Most are phototrophic, although some are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Īlgae lack the various structures that characterize land plants, such as the phyllids (leaf-like structures) of bryophytes, rhizoids of nonvascular plants, and the roots, leaves, and other organs found in tracheophytes ( vascular plants). Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction. Diatoms and brown algae are examples of algae with secondary chloroplasts derived from an endosymbiotic red alga. Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria. Īlgae constitute a polyphyletic group since they do not include a common ancestor, and although their plastids seem to have a single origin, from cyanobacteria, they were acquired in different ways. Although cyanobacteria are often referred to as "blue-green algae", most authorities exclude all prokaryotes from the definition of algae. Likewise, the colorless Prototheca under Chlorophyta are all devoid of any chlorophyll. One definition is that algae "have chlorophyll a as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells".
No definition of algae is generally accepted. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and stoneworts.
Most are aquatic and autotrophic (they generate food internally) and lack many of the distinct cell and tissue types, such as stomata, xylem and phloem that are found in land plants. Included organisms range from unicellular microalgae, such as Chlorella, Prototheca and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 metres (160 ft) in length. It is a polyphyletic grouping that includes species from multiple distinct clades. Diverse group of photosynthetic eukaryotic organismsĪlgae ( / ˈ æ l dʒ iː, ˈ æ l ɡ iː/ singular alga / ˈ æ l ɡ ə/) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms.
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