Algae

Article By:

Silva, Paul C. Department of Botany, University of California, Berkeley, California.

Moe, Richard L. Department of Botany, University of California, Berkeley, California.

Last reviewed:February 2021

DOI:https://doi.org/10.1036/1097-8542.022000

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• Algae, published June 2014:Download PDF Get Adobe Acrobat Reader

An informal assemblage of predominantly aquatic eukaryotes that carry out oxygen-evolving photosynthesis, but lack specialized water-conducting and food-conducting tissues. Algae (Fig. 1) are eukaryotic organisms (having an organized nucleus). Previously, taxonomists thought that algae were plants. However, algae are now considered to be plantlike protists (eukaryotes that are not fungi, animals, or plants). With regard to general form and structure, algae range from unicells that are 1–2 μm in diameter to huge thalli [vegetative bodies; for example, kelps measuring 30 m (100 ft) in length] with functionally and structurally distinctive tissues and organs. Unicells may be solitary or colonial, attached or free-living, with or without a protective cover, and motile or nonmotile. Colonies may be irregular or they may display a distinctive pattern, with the latter type being flagellate or nonmotile. Multicellular algae form packets, branched or unbranched filaments, sheets that are one or two cells thick, or complex thalli, including some with organs resembling roots, stems, and leaves (as in the brown algal orders Fucales and Laminariales). Coenocytic algae, in which the protoplast is not divided into cells, range from microscopic spheres to thalli that are 10 m (33 ft) in length with a complex structure of intertwined siphons (as in the green algal order Bryopsidales). Algae differ from the next most advanced group of organisms, the bryophytes (nonvascular land plants), by their lack of multicellular sex organs sheathed with sterile cells and by their failure to retain an embryo within the female organ. Many colorless organisms are referable to the algae on the basis of their similarity to photosynthetic forms with respect to structure, life history, cell wall composition, and storage products. The study of algae is called algology (from the Latin alga, meaning sea wrack) or phycology (from the Greek phykos, meaning seaweed). See also: Bryophyta; Bryopsidales; Cell nucleus; Eukaryota; Fucales; Laminariales; Photosynthesis; Protist

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