Alga
Algae ( AL-jee, UK also AL-ghee; sg.: alga AL-gə) is an informal umbrella term for any organisms from a large and diverse group of photosynthetic autotrophs/mixotrophs that are not plants (embryophytes) and includes species from numerous unrelated clades. Such organisms range from microscopic unicellular microalgae (including cyanobacteria and phytoplankton such as Chlorella, Porphyridium and diatoms) to multicellular macroalgae such as ulvophytes (ultrastructural green algae), nori (bangiaceous red algae) and kelp (large brown algae), which may grow up to 50 metres (160 ft) in length. Most algae are aquatic (especially marine) organisms, some part of cohesive colonies; macroscopic marine algae with complex, multicellular colonial structures are called seaweeds. The most complex freshwater algae are the Charophyta, a group of green algae that includes Spirogyra (filamentous green algae) and stoneworts, (which resembles aquatic grass). Most algae are planktons carried passively by water, although some macroalgae have developed holdfast structures that provide sessile anchorage. Algae exhibit a wide range of reproductive strategies, from simple asexual mitotic proliferation to complex forms of sexual reproduction via spores.
Algae constitute a polyphyletic group because they do not include a common ancestor. Although eukaryotic algae with two-membraned chloroplasts (chlorophyll-bearing plastids that evolved via primary symbiogenesis with cyanobacteria) seem to form a paraphyletic group within the clade Archaeplastida, other algae with three or more chloroplastic membranes evolved from protists that acquired photosynthesis after phagocytotic endosymbiosis of archaeplastids (secondary symbiogenesis). Chlorophytes (green algae sensu stricto), rhodophytes (red algae) and glaucophytes (grey algae) are prominent examples of algae that have primary chloroplasts directly derived from endosymbiont cyanobacteria, while diatoms, cryptomonads, euglenoids and phaeophyceans (brown algae) are examples of algae with secondary chloroplasts derived from indirectly endosymbiont red algae or green algae, which can be considered a form of kleptoplasty.
Most algae are single-celled organisms who (even when colonied) lack the distinct, differentiated tissue types and specialized organs typically found in plants (who evolved from freshwater green algae), such as stomata, phyllids (leaf-like structures, e.g. thallus) and rhizoids of bryophytes (non-vascular plants sensu stricto), as well as the roots, leaves and xylemic/phloemic stems found in tracheophytes (vascular plants). Most algae are photoautotrophs and the main primary producers of aquatic ecosystems, although some algae are mixotrophs that derive metabolic energy both from internal photosynthesis and from foraging external nutrients via osmotrophy, myzotrophy or phagotrophy. Some unicellular species of green algae, many golden algae, euglenids, dinoflagellates and others have become heterotrophs (also called colorless or apochlorotic algae), sometimes parasitic, relying entirely on external energy sources and have limited or no photosynthetic apparatus. Some other heterotrophic organisms, such as the apicomplexans, are also derived from cells whose ancestors possessed chlorophyllic plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from cyanobacteria that produce oxygen as a byproduct of splitting water molecules, unlike other organisms that conduct anoxygenic photosynthesis such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dated to 1.6 to 1.7 billion years ago.
Because of the wide range of types of algae, there is a correspondingly wide range of industrial and traditional applications in human society. Traditional seaweed farming practices have existed for thousands of years and have strong traditions in East Asian food cultures. More modern algaculture applications extend the food traditions for other applications, including cattle feed, using algae for bioremediation or pollution control, transforming sunlight into algae fuels or other chemicals used in industrial processes, and in medical and scientific applications. A 2020 review found that these applications of algae could play an important role in carbon sequestration to mitigate climate change while providing lucrative value-added products for global economies.
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