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Chlorophyta

Chlorophyta (green algae) A division of algae which are typically green in colour. They contain chlorophylls a and b and the storage product (starch) is formed in chloroplasts rather than in the cytoplasm. The organisms take many forms, ranging from unicellular to relatively complex multicellular plants. They are found mainly in freshwater habitats, and their distribution is cosmopolitan. They are known from the Precambrian to the present, and the earliest eukaryotes were probably of this class. The classification of the green algae is controversial. The division Chlorophyta is sometimes regarded as containing only 2 classes (Chlorophyceae and Charophyceae), but more recently it has been divided into 5 classes.

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Chlorophyta

Chlorophyta (green algae) A large phylum of algae, the members of which possess chlorophylls a and b, store food reserves as starch, and have cellulose cell walls. In these respects they resemble plants more closely than do any of the other algal phyla, although they are still classified as protoctists. The Chlorophyta are widely distributed and diverse in form. Unicellular forms may occur singly (sometimes with undulipodia (flagella) for motility) or in colonies, while multicellular forms may be filamentous (e.g. Spirogyra) or platelike (e.g. Ulva).

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Chlorophyta

Chlorophyta (green algae) Division of algae which are typically green in colour. In common with higher land plants, green algae include chlorophylls a and b among their principal pigments, have cellulose as the main constituent of cell walls, and form food reserves of starch. Consequently it is believed that the ancestors of land plants must have belonged to this group. The organisms take many forms, ranging from unicellular to relatively complex multicellular plants. They are found today mainly in freshwater habitats and their distribution is cosmopolitan. They are known from the Precambrian onwards, and the earliest eukaryotes were probably of this division. Marine, lime-secreting green algae have contributed to algal limestone reefs since the Cambrian. Fossil genera include Palaeoporella (similar to modern Halimeda), and Coelosphaeridium (similar to modern Acetabularia). See also CHAROPHYCEAE.

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Chlorophyta

Chlorophyta (klōrŏf´ətə), phylum (division) of the kingdom Protista consisting of the photosynthetic organisms commonly known as green algae. The organisms are largely aquatic or marine. The various species can be unicellular, multicellular, coenocytic (having more than one nucleus in a cell), or colonial. Those that are motile have two apical or subapical flagella. A few types are terrestrial, occurring on moist soil, on the trunks of trees, on moist rocks, and even in snowbanks. Various species are highly specialized, some living exclusively on turtles, sloths, or within the gill mantles of marine mollusks.

It is generally accepted that early chlorophytes gave rise to the plants. Cells of the Chlorophyta contain organelles called chloroplasts in which photosynthesis occurs; the photosynthetic pigments chlorophyll a and chlorophyll b, and various carotenoids, are the same as those found in plants and are found in similar proportions. Chlorophytes store their food in the form of starch in plastids and, in many, the cell walls consist of cellulose. Unlike in plants, there is no differentiation into specialized tissues among members of the division, even though the body, or thallus, may consist of several different kinds of cells. There are four evolutionary lineages of green algae. Most living species are grouped in classes that are coextensive with three of these lineages.

Class Chlorophyceae

This group contains the largest number of species of the division. They can have two or more flagella, near the apex of the cell. Mitosis in this class involves phycoplasts, microtubules that develop between and separate the daughter nuclei. This characteristic is not seen in any other organism, implying that no organisms have descended from this class. There are a variety of asexual and sexual reproductive techniques. Sexual reproduction is characterized by the formation of a zygospore (a dormant diploid zygote protected by a thick wall) that later undergoes meiosis.

The class includes unicellular organisms such as those in the genus Chlamydomonas with their two apical flagella and nonmotile organisms such as Chlorella, which is being cultivated for use as a dietary supplement. Colonial genera of Chlorophyceae include Hydrodictyon (the "water net" ) and the so-called volvocine line of flagellated specimens that range from simple colonies of Gonium to the intricate spinning spheres of Volvox, which can consist of up to 60,000 cells and exhibit some cellular specialization. The most complex of the class are the filamentous members, some of which exhibit features that are seen primarily in plants. Despite this similarity the class is not believed to have been the evolutionary source of plants.

