Phaeophyta

Phaeophyta , phylum (division) of the kingdom Protista consisting of those organisms commonly called brown algae. Many of the world's familiar seaweeds are members of Phaeophyta. There are approximately 1,500 species. Like the chrysophytes (see Chrysophyta ), brown algae derive their color from the presence, in the cell chloroplasts, of several brownish carotenoid pigments, including fucoxanthin, in addition to the photosynthetic pigments chlorophyll a and c. With only a few exceptions, brown algae are marine, growing in the colder oceans of the world, many in the tidal zone, where they are subjected to great stress from wave action; others grow in deep water. Among the brown algae are the largest of all algae, the giant kelps, which may reach a length of over 100 ft (30 m). Fucus (rockweed), Sargassum (gulfweed), and the simple filamentous Ectocarpus are other examples of brown algae.

The cell wall of the brown algae consists of a cellulose differing chemically from that of plants. The outside is covered with a series of gelatinous pectic compounds, generically called algin; this substance, for which the large brown algae, or kelps, of the Pacific coast are harvested commercially, is used industrially as a stabilizer in emulsions and for other purposes. The normal food reserve of the brown algal cell is a soluble polysaccharide called laminarin; mannitol and oil also occur as storage products. The body, or thallus, of the larger brown algae may contain tissues differentiated for different functions, with stemlike, rootlike, and leaflike organs, the most complex structures of all algae.

Some groups of brown algae have evolved an interesting type of alternation of generations, in which physiologically independent haploid gametophyte plants produce gametes, the fusion of which initiates the diploid sporophyte generation. The mature sporophyte plant produces, through meiosis , haploid spores, which develop into new gametophytes. The two generations, or phases, may be indistinguishable in size and form, or they may differ greatly. The genus Ectocarpus, for example, is found growing attached to larger algae. It has similar-looking gametophyte and sporophyte plants. In the kelps, however, the gametophyte is only a microscopic filament, in contrast to the occasionally tree-sized sporophyte.

Bibliography: See H. C. Bold and M. J. Wynne, Introduction to the Algae: Structure and Reproduction (1985); C. A. Lembi and J. R. Waaland, Algae and Human Affairs (1988); C. van den Hoek, Algae: an Introduction to Phycology (1994).

Phaeophyta (brown algae) A phylum of algae in which the green chlorophyll pigments are usually masked by the brown pigment fucoxanthin. Brown algae are usually marine (being abundant in cold water) and many species, such as the wracks (Fucus), inhabit intertidal zones. They vary in size from small branched filaments to ribbon-like bodies (known as kelps) many metres long.

Phaeophyta (brown algae) A division of algae which includes no single-celled species; almost all are marine, growing mostly in the intertidal regions (but species of Bodenella and Heribaudiella occur in fresh water). They are the dominant seaweeds in the colder waters of the northern hemisphere. They are typically olive-brown or greenish in colour (at least when wet) owing to the presence of the pigment fucoxanthin in the chloroplasts. There are at least 13 orders, and many genera.