Brown algae

Brown algae (sg.: alga) are a large group of photoautotrophic, multicellular SAR comprising the class Phaeophyceae. They include many seaweeds located in colder waters of the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and polar regions. Many brown algae, such as members of the order Fucales, commonly grow along rocky seashores. Most brown algae live in marine environments, where they play an important role both as food and as a potential habitat. For instance, Macrocystis, a kelp of the order Laminariales, may reach 60 m (200 ft) in length and forms prominent underwater kelp forests that contain a high level of biodiversity. Another example is Sargassum, which creates unique floating mats of seaweed in the tropical waters of the Sargasso Sea that serve as the habitats for many species. Some members of the class, such as kelps, are used by humans as food.

Between 1,500 and 2,000 species of brown algae are known worldwide. Some species, such as Ascophyllum nodosum, have become subjects of extensive research in their own right due to their commercial importance. They also have environmental significance through carbon fixation.

Brown algae belong to the Stramenopiles, a clade of eukaryotic organisms belonging to the SAR supergroup, and are thus not closely related to red algae, green algae, nor green plants (which are all archaeplastids). The chloroplasts of brown algae are surrounded by four membranes, suggesting that they were acquired secondarily from a symbiotic relationship between a stramenopile and possibly a red or green alga. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name. Brown algae are unique among Stramenopiles in developing into multicellular forms with differentiated tissues. They still reproduce by means of flagellated spores and gametes that closely resemble cells of other Stramenopiles. Genetic studies show its closest relative to be the species Schizocladia isciensis, followed by the yellow-green algae.