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Algae (; singular alga ) is an informal term for a large and diverse group of
photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's metabolic activities. This chemical energy is stored in ...

photosynthetic
eukaryotic Eukaryotes () are organisms whose cells have a nucleus enclosed within a nuclear envelope. Eukaryotes belong to the domain Eukaryota or Eukarya; their name comes from the Greek εὖ (''eu'', "well" or "good") and κάρυον (''karyon'', "nu ...
organism In biology, an organism (from Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the properties of life. It is a synonym for "life form". Organisms are classified by taxonomy into groups such as ...
s. It is a
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grouping that includes species from multiple distinct
clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic—that is, composed of a common ancestor and all its lineal descendants - on a phylogenetic tree. R ...
s. Included organisms range from
unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Unicellular organisms fall into two general categories: prokaryotic org ...
microalgae Microalgae or microphytes are microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine systems, living in both the water column and sediment. They are unicellular species which exist individual ...
, such as ''
Chlorella ''Chlorella'' is a genus of about thirteen species of single-celled green algae belonging to the division Chlorophyta. The cells are spherical in shape, about 2 to 10 μm in diameter, and are without flagella. Their chloroplasts contain the gre ...
,''
Prototheca ''Prototheca'' is a genus of algae in the family Chlorellaceae.See the NCBIbr>webpage on Prototheca Data extracted from the All the species within this genus, even though classified as green algae, have forfeited their photosynthetic ability and ...
and the
diatom Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart'; and the root of ''tém-n-ō'', 'I cut'.) are a major group of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms mak ...
s, to
multicellular Multicellular organisms are organisms that consist of more than one cell, in contrast to unicellular organisms. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and ...
forms, such as the
giant kelp ''Macrocystis pyrifera'', commonly known as giant kelp or giant bladder kelp, is a species of kelp (large brown algae), and one of four species in the genus ''Macrocystis''. Despite its appearance, it is not a plant; it is a heterokont. Giant kel ...
, a large
brown alga The brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Most brown algae live in marine environments, where the ...
which may grow up to in length. Most are aquatic and
autotrophic An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", 3r ...
and lack many of the distinct cell and tissue types, such as
stoma In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates") is a pore, found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bord ...

stoma
ta,
xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word "xylem" is derived from th ...
and
phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to parts of the plant where needed. This transport proce ...
, which are found in
land plants The Embryophyta () or land plants are the most familiar group of green plants that form vegetation on earth. Embryophyta is a clade within the Phragmoplastophyta, a larger clade that also includes several green algae groups (including the Charophy ...
. The largest and most complex marine algae are called
seaweed Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of ''Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as k ...
s, while the most complex freshwater forms are the
Charophyta The Charophyta () or charophytes () is a group of freshwater green algae, sometimes treated as a division, but also as a superdivision, or an unranked clade. The terrestrial plants, the Embryophyta most likely emerged within Charophyta, possibly ...
, a
division Division or divider may refer to: Mathematics *Division (mathematics), the inverse of multiplication *Division algorithm, a method for computing the result of mathematical division Military *Division (military), a formation typically consisting o ...
of green algae which includes, for example, ''
Spirogyra ''Spirogyra'' (common names include water silk, mermaid's tresses, and blanket weed) is a filamentous charophyte green alga of the order Zygnematales, named for the helical or spiral arrangement of the chloroplasts that is characteristic of the ...
'' and
stonewort Charales is an order of freshwater green algae in the division Charophyta, class Charophyceae, commonly known as stoneworts. Linnaeus established the genus ''Chara'' in 1753. Description The Charales grow in freshwater and brackish environments ...
s. No definition of algae is generally accepted. One definition is that algae "have
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll is ...
as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells". Likewise, the colorless
Prototheca ''Prototheca'' is a genus of algae in the family Chlorellaceae.See the NCBIbr>webpage on Prototheca Data extracted from the All the species within this genus, even though classified as green algae, have forfeited their photosynthetic ability and ...
under
Chlorophyta Green algae on coastal rocks at Shihtiping in Taiwan Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular auth ...
are all devoid of any chlorophyll. Although
cyanobacteria Cyanobacteria , also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' comes from their color ( el, κυανός, translit=kyanós, translation=blue), giving them their ot ...
are often referred to as "blue-green algae", most authorities exclude all
prokaryotes A prokaryote is a typically unicellular organism that lacks a nuclear membrane-enclosed nucleus. The word ''prokaryote'' comes from the Greek (, 'before') and (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connections". Pearson Education ...
from the definition of algae. Algae constitute a
polyphyletic 300px, Cladogram of the primates, showing a monophyly (the simians, in yellow), a paraphyly">monophyly.html" style="text-decoration: none;"class="mw-redirect" title="primates, showing a monophyly">primates, showing a monophyly (the simians, ...
group since they do not include a common ancestor, and although their
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracelluar endosymbiotic Cyanobacte ...
s seem to have a single origin, from cyanobacteria, they were acquired in different ways.
Green algae#REDIRECT Green algae#REDIRECT Green algae {{R from other capitalisation ...
{{R from other capitalisation ...

