# Algae

By [KlOP](https://paragraph.com/@klop) · 2023-05-30

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**Algae** ([UK](https://en.wikipedia.org/wiki/British_English): [/ˈælɡiː/](https://en.wikipedia.org/wiki/Help:IPA/English), [US](https://en.wikipedia.org/wiki/American_English): [/ˈældʒiː/](https://en.wikipedia.org/wiki/Help:IPA/English);[\[3\]](https://en.wikipedia.org/wiki/Algae#cite_note-3) singular **alga** [/ˈælɡə/](https://en.wikipedia.org/wiki/Help:IPA/English)) is an informal term for a large and diverse group of [photosynthetic](https://en.wikipedia.org/wiki/Photosynthesis), [eukaryotic](https://en.wikipedia.org/wiki/Eukaryotic) organisms. It is a [polyphyletic](https://en.wikipedia.org/wiki/Polyphyletic) grouping that includes species from multiple distinct [clades](https://en.wikipedia.org/wiki/Clade). Included organisms range from unicellular [microalgae](https://en.wikipedia.org/wiki/Microalgae), such as [_Chlorella_](https://en.wikipedia.org/wiki/Chlorella), [_Prototheca_](https://en.wikipedia.org/wiki/Prototheca) and the [diatoms](https://en.wikipedia.org/wiki/Diatom), to [multicellular](https://en.wikipedia.org/wiki/Multicellular) forms, such as the [giant kelp](https://en.wikipedia.org/wiki/Macrocystis_pyrifera), a large [brown algae](https://en.wikipedia.org/wiki/Brown_algae) which may grow up to 50 metres (160 ft) in length. Most are aquatic and lack many of the distinct cell and tissue types, such as [stomata](https://en.wikipedia.org/wiki/Stoma), [xylem](https://en.wikipedia.org/wiki/Xylem) and [phloem](https://en.wikipedia.org/wiki/Phloem) that are found in [land plants](https://en.wikipedia.org/wiki/Embryophyte). The largest and most complex marine algae are called [seaweeds](https://en.wikipedia.org/wiki/Seaweed), while the most complex freshwater forms are the [_Charophyta_](https://en.wikipedia.org/wiki/Charophyta), a [division](https://en.wikipedia.org/wiki/Phylum) of [green algae](https://en.wikipedia.org/wiki/Green_algae) which includes, for example, [_Spirogyra_](https://en.wikipedia.org/wiki/Spirogyra) and [stoneworts](https://en.wikipedia.org/wiki/Stonewort).

No definition of algae is generally accepted.\[[_citation needed_](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)\] One definition is that algae "have [chlorophyll](https://en.wikipedia.org/wiki/Chlorophyll) as their primary [photosynthetic pigment](https://en.wikipedia.org/wiki/Photosynthetic_pigment) and lack a sterile covering of cells around their [reproductive cells](https://en.wikipedia.org/wiki/Gamete)".[\[4\]](https://en.wikipedia.org/wiki/Algae#cite_note-4) On the other hand, the colorless _Prototheca_ under [_Chlorophyta_](https://en.wikipedia.org/wiki/Chlorophyta) are all devoid of any chlorophyll. Although [cyanobacteria](https://en.wikipedia.org/wiki/Cyanobacteria) are often referred to as "blue-green algae", most authorities exclude all [prokaryotes](https://en.wikipedia.org/wiki/Prokaryote), including cyanobacteria, from the definition of algae.[\[5\]](https://en.wikipedia.org/wiki/Algae#cite_note-IntroBot-5)[\[6\]](https://en.wikipedia.org/wiki/Algae#cite_note-6)

Algae constitute a polyphyletic group[\[5\]](https://en.wikipedia.org/wiki/Algae#cite_note-IntroBot-5) since they do not include a common ancestor, and although their [plastids](https://en.wikipedia.org/wiki/Plastid) seem to have a single origin, from cyanobacteria,[\[7\]](https://en.wikipedia.org/wiki/Algae#cite_note-keeling-7) they were acquired in different ways. Green algae are examples of algae that have primary [chloroplasts](https://en.wikipedia.org/wiki/Chloroplast) derived from [endosymbiotic](https://en.wikipedia.org/wiki/Endosymbiotic_theory) cyanobacteria. [Diatoms](https://en.wikipedia.org/wiki/Diatom) and [brown algae](https://en.wikipedia.org/wiki/Brown_algae) are examples of algae with secondary chloroplasts derived from an endosymbiotic [red alga](https://en.wikipedia.org/wiki/Red_alga).[\[8\]](https://en.wikipedia.org/wiki/Algae#cite_note-8) Algae exhibit a wide range of reproductive strategies, from simple [asexual](https://en.wikipedia.org/wiki/Asexual_reproduction) cell division to complex forms of [sexual reproduction](https://en.wikipedia.org/wiki/Sexual_reproduction).[\[9\]](https://en.wikipedia.org/wiki/Algae#cite_note-9)

