BROWN ALGAE

members of the phylum or division Phaeophyta, comprising about 1,500 species, common in cold waters along continental coasts; freshwater species are rare. The colour of brown algae, which varies from dark brown to olive green, depends upon the proportion of brown pigment (fucoxanthin) to green pigment (chlorophyll). Gas-filled bladders (pneumatocysts) keep photosynthetic parts of the algal thallus floating on or near the surface of the water. Brown algae vary also in form and size, from the small filamentous epiphytes (Ectocarpus) to complex giant kelps that range in size from 1 m to more than 100 m (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 (kombu) in the Orient. Brown algae of the order Laminariales are popularly called kelp.