|
|domain=Eukaryota, Bacteria |image=NSW seabed 1.JPG |image_width=263px |image_caption=A variety of algae growing on the sea bed in shallow waters |includes= * Archaeplastida * * Chlorophyta (green algae) * * Rhodophyta (red algae) * * Glaucophyta * * Charophyta * Rhizaria, Excavata * * Chlorarachniophytes * * Euglenids * Chromista, Alveolata * * Heterokonts * * * Bacillariophyceae (Diatoms) * * * Axodines * * * Bolidomonas * * * Eustigmatophyceae * * * Phaeophyceae (brown algae) * * * Chrysophyceae (golden algae) * * * Raphidophyceae * * * Synurophyceae * * * Xanthophyceae (yellow-green algae) * * Cryptophyta * * Dinoflagellata * * Haptophyta * Cyanobacteria (blue-green algae) |excludes= * Bacteria (non-photosynthetic) * Protista (non-photosynthetic) * Animalia * Embryophyta * Fungi}} Algae ( or ; singular ''alga'' ) is an informal term for a large, diverse group of eukaryotes that are not related and are thus polyphyletic. Included organisms range from unicellular genera, such as ''Chlorella'' and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga that may grow up to 50 meters in length. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata, xylem and phloem, that are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae that includes, for example, ''Spirogyra'' and the stoneworts. There is no generally accepted definition of algae. One definition is that algae "have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells". Some authors exclude all prokaryotes and thus do not consider cyanobacteria (blue-green algae) as algae. Algae constitute a polyphyletic group〔 since they do not include a common ancestor, and although their plastids seem to have a single origin, from cyanobacteria, they were acquired in different ways. Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria. Diatoms and brown algae are examples of algae with secondary chloroplasts derived from an endosymbiotic red alga. Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction.〔Smithsonian National Museum of Natural History; Department of Botany. http://botany.si.edu/projects/algae/introduction.htm〕 Algae lack the various structures that characterize land plants, such as the phyllids (leaf-like structures) of bryophytes, rhizoids in nonvascular plants, and the roots, leaves, and other organs that are found in tracheophytes (vascular plants). Most are phototrophic, although some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular species of green algae, many golden algae, euglenids, dinoflagellates and other algae have become heterotrophs (also called colorless or apochlorotic algae), sometimes parasitic, relying entirely on external energy sources and have limited or no photosynthetic apparatus.〔Pringsheim, E.G. 1963. ''Farblose Algen. Ein beitrag zur Evolutionsforschung''. Gustav Fischer Verlag, Stuttgart. 471 pp., species:Algae#Pringsheim .281963.29.〕 Some other heterotrophic organisms, like the apicomplexans, are also derived from cells whose ancestors possessed plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from cyanobacteria that produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago. ==Etymology and study== The singular ''alga'' is the Latin word for "seaweed" and retains that meaning in English. The etymology is obscure. Although some speculate that it is related to Latin ''algēre'', "be cold", there is no known reason to associate seaweed with temperature. A more likely source is ''alliga'', "binding, entwining." The Ancient Greek word for seaweed was ''φῦκος'' (fūkos or phykos), which could mean either the seaweed (probably red algae) or a red dye derived from it. The Latinization, ''fūcus'', meant primarily the cosmetic rouge. The etymology is uncertain, but a strong candidate has long been some word related to the Biblical ''פוך'' (pūk), "paint" (if not that word itself), a cosmetic eye-shadow used by the ancient Egyptians and other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, blue. Accordingly, the modern study of marine and freshwater algae is called either phycology or algology, depending on whether the Greek or Latin root is used. The name ''Fucus'' appears in a number of taxa. 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「Algae」の詳細全文を読む スポンサード リンク
|