Brown algae are found in the class Phaeophyceae and a vast majority of the known species lives in saltwater. A lot of the seaweeds that grows in the cool waters of the Northern Hemisphere are a part of this class, e.g. kelp and the members of the genus Sargassum, but you can also find brown algae in tropical waters. The largest forms are however only found in cool waters. Brown algae are always multicellular, never unicellular or colonial. They have a large surface area since this makes the process of absorbing nutrients from the water more efficient.
Brown algae might look similar to land plants, but there are many notable differences. Land living plants typically use chlorophyll a and chlorophyll b to carry out photosynthesis, while brown algae relay on chlorophyll c and have cells containing fucoxanthin. The pigment fucoxanthin is what gives the brown algae their greenish-brown colour.
Generally speaking, brown algae grow attached to a surface, such as a rock or a cliff, instead of drifting around in the water. Many species will grow from the ocean floor until they reaches the surface. The largest species of the genus Macrocystis can exceed 60 meters (200 feet) in length. The smallest ones are microscopic and may grow as epiphytes on underwater vegetation. (An epiphyte is an organism that grows upon or attached to a living plant.)
Brown algae is not only a valuable source of food for may species, they can also dominate an ecosystem by forming large underwater forests, e.g. the famous kelp forests primarily found in polar and temperate costal waters. The legendary Sargasso Sea in the middle of the North Atlantic is characterized by the presence of brown algae from the genus Sargassum that floats en masse on the surface.
Brown algae have developed to fit into a wide range of habitats and ecological niches and are for instance found in rock pools, in the intertidal zones and in the turbulent tidal splash zone. Brown algae living in coastal environments subjected to tidal movements will survive being regularly exposed to air. Brown algae is also known to appreciate the environment formed by nutrient rich cold water up wellings and inflows from land.
Some species of brown algae use floatation bladders to keep their blades close to the surface of the water, since they wish to absorb as much light as possible. The floatation bladders are filled with gas to keep them afloat and this gas can be toxic to humans.
The oldest examples of fossilized brown algae date back to the Mesozoic Era (251 to 180 million years ago), but they might have been around during the Jurassic Period as well (200 to 145 million years ago). Since brown algae tend to be soft, fossils are rare.
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