Brown algae

The Phaeophyceae or brown algae, (singular: alga) is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments both as food, and for the habitats they can form. For instance Macrocystis, a member of the Laminariales or kelps, may reach 60 m in length, and forms prominent underwater forests. Another example is Sargassum, which creates unique habitats in the tropical waters of the Sargasso Sea. This is one of the few areas where a large biomass of brown algae may be found in tropical waters. Many brown algae such as members of the order Fucales are commonly found along rocky seashores. Some members of the division are used as food for humans.

Worldwide there are about 1500 — 2000 brown seaweed species.

Brown algae belong to a very large group, the Heterokontophyta, a eukaryotic group of 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 flagellate spores, which closely resemble other heterokont cells. Genetic studies show their closest relatives to be the yellow-green algae.

Phaeophyta first appear in the fossil record in the Mesozoic, possibly as early as the Jurassic. Their occurrence as fossils is rare due to their generally soft-bodied habit, and scientists continue to debate the identification of some finds. Other algae groups, such as the red algae and green algae have a number of calcareous members, which are more likely to leave evidence in the fossil record than the soft bodies of the brown algae. Miocene fossils of a soft-bodied brown macro algae, Julescrania, have been found well-preserved in Monterey Formation diatomites, but few other certain fossils, particularly of older specimens are known in the fossil record. 

Taxonomy
This is a list of the orders in the class Phaeophyceae:
 * Ascoseirales Petrov
 * Choristocarpales
 * Cutleriales Oltmanns
 * Desmarestiales Setchell & Gardner
 * Dictyotales Kjellman
 * Ectocarpales Setchell & Gardner
 * Fucales Kylin
 * Ishigeales
 * Laminariales Migula
 * Ralfsiales Nakamura
 * Scytothamnales A. F. Peters & M. N. Clayton
 * Sphacelariales Oltmanns
 * Sporochnales Sauvageau
 * Syringodermatales E. C. Henry
 * Tilopteridale Bessey

A few species, such as Botrydium stoloniferum, are placed incertae sedis, or of uncertain position, as to order in this classification scheme.

Life cycle
The life cycle shows great variability from one group to another. However the life cycle of Laminaria consists of the diploid generation, that is the large plant well know to most people. It produces sporangia from specialised microscioic structures, these divide meiotically (meiosis) before they are released. As they are haploid there are equal numbers of male and female spores. With the exception of the Fucales all brown algae have a life cycle which consists of an alternation between morphologically haploid and diploid plants referred to as Monomorphic. A dimorphic life cycle consists of an alteration between dissimilar haploid and diploid plants.