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Appearance
Brown algae (Phaeophyta), belongs to the Heterokontophyta group of organisms. The vast majority of species within the group get their color from the brown pigment fucoxanthin. The amount of fucoxanthin present in the algae determines the various shades of the different species. Pigmentation ranges from olive-green to dark brown. Brown algae takes a wide variety of forms, ranging from delicate, feather-like filaments to vast leathery strings of kelp that snake up to 100 feet in length.
Biology
Brown algae are the most complex forms of algae. Species typically store food as complex polysaccharides, sugars and higher alcohols. True starch is absent. Cell walls are made of cellulose and alginic acid. Many brown algae exhibit complex, differentiated tissues, including a holdfast that secures the organism to the surface on which it grows, air bladders to aid with buoyancy, a supporting stalk, and spore-producing reproductive tissues. Brown algae shed reproductive cells into water. All species are multi-cellular.
Kelp
The largest variety of brown algae is the giant kelp, or Macrocystis pyrifera, also known as giant bladder kelp. In his examination of the species, Professor David John of The British Natural History Museum notes that the fronds are capable of growing to 45m in one season -- the world's fastest linear growth. Giant kelp is buoyed by small gas-filled bladders which spread out on the surface to form a dense subsurface canopy.
Other Varieties
The many species of brown algae provide food and shelter for various forms of aquatic sea life. They are also used by humans in food, medicine and as fertilizers. Other notable varieties of brown algae include Sargassum (a large brown seaweed after which the Sargasso Sea is named) and bladder wrack or rockweed (the seaweed from which French chemist Bernard Courtois originally discovered iodine in 1811).