Brown algae. Image: NOAA.
Brown algae are organisms belonging to the phylum Phaeophyta of the kingdom Protista. Of the approximately 1,500 species of brown algae known, almost all are marine. They include many seaweeds, including the familiar wracks and bladder wrack, and the kelps, including the largest of all brown algae, Macrocystis – a genus of kelp that grows to more than 100 m (320 ft) at up to 0.5 m (18 in) per day. Sargassum forms vast floating masses in the Sargasso Sea in the mid-Atlantic, with their own distinctive communities of animals and microorganisms.
Brown algae are very diverse in form and size, ranging from less than 1 mm long to some species that are among the largest photosynthetic organisms on Earth. The largest kelps may grow to more than 60 m in length, forming dense underwater forests in colder waters. Many microscopic brown algae grow as epiphytes on underwater vegetation, forming networks of branched filaments, or broad encrustations. All species are multicellular and do not form colonies. Their life cycles are complex, involving alternation of generations. In general, they are not free-floating organisms, but are attached to rock, coral, or other firm surfaces. A species of Sargassum is exceptional in being pelagic (open-sea dwelling), accumulating in large quantities in the Sargasso Sea near the West Indies and staying afloat by means of gas-filled bladders.
The main food reserve of brown algae is the polysaccharide laminarin. The cell wall consists of two layers: an inner one of cellulose and an outer one of mucilaginous pectic material. Asexual reproduction occurs by fragmentation of thallus or by mostly laterally biflagellate zoospores. Sexual reproduction is isogamous, gametes motile, or anisogamous.