Phaeophyte

Brown algae Temporal range: 150–0 Ma PreЄ Є O S D C P T J K Pg N
Giant kelp (Macrocystis pyrifera)
Scientific classification
Domain:	Eukaryota
(unranked):	SAR
Superphylum:	Heterokonta
Phylum:	Ochrophyta
Class:	Phaeophyceae Kjellman, 1891
Orders
see Classification.
Synonyms
Fucophyceae Warming, 1884 Melanophyceae Rabenhorst, 1863 Phaeophyta

see Classification.

The Phaeophyceae or brown algae (singular: alga), are a large group of mostly marine multicellular algae, including many seaweeds located in colder Northern Hemisphere waters. They play an important role in marine environments, both as food and as habitats. For instance, Macrocystis, a kelp of the order Laminariales, may reach 60 m (200 ft) in length and forms prominent underwater kelp forests. Kelp forests like these 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. Many brown algae, such as members of the order Fucales, commonly grow along rocky seashores. Some members of the class, such as kelp, are used as food for humans.

Worldwide, over 1500–2000 species of brown algae are known. Some species, such as Ascophyllum nodosum, are important in commercial use because they have become subjects of extensive research in their own right. They have environmental importance too through Carbon fixation. In the ecosystem, there is sometimes a symbiotic relationship between coral and algae. Coral will host algae and use its carbon fixation for it own good. This relationship is not always mutually beneficial, as the coral often loses its pigments and begins bleaching.

Brown algae belong to the group Heterokontophyta, a large group of eukaryotic organisms distinguished most prominently by having chloroplasts surrounded by four membranes, suggesting an origin from a symbiotic relationship between a basal eukaryote and another eukaryotic organism. 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 heterokonts in developing into multicellular forms with differentiated tissues, but they reproduce by means of flagellated spores and gametes that closely resemble cells of other heterokonts. Genetic studies show their closest relatives to be the yellow-green algae.