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Dinoflagellates are microorganisms that are regarded as algae. Their wide array of exotic shapes and, sometimes, armored appearance is distinct from other algae. The closest microorganism in appearance are the diatoms .

Dinoflagellates are single-celled organisms. There are nearly 2000 known living species. Some are bacterial in size, while the largest, Noctiluca, can be up to two millimeters in size. This is large enough to be seen by the unaided eye.

Ninety per cent of all known dinoflagellates live in the ocean, although freshwater species also exist. In fact, dinoflagellates have even been isolated from snow. In these environments, the organisms can exist as free-living and independent forms, or can take up residence in another organism. A number of photosynthetic dinoflagellates inhabit sponges, corals, jellyfish, and flatworms. The association is symbiotic. The host provides a protective environment and the growth of the dinoflagellates impart nutritive carbohydrates to the host.

As their name implies, flagella are present. Indeed, the term dinoflagellate means whirling flagella. Typically, there are two flagella. One of these circles around the body of the cell, often lying in a groove called the cingulum. The other flagellum sticks outward from the surface of the cell. Both flagella are inserted into the dinoflagellate at the same point. The arrangement of the flagella can cause the organism to move in a spiral trajectory.

The complex appearance, relative to other algae and bacteria , is carried onward to other aspects of dinoflagellate behavior and growth. Some dinoflagellates feed on other microorganisms, while others produce energy using photosynthesis. Still other dinoflagellates can do both. The life cycle of the organisms is also complex, involving forms that are immobile and capable of movement and forms that are capable of sexual or asexual reproduction (bacteria, for example, reproduce asexually, by the self-replication of their genetic material and other constituents). Dinoflagellates are primarily asexual in reproduction.

Some dinoflagellates contain plates of cellulose that lie between the two surface membranes that cover the organism. These plates function as protective armor.

Dinoflagellates are noteworthy for several reasons. They are one of the bedrocks of the food chain, particularly in the oceans and lakes of the world. Their numbers can be so great that they are evident as a mass of color on the surface of the water. Sometimes satellite cameras can even visualize these blooms. This abundant growth can consume so much oxygen that survival of other species in the area is threatened. As well, some dinoflagellates can produce toxins that can find their way into higher species, particularly those such as shellfish that feed by filtering water through them. Paralytic shellfish poisoning, which harms the neurological system of humans, is an example of a malady associated with the consumption of clams, mussels, and oysters that are contaminated with dinoflagellate toxins known as saxitoxin and brevitoxin. Saxitoxin is extremely potent, exerting its effect on the neurological system at concentrations 10,000 times lower than that required by cocaine. Another example of a dinoflagellaterelated malady is a disease called ciguatera, which results from eating toxin-contaminated fish.

A third distinctive feature of dinoflagellates concerns their nucleus . The deoxyribonucleic acid shares some features with the DNA of eukaryotes , such as the presence of repeated stretches of DNA. But, other eukaryotic features, such as the supportive structures known as histones, have as yet not been detected. Also, the amount of DNA in dinoflagellates is far greater than in eukaryotes. The nucleus can occupy half the volume of the cell.

As with other microorganisms, dinoflagellates have been present on the Earth for a long time. Fossils of Arpylorus antiquus have been found in rock that dates back 400 million years. And, fossils that may be dinoflagellate cysts have been found in rock that is almost two billion years old. Current thought is that dinoflagellates arose when a bacterium was swallowed but not digested by another microorganism. The bacteria became symbiotic with the organism that swallowed them. This explanation is also how mitochondria are thought to have arisen.

Dinoflagellates cysts are analogous to the cysts formed by other microorganisms. They function to protect the genetic material during periods when conditions are too harsh for growth. When conditions become more favorable, resuscitation of the cyst and growth of the dinoflagellate resumes.

Dinoflagellates are sometimes referred to as Pyrrhophyta, which means fire plants. This is because of their ability to produce biological luminescence, akin to that of the firefly. Often, these luminescent dinoflagellates can be seen in the wake of ocean-going ships at night.

See also Bioluminescence; Red tide; Snow blooms

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Dinoflagellata (dī´nōflăj´əlät´ə, –lā´tə), phylum (division) of unicellular, mostly marine algae, called dinoflagellates. In some classification systems this division is called Pyrrhophyta. There are approximately 2,000 species of dinoflagellates. Most have two flagella that lie perpendicular to one another and cause them to spin as they move through the water. Most have walls, or thecae, that are rigid and armorlike and sometimes take on fantastic shapes. The plates that make up these walls are actually located inside the plasma membrane rather than outside, as cell walls are. Some species are heterotrophic, but many are photosynthetic organisms containing chlorophyll a and chlorophyll c. The green of these chlorophylls may be masked by various other pigments. Still other species are symbionts, living inside such organisms as jellyfish and corals. Food reserves are largely starch.

Reproduction for most dinoflagellates is asexual, through simple division of cells following mitosis. They are unusual in that in each cell, the chromosomes remain compact between divisions, instead of stretching out into slender threads, as in most other organisms. The chromosomes are constricted at regular intervals and do not have centromeres, or fiber-attachment centers. There is no spindle, yet the very numerous chromosomes are divided equally at the time of mitosis.

The dinoflagellates are important constituents of plankton, and as such are primary food sources in warmer oceans. Many forms are phosphorescent; they are largely responsible for the phosphorescence visible at night in tropical seas. The phenomenon known as red tide occurs when the rapid reproduction of certain dinoflagellate species results in large brownish red algal blooms. Some of these organisms are highly toxic and can kill fish and shellfish and kill or weaken the animals (including humans) that eat them in their turn or, in some cases, are merely exposed to water containing the organisms.

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dinoflagellates A division (Dinoflagellata) of protists that are heterotrophs but closely allied to brown algae and diatoms (they are sometimes classified as algae). Many have brown or yellow chromoplasts containing xanthophyll and chlorophylls a and c; others are colourless. Typically, dinoflagellates have two flagella, one propelling water to the rear and providing forward motion, attached just behind the centre of the body and directed posteriorly, the other causing the body to rotate and move forwards, forming a transverse ring or spiral of several turns around the centre of the body. Some dinoflagellates are naked, others are covered with a membrane or plates of cellulose. Many species are capable of emitting light, and these are the main contributors to bioluminescence in the sea. Most are planktonic, some in fresh water but most in marine environments, and some live in symbiosis with animals (e.g. the flatworm Amphiscolops, sea anemones, and corals) with which they exchange nutrients. Some are colonial. There are many species.

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Dinomastigota (Dinoflagellata) A phylum of mostly single-celled organisms classified in the kingdom Protoctista. They are abundant in the marine plankton; many are photoautotrophs, containing brown xanthophyll pigments in addition to chlorophyll. Dinoflagellates characteristically have two undulipodia (flagella) for locomotion and most have a rigid cell wall of cellulose encrusted with silica. Some species (e.g. Noctiluca miliaris) are bioluminescent.

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dinoflagellates See DINOPHYCEAE.