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The Phaeophyceae or brown algae (singular: ''alga''), is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments, both as food and for the habitats they form. For instance ''Macrocystis'', a kelp of the order Laminariales, may reach 60 m in length, and forms prominent underwater forests. Another example is ''Sargassum'', which creates unique habitats in the tropical waters of the Sargasso Sea. Many brown algae, such as members of the order Fucales, commonly grow along rocky seashores. Some members of the class, such as kelp, are used as food for humans. Worldwide there are about 1500–2000 species of brown algae.〔 〕 Some species are of sufficient commercial importance, such as ''Ascophyllum nodosum'', that they have become subjects of extensive research in their own right.〔 〕 Brown algae belong to a very large group, the Heterokontophyta, a eukaryotic group of organisms distinguished most prominently by having chloroplasts surrounded by four membranes, suggesting an origin from a symbiotic relationship between a basal eukaryote and another eukaryotic organism. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name. Brown algae are unique among heterokonts in developing into multicellular forms with differentiated tissues, but they reproduce by means of flagellated spores and gametes that closely resemble cells of other heterokonts. Genetic studies show their closest relatives to be the yellow-green algae. ==Morphology== Brown algae exist in a wide range of sizes and forms. The smallest members of the group grow as tiny, feathery tufts of threadlike cells no more than a few centimeters long.〔 〕 Some species have a stage in their life cycle that consists of only a few cells, making the entire alga microscopic. Other groups of brown algae grow to much larger sizes. The rockweeds and leathery kelps are often the most conspicuous algae in their habitats.〔 〕 Kelps can range in size from the two-foot-tall sea palm ''Postelsia'' to the giant kelp ''Macrocystis pyrifera'', which grows to over 45 m (150 ft) long〔 〕〔 〕 and is the largest of all the algae. In form, the brown algae range from small crusts or cushions〔 〕 to leafy free-floating mats formed by species of ''Sargassum''. They may consist of delicate felt-like strands of cells, as in ''Ectocarpus'', or of foot-long flattened branches resembling a fan, as in ''Padina''. Regardless of size or form, two visible features set the Phaeophyceae apart from all other algae. First, members of the group possess a characteristic color that ranges from an olive green to various shades of brown. The particular shade depends upon the amount of fucoxanthin present in the alga.〔 Second, all brown algae are multicellular. There are no known species that exist as single cells or as colonies of cells,〔 and the brown algae are the only major group of seaweeds that does not include such forms. However, this may be the result of classification rather than a consequence of evolution, as all the groups hypothesized to be the closest relatives of the browns include single-celled or colonial forms. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「brown algae」の詳細全文を読む スポンサード リンク
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