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Brown Algae

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.

Life Cycle

The life cycle of a representative species Laminaria. Most Brown Algae follow this form of sexual reproduction.

Most brown algae, with the exception of the Fucales, perform sexual reproduction through sporic meiosis. Between generations, the algae go through separate sporophyte (diploid) and gametophyte (haploid) phases. The sporophyte stage is often the more visible of the two, though some species of brown algae have similar diploid and haploid phases. Free floating forms of brown algae often do not undergo sexual reproduction until they attach themselves to substrate. The haploid generation consists of male and female gametophytes. The fertilization of egg cells varies between species of brown algae, and may be isogamous, oogamous, or anisogamous.  Fertilization may take place in the water with eggs and motile sperm, or within the oogonium itself.

Certain species of brown algae can also perform asexual reproduction through the production of motile diploid zoospores. These zoospores form in plurilocular sporangium, and can mature into the sporophyte phase immediately.

A closeup of a Fucus' conceptacle, showing the gametes coming together to form a fertilized zygote.

In a representative species Laminaria, there is a conspicuous diploid generation and smaller haploid generations. Meiosis takes place within several unilocular sporangium along the algae's blade, each one forming either haploid male or female zoospores. The spores are then released from the sporangia and grow to form male and female gametophytes. The female gametophyte produces an egg in the oogonium, and the male gametophyte releases motile sperm that fertilize the egg. The fertilized zygote then grows into the mature diploid sporophyte.

In the order Fucales, sexual reproduction is oogamous, and the mature diploid is the only form for each generation. Gametes are formed in specialized conceptacles that occur scattered on both surfaces of the receptacle, the outer portion of the blades of the parent plant. Egg cells and motile sperm are released from separate sacs within the conceptacles of the parent algae, combining in the water to complete fertilization. The fertilized zygote settles onto a surface and then differentiates into a leafy thallus and a finger-like holdfast. Light regulates differentiation of the zygote into blade and holdfast.

Importance and Uses

Brown algae include a number of edible seaweeds. All brown algae contain alginic acid (alginate) in their cell walls, which is extracted commercially and used as a industrial thickening agent in food and for other uses. One of these products is used in a Lithium Ion batteries. Alginic acid is used as a stable component of a battery anode. This polysaccharide is a major component of brown algae, and is not found in land plants.

Alginic acid can also be used in aquaculture. For example, alginic acid enhances the immune system of rainbow trout. Younger fish are more likely to survive when given a diet with alginic acid.

Brown algae including kelp beds also fix a significant portion of the earth's carbon dioxide yearly through photosynthesis.

Sargachromanol G, an extract of Sargassum siliquastrum, has been shown to have anti-inflammatory effects.