simple hairs that can be of varying lengths. The flagellar movement produces forward propulsion and also a turning force. The longitudinal flagellum is relatively conventional in appearance, with few or no hairs. It beats with only one or two periods to its wave. The flagella lie in surface grooves: the transverse one in the cingulum and the longitudinal one in the sulcus, although its distal portion projects freely behind the cell. In dinoflagellate species with desmokont flagellation (e.g., Prorocentrum), the two flagella are differentiated as in dinokonts, but they are not associated with grooves.