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Algae are a very large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. Most are photosynthetic and "simple" because they lack the many distinct cell and organ types found in land plants. The largest and most complex marine forms are called seaweeds. Though the prokaryotic cyanobacteria are informally referred to as blue-green algae, this usage is incorrect since they are regarded as bacteria. The term algae is now restricted to eukaryotic organisms. All true algae therefore have a nucleus enclosed within a membrane and plastids bound in one or more membranes. Algae constitute a polyphyletic group since they do not include a common ancestor, although their plastids seem to have a single origin. Diatoms are also examples of algae. Algae exhibit a wide range of reproductive strategies, from simple, asexual cell division to complex forms of sexual reproduction. Algae lack the various structures that characterize land plants, such as the leaf-like phyllids of bryophytes, rhizoids in nonvascular plants, and the roots, leaves, and other organs that are found in tracheophytes. Many are phototrophic, although some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular species rely entirely on external energy sources and have limited or no photosynthetic apparatus. (via Freebase)