Green algae
are examples of algae that have primary
chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water ...

chloroplast
s derived from
endosymbiotic An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within", σύν ' ...

endosymbiotic
cyanobacteria.
Diatom Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart'; and the root of ''tém-n-ō'', 'I cut'.) are a major group of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms mak ...
s and brown algae are examples of algae with secondary chloroplasts derived from an
endosymbiotic An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within", σύν ' ...
red alga Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...
. Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete (such as a sperm or egg cell) with a single set of chromosomes (haploid) combines with another to produce an organism composed of cells with two set ...
. Algae lack the various structures that characterize land plants, such as the phyllids (leaf-like structures) of
bryophyte Bryophytes are an informal group consisting of three divisions of non-vascular land plants (embryophytes): the liverworts, hornworts and mosses. They are characteristically limited in size and prefer moist habitats although they can survive in drie ...
s,
rhizoid Rhizoids are protuberances that extend from the lower epidermal cells of bryophytes and algae. They are similar in structure and function to the root hairs of vascular land plants. Similar structures are formed by some fungi. Rhizoids may be unicel ...
s in nonvascular plants, and the
root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They most often lie below the surfac ...
s,
leaves A leaf (plural leaves) is the principal lateral appendage of the vascular plant stem, usually borne above ground and specialized for photosynthesis. The leaves and stem together form the shoot. Leaves are collectively referred to as foliag ...

leaves
, and other
organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma, the tissue peculiar to (or at least archetyp ...
found in tracheophytes (
vascular plants Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from the Greek ''trācheia''), form a large group of plants ( 300,000 accepted known species) that are defined as land plants with lignified tissues ( ...
). Most are
phototroph Terrestrial and aquatic phototrophs: plants grow on a fallen log floating in algae-rich water Phototrophs (''Gr'': φῶς, φωτός = light, τροϕή = nourishment) are organisms that carry out photon capture to produce complex organic compo ...
ic, although some are
mixotrophA mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs compri ...
ic, deriving energy both from photosynthesis and uptake of organic carbon either by
osmotrophy Osmotrophy is a feeding mechanism involving the movement of dissolved organic compounds by osmosis for nutrition. Organisms that use osmotrophy are called osmotrophs. Some mixotrophic microorganisms use osmotrophy to derive some of their energy. Os ...
, myzotrophy, or
phagotrophy Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is cal ...

phagotrophy
. Some unicellular species of
green algae#REDIRECT Green algae#REDIRECT Green algae {{R from other capitalisation ...
{{R from other capitalisation ...