Class Charophyceae

Charophyceae are of great fossil age; the stoneworts date as far back as the late Silurian period. Cells of this class are asymetrical. Those that are motile have two flagella, at right angles near the apex of the cell. Sexual reproduction in this class, as in Chlorophyceae, is characterized by the formation of a zygospore and zygotic meiosis. Unlike in the other two common classes of green algae, but as with plants, the nuclear envelope disintegrates when mitosis begins. During cell division the mitotic spindle is present; in some a phragmoplast similar to those seen in plants aids in the formation of a cell plate. Plants are thought to have evolved from early species of Charophyceae.

The class includes Spirogyra, familiar filamentous algae that float on ponds and lakes in slimy masses. The desmids are single cells noted for their extraordinary symmetry and geometrical beauty. They are found only in fresh (usually still) water and often take an important place in the food chains of small nutrient-poor ponds and peat bogs. The stoneworts consist of a complex branched thallus with an erect stemlike structure and many whorls of short branches. They occur in shallow fresh or brackish water and especially in water rich in calcium, where they become stiff and lime-encrusted, a characteristic that has made them plentiful in the fossil record.

Class Ulvophyceae

Ulvophyceae contains marine organisms that take a variety of shapes that may consist of a few cells, long filaments, thin sheets of cells, or coenocytic cells. Most approach being radially symmetrical. They have an alternation of generations and unlike in the other classes, meiosis occurs in the spores rather than the zygotes. When present, there can be two or more apical flagella. During mitosis, the nuclear envelope and the mitotic spindle persist, as they do in the Charophyceae.

The class Ulvophyceae includes sea lettuce, or Ulva, bright green, leaflike algae that grows in shallow waters on rocks and piers. Ventricaria is an egg-shaped, coenocytic alga, familiar in warm seas. Some organisms of Ulvophyceae produce toxins that discourage predation. The chloroplasts of some others become symbionts after they are retained in the bodies of sea slugs that eat the algae. They continue to perform photosynthesis, providing the slug with needed oxygen.

See also seaweed.

Bibliography

See H. C. Bold and M. J. Wynne, Introduction to the Algae: Structure and Reproduction (1985); C. A. Lembi and J. R. Waaland, Algae and Human Affairs (1988); C. van den Hoek, Algae: an Introduction to Phycology (1994).

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Chlorophyta

Chlorophyta

Chlorophyta are microorganisms that are grouped in the kingdom called Protista. The microbes are plant-like, in that they are able to manufacture energy from sunlight. The microbes are also commonly known as green algae

Depending on the species, Chlorophyta can be single-celled, multicelled, and can associate together in colonies. The environmental diversity of Chlorophyta is vast. Many types live in marine and fresh water. Terrestrial habitats include tree trunks, moist rocks, snowbanks, and creatures including turtles, sloths and mollusks. There are some 8,000 species of chlorophytes, ranging in size from microscopic to visibly large.

There are three classes of Chlorophyta. The first class, which contains the greatest number of organisms, is called Chlorophyceae. A notable example of an organism from this class is Chlorella, which is economically important as a dietary supplement. Another member of the class is Volvox, a spherical organized community containing upwards of 60,000 cells.

The second class is called Charophyceae. Members of this class have existed since prehistoric times, as evidenced by fossil finds. An example of this class is Spirogyra, which form slimy filaments on the surface of freshwater.

The third class is called Ulvophyceae. These are marine organisms. Some become associated with sea slugs where they provide the slug with oxygen and are in turn provided with protection and nutrients. Species of a calcium-rich green algae called Halimeda form the blinding white sand beaches of the Caribbean when they wash up onshore and become bleached by the sun. Another example from this class is Ulva that grows on rocks and wharves as green, leafy-appearing clusters.

Chlorophyta contain structures that are called chloroplasts. Within the chloroplasts two pigments (chlorophyll a and chlorophyll b) are responsible for the conversion of sunlight to chemical energy. The energy is typically stored as starch, and in their cell walls, which are composed of a material called cellulose. The stored material can be used for energy as needed. This process of energy generation is similar to that which occurs in plants. There is an evolutionary basis for this similarity. Available evidence indicates that members of Chlorophyta were the precursors of plants. Chlorophyte fossils date from over one billion years ago, before the development of plants.

See also Photosynthesis

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