Brown algae are common on coral reefs and are conspicuous in reef flat habitats. in the In the Caribbean several species of brown algae (eg. Dictyota sp.) are becoming pests, replacing the coral Acropora palmata . On the Great Barrier Reef species of the brown algae Sargassum may be prominent on inshore reefs. At certain seasons two other large brown algae may dominate the top of emergent coral: Chnoospora and Hydroclathrus.

• To be a plant with chlorophyll a and c.
• A carotenoid pigment (fucoxanthin) which makes these algae appear brown.
• Motile reproductive cells with two different flagella.
• To store the products of photosynthesis in the cytoplasm in the form of granules of laminarin.
• No supportive cells around gametes.
• Four membranes around the chloroplasts.

Sea weed

Brown algae can range in size from thin filaments to fleshy forms and include the gigantic kelps that grow 100 m long. They are usually brown or yellowish-brown and have a range of structures and symmetry. They are solitary plants that can form encrusting growths on rocks, but are more commonly medium sized fleshy plants attached to the bottom with a structure called a holdfast. One group of species in the genus Sargassum have air bladders and spend most of their life free floating in oceanic gyres. The Sargasso Sea is named after this group of brown algae.

Brown algae live in temperate and tropical oceans from intertidal zones to shallow coastal depths. The large kelps are mainly temperate or boreal. They are particularly conspicuous in the intertidal region of cold temperate regions (eg. New Zealand, Atlantic coasts of Northern Europe and Canada). In temperate Australia, a common brown algae is the kelp Ecklonia radiata which is found just below the low water mark on rocky coasts and grows up to 2m in length. A second species, the bull kelp Durvillaea potatorum,is common in intertidal habitats in Victoria and Tasmania where it is eaten by Koori people. On coral reefs brown algae are found in all reef zones, but are most conspicuous in reef flat and on reef top (at certain seasons).

All brown algae are photosynthetic and autotrophic - they get all their energy requirements from photosynthesis.

Brown algae are eaten by herbivorous fish and other reef herbivores such as gastropods. They are important members of the EAC (see Green Algae) which may be responsible for producing up to 60% of all the food generated on a coral reef.

Brown algae reproduce asexually and sexually. In sexual reproduction simple zoospores (isogamy and anisogamy) or sperm and eggs are produced. In some Orders (eg. Fucales), there are complex structures (sori) which produce the gametes. Asexual reproduction is by division of cells, much like other plants.

Brown algae may often be found growing on other plants, as epiphytes. Kelp holdfasts provide a safe habitat for a range of animals, such as worms, crabs, bivalves and sponges, which live amongst the thick cords of the holdfast. Kelp beds provide shelter for many plants and animals, which live below the dense canopy of fronds.

Some brown algae are eaten by Japanese and other peoples (including probably the Kooris and Murrays of Australia). Large kelps are today important in the production of alginates which find a ready use in the food and manufacturing industries. They are used as fillers and stabilisers in products such as ice cream and toothpaste. Brown algae tend to suffer when conditions become eutrophic and be replaced by green and blue-green algae. An exception to this however, is occurring in the Caribbean (see above) where brown algae are becoming a weed species as the coral Acropora palmata dies off.