green algae, many
golden algae The Chrysophyceae, usually called chrysophytes, chrysomonads, golden-brown algae or golden algae are a large group of algae, found mostly in freshwater. Golden algae is also commonly used to refer to a single species, ''Prymnesium parvum'', which ...
,
euglenid Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typical of ...
s,
dinoflagellate The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are single-celled eukaryotes constituting the phylum Dinoflagellata. Usually considered algae, dinoflagellates are mostly marine plankton, but they ...
s, and other algae have become
heterotroph A heterotroph (; from Ancient Greek "other" and "nutrition") is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are prim ...
s (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 plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from
cyanobacteria Cyanobacteria , also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' comes from their color ( el, κυανός, translit=kyanós, translation=blue), giving them their ot ...
that produce
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well a ...
as a by-product of photosynthesis, unlike other photosynthetic bacteria such as
purple Purple refers to any of a variety of colors with hue between red and blue. Purple is closely associated with violet. In optics, purple and violet refer to colors that look similar, but purples are mixtures of red light and blue or violet li ...
and
green sulfur bacteria The green sulfur bacteria (Chlorobiaceae) are a family of obligately anaerobic photoautotrophic bacteria. Together with the non-photosynthetic Ignavibacteriaceae, they form the phylum Chlorobi. Green sulfur bacteria are nonmotile (except ''Chloro ...
. Fossilized filamentous algae from the
Vindhya The Vindhya Range (also known as Vindhyachal) () is a complex, discontinuous chain of mountain ridges, hill ranges, highlands and plateau escarpments in west-central India. Technically, the Vindhyas do not form a single mountain range in the geo ...

Vindhya
basin have been dated back to 1.6 to 1.7 billion years ago. Because of the wide range of types of algae, they have increasing different industrial and traditional applications in human society. Traditional
seaweed farming Seaweed farming or kelp farming is the practice of cultivating and harvesting seaweed. In its simplest form, it consists of the management of naturally found batches. In its most advanced form, it consists of fully controlling the life cycle of th ...
practices have existed for 1000s of years and have strong traditions in East Asia food cultures. More modern
algaculture Algaculture is a form of aquaculture involving the farming of species of algae. The majority of algae that are intentionally cultivated fall into the category of microalgae (also referred to as phytoplankton, microphytes, or planktonic algae). Macr ...
applications extend the food traditions for other applications include cattle feed, using algae for
bioremediation Bioremediation is a process used to treat contaminated media, including water, soil and subsurface material, by altering environmental conditions to stimulate growth of microorganisms and degrade the target pollutants. Cases where bioremediation is ...
or pollution control, transforming sunlight into
algae fuel Algae fuel, algal biofuel, or algal oil is an alternative to liquid fossil fuels that uses algae as its source of energy-rich oils. Also, algae fuels are an alternative to commonly known biofuel sources, such as corn and sugarcane. When made from ...
s 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 Carbon sequestration or carbon dioxide removal (CDR) is the long-term removal, capture or sequestration of carbon dioxide from the atmosphere to slow or reverse atmospheric CO2 pollution and to mitigate or reverse global warming. Carbon dioxid ...
in order to mitigate climate change while providing valuable value-add products for global economies.


Etymology and study

The singular is the Latin word for 'seaweed' and retains that meaning in English. The
etymology Etymology ()The New Oxford Dictionary of English (1998) – p. 633 "Etymology /ˌɛtɪˈmɒlədʒi/ the study of the class in words and the way their meanings have changed throughout time". is the study of the history of words. By extension, t ...
is obscure. Although some speculate that it is related to Latin , 'be cold', no reason is known to associate seaweed with temperature. A more likely source is , 'binding, entwining'. The
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic period ...
word for 'seaweed' was (), which could mean either the seaweed (probably red algae) or a red dye derived from it. The Latinization, , meant primarily the cosmetic rouge. The etymology is uncertain, but a strong candidate has long been some word related to the Biblical (), 'paint' (if not that word itself), a cosmetic eye-shadow used by the ancient Egyptians and other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, or blue. Accordingly, the modern study of marine and freshwater algae is called either
phycology Phycology (from Greek , ''phykos'', "seaweed"; and , ''-logia'') is the scientific study of algae. Also known as algology, phycology is a branch of life science. Algae are important as primary producers in aquatic ecosystems. Most algae are euka ...
or algology, depending on whether the Greek or Latin root is used. The name ''fucus'' appears in a number of
taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name ...
.


Classifications

The committee on the International Code of Botanical Nomenclature has recommended certain suffixes for use in the classification of algae. These are -phyta for division, ''-phyceae'' for class, ''-phycideae'' for subclass, ''-ales'' for order, ''-inales'' for suborder, ''-aceae'' for family, ''-oidease'' for subfamily, a Greek-based name for genus, and a Latin-based name for species.


Algal characteristics basic to primary classification

The primary classification of algae is based on certain morphological features. The chief among these are (a) pigment constitution of the cell, (b) chemical nature of stored food materials, (c) kind, number, point of insertion and relative length of the flagella on the motile cell, (d) chemical composition of cell wall and (e) presence or absence of a definitely organized nucleus in the cell or any other significant details of cell structure.