Algae lack the various structures that characterize land plants, such as the phyllids (leaf-like structures) of [bryophytes](https://en.wikipedia.org/wiki/Bryophyte), [rhizoids](https://en.wikipedia.org/wiki/Rhizoid) of [non-vascular plants](https://en.wikipedia.org/wiki/Non-vascular_plant), and the [roots](https://en.wikipedia.org/wiki/Root), [leaves](https://en.wikipedia.org/wiki/Leaf), and other [organs](https://en.wikipedia.org/wiki/Organ_\(biology\)) found in [tracheophytes](https://en.wikipedia.org/wiki/Tracheophyte) ([vascular plants](https://en.wikipedia.org/wiki/Vascular_plant)). Most are [phototrophic](https://en.wikipedia.org/wiki/Phototroph), although some are [mixotrophic](https://en.wikipedia.org/wiki/Mixotroph), deriving energy both from photosynthesis and uptake of organic carbon either by [osmotrophy](https://en.wikipedia.org/wiki/Osmotrophy), [myzotrophy](https://en.wikipedia.org/wiki/Myzocytosis), or [phagotrophy](https://en.wikipedia.org/wiki/Phagocytosis). Some unicellular species of [green algae](https://en.wikipedia.org/wiki/Green_algae), many [golden algae](https://en.wikipedia.org/wiki/Golden_algae), [euglenids](https://en.wikipedia.org/wiki/Euglenid), [dinoflagellates](https://en.wikipedia.org/wiki/Dinoflagellate), and other algae have become [heterotrophs](https://en.wikipedia.org/wiki/Heterotroph) (also called colorless or apochlorotic algae), sometimes parasitic, relying entirely on external energy sources and have limited or no photosynthetic apparatus.[\[10\]](https://en.wikipedia.org/wiki/Algae#cite_note-10)[\[11\]](https://en.wikipedia.org/wiki/Algae#cite_note-11)[\[12\]](https://en.wikipedia.org/wiki/Algae#cite_note-12) Some other heterotrophic organisms, such as the [apicomplexans](https://en.wikipedia.org/wiki/Apicomplexans), are also derived from cells whose ancestors possessed [plastids](https://en.wikipedia.org/wiki/Plastid), but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from [cyanobacteria](https://en.wikipedia.org/wiki/Cyanobacteria) that produce [oxygen](https://en.wikipedia.org/wiki/Oxygen) as a by-product of photosynthesis, unlike other photosynthetic bacteria such as [purple](https://en.wikipedia.org/wiki/Purple_sulfur_bacteria) and [green sulfur bacteria](https://en.wikipedia.org/wiki/Green_sulfur_bacteria). Fossilized filamentous algae from the [Vindhya](https://en.wikipedia.org/wiki/Vindhya) basin have been dated back to 1.6 to 1.7 billion years ago.[\[13\]](https://en.wikipedia.org/wiki/Algae#cite_note-13)

Because of the wide range of types of algae, they have increasing different industrial and traditional applications in human society. Traditional [seaweed farming](https://en.wikipedia.org/wiki/Seaweed_farming) practices have existed for thousands of years and have strong traditions in [East Asia](https://en.wikipedia.org/wiki/East_Asia) food cultures. More modern [algaculture](https://en.wikipedia.org/wiki/Algaculture) applications extend the [food traditions](https://en.wikipedia.org/wiki/Edible_seaweed) for other applications include cattle feed, using algae for [bioremediation](https://en.wikipedia.org/wiki/Bioremediation) or pollution control, transforming sunlight into [algae fuels](https://en.wikipedia.org/wiki/Algae_fuel) 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](https://en.wikipedia.org/wiki/Carbon_sequestration) in order to [mitigate climate change](https://en.wikipedia.org/wiki/Climate_change_mitigation) while providing valuable value-add products for global economies.[\[14\]](https://en.wikipedia.org/wiki/Algae#cite_note-14)

[https://opensea.io/assets/0xe4bDcC11A6b9EB941FE3A761D174714Eb626d44a/1](https://opensea.io/assets/0xe4bDcC11A6b9EB941FE3A761D174714Eb626d44a/1)

Etymology and study
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The singular _alga_ is the Latin word for 'seaweed' and retains that meaning in English.[\[15\]](https://en.wikipedia.org/wiki/Algae#cite_note-15) The [etymology](https://en.wikipedia.org/wiki/Etymology) is obscure. Although some speculate that it is related to Latin _algēre_, 'be cold',[\[16\]](https://en.wikipedia.org/wiki/Algae#cite_note-16) no reason is known to associate seaweed with temperature. A more likely source is _alliga_, 'binding, entwining'.[\[17\]](https://en.wikipedia.org/wiki/Algae#cite_note-17)

The [Ancient Greek](https://en.wikipedia.org/wiki/Ancient_Greek) word for 'seaweed' was φῦκος (_phŷkos_), which could mean either the seaweed (probably red algae) or a red dye derived from it. The Latinization, _fūcus_, meant primarily the cosmetic rouge. The etymology is uncertain, but a strong candidate has long been some word related to the [Biblical](https://en.wikipedia.org/wiki/Biblical_Hebrew) פוך (_pūk_), 'paint' (if not that word itself), a [cosmetic eye-shadow](https://en.wikipedia.org/wiki/Kohl_\(cosmetics\)) used by the [ancient Egyptians](https://en.wikipedia.org/wiki/Ancient_Egypt) and other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, or blue.[\[18\]](https://en.wikipedia.org/wiki/Algae#cite_note-18)

Accordingly, the modern study of marine and freshwater algae is called either [phycology](https://en.wikipedia.org/wiki/Phycology) or algology, depending on whether the Greek or Latin root is used. The name _fūcus_ appears in a number of [taxa](https://en.wikipedia.org/wiki/Taxon).

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*Originally published on [KlOP](https://paragraph.com/@klop/algae)*