| Abstract from DBPedia | Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and polar regions. They are dominant on rocky shores throughout cooler areas of the world. Most brown algae live in marine environments, where they play an important role both as food and as a potential habitat. For instance, Macrocystis, a kelp of the order Laminariales, may reach 60 m (200 ft) in length and forms prominent underwater kelp forests. Kelp forests like these contain a high level of biodiversity. Another example is Sargassum, which creates unique floating mats of seaweed in the tropical waters of the Sargasso Sea that serve as the habitats for many species. Many brown algae, such as members of the order Fucales, commonly grow along rocky seashores. Some members of the class, such as kelps, are used by humans as food. Between 1,500 and 2,000 species of brown algae are known worldwide. Some species, such as Ascophyllum nodosum, have become subjects of extensive research in their own right due to their commercial importance. They also have environmental significance through carbon fixation. Brown algae belong to the group Heterokontophyta, a large group of eukaryotic 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.褐藻（かっそう、英語: brown algae）はワカメやコンブ、ヒジキ、モズクなどを含む藻類の一群、またはこれに属する藻類のことである。分類学的には、オクロ植物門（不等毛植物門）の褐藻綱（学名: Phaeophyceae）にまとめられる。 全ての種が原形質連絡をもつ多細胞性の体をもち、比較的複雑な組織・器官分化を示すものや、長さ数十メートルに達するものもいる（右図）。陸上植物などとは独立に多細胞化を遂げたグループであるが、原生生物の中では最も複雑な多細胞体をもつ。細胞壁はセルロースやアルギン酸、フコイダンなどを含む。珪藻など他の不等毛藻と同様、二次共生した紅藻に由来する葉緑体をもつ。藻体はふつう褐色をしており、葉緑体は光合成色素としてクロロフィルa、クロロフィルc、フコキサンチンなどを含む。多くは単相の配偶体と複相の胞子体の間で世代交代を行うが、複相の世代のみをもつものもいる。ほとんどの種は沿岸域に生育する海藻であり、特にやの大型種は藻場を形成し、沿岸域の生態系の重要な構成要素となっている（右図）。上記のように食用として身近な海藻が含まれ、また細胞壁成分であるアルギン酸は食品添加物などに広く利用されている。 (Source: http://dbpedia.org/resource/Brown_algae) |
