Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many of the distinct organs found in land plants. All true algae have a nucleus enclosed within a membrane and bound in one or more membranes. Algae constitute a paraphyletic and polyphyletic group: they do not represent a single evolutionary direction or line, but a level or grade of organization that may have developed several times in the early history of life on Earth]].

[edit] Ecology

Algae are most prominent in bodies of water but are also common in terrestrial environments. However, terrestrial algae are usually rather inconspicuous and far more common in moist, tropical regions than dry ones, because algae lack vascular tissues and other adaptations to live on land.

The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column provide the food base for most marine food chains. In very high densities these algae may discolor the water and outcompete, poison, or asphyxiate other life forms. Seaweeds grow mostly in shallow marine waters, however some have been recorded to a depth of 300 m.Some are used as human food or harvested for useful substances such as agar, carrageenan, or fertilizer.