green algae(redirected from green alga)
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green algae:see algaealgae
[plural of Lat. alga=seaweed], a large and diverse group of primarily aquatic plantlike organisms. These organisms were previously classified as a primitive subkingdom of the plant kingdom, the thallophytes (plants that lack true roots, stems, leaves, and flowers).
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, phylum (division) of the kingdom Protista consisting of the photosynthetic organisms commonly known as green algae. The organisms are largely aquatic or marine.
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(Chlorophyta), a phylum of lower plants characterized by a green color, which arises because of a predominance of chlorophyll in their cells. Green algae contain the same pigments as higher plants (chlorophyll a and b, carotene, and xanthophyll) and in almost the same ratios.
There are unicellular, colonial, and multicellular green algae. The multicellular plants are mostly threadlike; some are lamellate. Some green algae have a so-called noncellular structure; that is, their body, despite its large size and sometimes complex outer segmentation, is not divided into cells. The mobile unicellular and colonial forms, as well as the green algae zoospores and gametes, have two to four (occasionally more) flagella and a light-sensitive ocellus. Green algae cells are uninuclear or, less frequently, multinuclear, in most cases covered with a predominantly cellulose envelope. The chromatophores often have pyrenoids. Reproduction is asexual (by means of zoospores and immobile spores), sexual (isogamy, heterogamy, oogamy, conjugation), and vegetative (unicellular organisms reproducing by body division, multicellular threadlike organisms by means of parts of the thallus). The vegetative cells fulfill the functions of the sexual and asexual reproductive organs. Some Dasycladophyceae have sporangia as a special formation; gametangia are found among some Dasycladophyceae and Siphonophyceae. Depending upon external conditions, in some green algae the same individual may have organs for either asexual or sexual reproduction; others have sporophytes and gametophytes, which can have identical or different structures. In green algae, meiosis occurs in the zygote.
Green algae are divided into two subphyla: Chlorophytina, whose sexual process is in the form of fusion of gametes, and Conjugatophytina. The first subphylum is divided into six classes: Protococcophyceae or Chlorococcophyceae, Volvocophyceae, Ulotrichophyceae, Siphonocladophyceae, Dasycladophyceae, and Siphonophyceae. There are approximately 5,700 known species of green algae, belonging to 360 genera. They are distributed mainly in freshwater environments but are also found in seas. A small number of green algae have adapted to life on tree trunks or in the soil (Protococcus, Trentepohlia). Unicellular and colonial green algae are included among plankton, and they color the water when they grow in large numbers. Sea lettuce (Ulva) and monostromatic algae are used as food in East Asia. In many countries highly profitable methods have been devised for the intensive cultivation of unicellular green algae (Chlorella, Scenedesmus) as a source of food and animal fodder, for the biological purification of sewage, and for the regeneration of the air in closed ecosystems such as spaceships and submarines.
IU. E. PETROV