|any algae of the phylum Phaeophyta, such as the wracks and kelps, which contain a brown pigment in addition to chlorophyll|
|brown alga (broun) Pronunciation Key
Any of various photosynthetic protists belonging to the phylum Phaeophyta, almost all of which live in marine environments. Brown algae have chlorophylls a and c as well large quantities of the pigment fucoxanthin, which gives the group their characteristic brownish colors. This pigment absorbs the range of blue light frequencies available to brown algae when they are submerged and allows them to live in deeper waters than green algae. The brown algae store their food in a compound called laminarin and transport it throughout their bodies in a compound called mannitol (unlike plants and green algae). The cell walls of brown algae are made of cellulose and algin. Their bodies vary from small filaments to the immense leaflike thalli of kelp. Species of brown algae dominate shoreline and coastal ecosystems in cooler waters, and huge masses of the brown alga Sargassum cover the warm Sargasso sea in the Atlantic. See more at alga.
members of the class Phaeophyceae (division Chromophyta), comprising about 1,500 species, common in cold waters along continental coasts. Freshwater species are rare. Species colour varies from dark brown to olive green, depending upon the proportion of brown pigment (fucoxanthin) to green pigment (chlorophyll). Some brown seaweeds have gas-filled bladders (pneumatocysts), which keep photosynthetic parts of the algal thallus floating on or near the surface of the water. Brown algae vary in form and size from small filamentous epiphytes (Ectocarpus) to complex giant kelps that range in size from 1 metre to more than 100 metres (3.3 to 330 feet; Laminaria, Macrocystis, Nerocystis). Rockweed, another type of brown algae, is found attached to rocky coasts in temperate zones (Fucus, Ascophyllum) or floating freely (Sargassum). Brown algae multiply by asexual and sexual reproduction; both the motile zoospores and gametes have two unequal flagella. Once a major source of iodine and potash, brown algae are still an important source of algin, a colloidal gel used as a stabilizer in the baking and ice-cream industries. Certain species are also used as fertilizer, and several are eaten as a vegetable (e.g., Laminaria, called kombu in Japan) in East Asia. Brown algae of the order Laminariales are popularly called kelps.
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