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Algae (pronounced /ˈældʒiː/ or /ˈælɡiː/; singular alga /ˈælɡə/, Latin for "seaweed") are a 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. The US Algal Collection is represented by almost 300,000 accessioned and inventoried herbarium specimens.[3] The largest and most complex marine forms are called seaweeds. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many distinct organs found in land plants.

Though the prokaryotic cyanobacteria (commonly referred to as blue-green algae) were traditionally included as "algae" in older textbooks, many modern sources regard this as outdated[4] as they are now considered to be bacteria.[5] The term algae is now restricted to eukaryotic organisms.[6] All true algae therefore have a nucleus enclosed within a membrane and plastids bound in one or more membranes.[4][7] Algae constitute a paraphyletic and polyphyletic group,[4] as they do not include all the descendants of the last universal ancestor nor do they all descend from a common algal ancestor, although their plastids seem to have a single origin.[1] Diatoms are also examples of algae.

Algae are found in the fossil record dating back to approximately 3 billion years in the Precambrian. They exhibit a wide range of reproductive strategies, from simple, asexual cell division to complex forms of sexual reproduction.[8]

Algae lack the various structures that characterize land plants, such as phyllids (leaves) and rhizoids in nonvascular plants, or leaves, roots, and other organs that are found in tracheophytes (vascular plants). Many are photoautotrophic, 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.

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