History of classification of algae

Although
Carolus Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement as Carl von LinnéBlunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the modern sys ...

Carolus Linnaeus
(1754) included algae along with lichens in his 25th class Cryptogamia, he did not elaborate further on the classification of algae.
Jean Pierre Étienne VaucherJean Pierre Étienne Vaucher (17 April 1763 – 5 or 6 January 1841) was a Swiss Protestant pastor and botanist who was a native of the Republic of Geneva. He studied theology at Geneva, and from 1795 to 1821 was a pastor at the Church of Saint- ...
(1803) was perhaps the first to propose a system of classification of algae, and he recognized three groups, Conferves, Ulves, and Tremelles. While
Johann Heinrich Friedrich Link Johann Heinrich Friedrich Link (2 February 1767 – 1 January 1851) was a German naturalist and botanist. Biography Link was born at Hildesheim as a son of the minister August Heinrich Link (1738–1783), who taught him love of nature through ...
(1820) classified algae on the basis of the colour of the pigment and structure,
William Henry Harvey William Henry Harvey, FRS FLS (5 February 1811 – 15 May 1866) was an Irish botanist and phycologist who specialised in algae. Biography Harvey was born at Summerville near Limerick, Ireland, in 1811, the youngest of 11 children. His father Jo ...
(1836) proposed a system of classification on the basis of the habitat and the pigment. J. G. Agardh (1849–1898) divided algae into six orders: Diatomaceae, Nostochineae, Confervoideae, Ulvaceae, Floriadeae and Fucoideae. Around 1880, algae along with fungi were grouped under Thallophyta, a division created by Eichler (1836). Encouraged by this,
Adolf Engler Heinrich Gustav Adolf Engler (25 March 1844 – 10 October 1930) was a German botanist. He is notable for his work on plant taxonomy and phytogeography, such as ''Die natürlichen Pflanzenfamilien'' (''The Natural Plant Families''), edited with Ka ...
and
Karl A. E. Prantl KARL (105.1 FM) is a radio station broadcasting a Classic Country format. Licensed to Tracy, Minnesota, the station serves the Marshall, Minnesota area. The station is currently owned by Linder Radio Group. It carries Westwood One's "Real Count ...
(1912) proposed a revised scheme of classification of algae and included fungi in algae as they were of opinion that fungi have been derived from algae. The scheme proposed by Engler and Prantl is summarised as follows: # Schizophyta # Phytosarcodina # Flagellata # Dinoflagellata # Bacillariophyta # Conjugatae # Chlorophyceae # Charophyta # Phaeophyceae # Rhodophyceae # Eumycetes (Fungi) The algae contain
chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water ...

chloroplast
s that are similar in structure to cyanobacteria. Chloroplasts contain circular
DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, g ...
like that in cyanobacteria and are interpreted as representing reduced endosymbiotic cyanobacteria. However, the exact origin of the chloroplasts is different among separate lineages of algae, reflecting their acquisition during different endosymbiotic events. The table below describes the composition of the three major groups of algae. Their lineage relationships are shown in the figure in the upper right. Many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost plastids entirely.
Phylogeny , based on completely sequenced genomes. A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary rela ...

Phylogeny
based on
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracelluar endosymbiotic Cyanobacte ...
not nucleocytoplasmic genealogy:
Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement as Carl von LinnéBlunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the modern sys ...
, in ''
Species Plantarum ' (Latin for "The Species of Plants") is a book by Carl Linnaeus, originally published in 1753, which lists every species of plant known at the time, classified into genera. It is the first work to consistently apply binomial names and was the st ...
'' (1753), the starting point for modern
botanical nomenclature Botanical nomenclature is the formal, scientific naming of plants. It is related to, but distinct from taxonomy. Plant taxonomy is concerned with grouping and classifying plants; botanical nomenclature then provides names for the results of this p ...
, recognized 14 genera of algae, of which only four are currently considered among algae. In ''
Systema Naturae ' (originally in Latin written ' with the ligature æ) is one of the major works of the Swedish botanist, zoologist and physician Carl Linnaeus (1707–1778) and introduced the Linnaean taxonomy. Although the system, now known as binomial nomencl ...
'', Linnaeus described the genera ''
Volvox ''Volvox'' is a polyphyletic genus of chlorophyte green algae in the family Volvocaceae. It forms spherical colonies of up to 50,000 cells. They live in a variety of freshwater habitats, and were first reported by Antonie van Leeuwenhoek in 1700. ...

Volvox
'' and ''
Corallina ''Corallina'' is a genus of red seaweeds with hard, abrasive calcareous skeletons in the family Corallinaceae. They are stiff, branched plants with articulations. Species
'', and a species of ''
Acetabularia ''Acetabularia'' is a genus of green algae in the family Polyphysaceae, Typically found in subtropical waters, ''Acetabularia'' is a single-celled organism, but gigantic in size and complex in form, making it an excellent model organism for study ...
'' (as ''
Madrepora Madrepora ("mother of pores") is a genus of stony corals, often found forming reefs or islands in tropical locations. The names Madrepore and Madreporaria were formerly applied universally to any stony coral of the family Scleractinia. They reprod ...
''), among the animals. In 1768,
Samuel Gottlieb Gmelin Samuel George Gottlieb Gmelin (23 June 1745 – 27 July 1774) was a German physician, botanist and explorer. Background Gmelin was born at Tübingen in a well known family of naturalists. His father was Johann Conrad Gmelin, an apothecary and surg ...
(1744–1774) published the ''Historia Fucorum'', the first work dedicated to marine algae and the first book on
marine biology Marine biology is the scientific study of the biology of marine life, organisms in the sea. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies spe ...
to use the then new binomial nomenclature of Linnaeus. It included elaborate illustrations of seaweed and marine algae on folded leaves. W. H. Harvey (1811–1866) and Lamouroux (1813) were the first to divide macroscopic algae into four divisions based on their pigmentation. This is the first use of a biochemical criterion in plant systematics. Harvey's four divisions are: red algae (Rhodospermae), brown algae (Melanospermae), green algae (Chlorospermae), and Diatomaceae. At this time, microscopic algae were discovered and reported by a different group of workers (e.g., O. F. Müller and Ehrenberg) studying the
Infusoria Infusoria is a collective term for minute aquatic creatures such as ciliates, euglenoids, protozoa, unicellular algae and small invertebrates that exist in freshwater ponds. Some authors (e.g., Bütschli) used the term as a synonym for Ciliophora. ...
(microscopic organisms). Unlike
macroalgae Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of ''Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as k ...
, which were clearly viewed as plants,
microalgae Microalgae or microphytes are microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine systems, living in both the water column and sediment. They are unicellular species which exist individual ...
were frequently considered animals because they are often motile. Even the nonmotile (coccoid) microalgae were sometimes merely seen as stages of the lifecycle of plants, macroalgae, or animals. Although used as a taxonomic category in some pre-Darwinian classifications, e.g., Linnaeus (1753), de Jussieu (1789), Horaninow (1843), Agassiz (1859), Wilson & Cassin (1864), in further classifications, the "algae" are seen as an artificial, polyphyletic group. Throughout the 20th century, most classifications treated the following groups as divisions or classes of algae: cyanophytes,
rhodophyte Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...
s,
chrysophyte The Chrysophyceae, usually called chrysophytes, chrysomonads, golden-brown algae or golden algae are a large group of algae, found mostly in freshwater. Golden algae is also commonly used to refer to a single species, ''Prymnesium parvum'', which ...
s, xanthophytes, bacillariophytes, phaeophytes,
pyrrhophytes
pyrrhophytes
( cryptophytes and dinophytes),
euglenophyte The euglenozoa are a large group of flagellate Excavata. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. There are two main subgroups, the euglenids and kinetoplastids. Euglen ...
s, and chlorophytes. Later, many new groups were discovered (e.g., Bolidophyceae), and others were splintered from older groups: charophytes and glaucophytes (from chlorophytes), many heterokontophytes (e.g., Synurophyceae, synurophytes from chrysophytes, or eustigmatophytes from xanthophytes), haptophytes (from chrysophytes), and chlorarachniophytes (from xanthophytes). With the abandonment of plant-animal dichotomous classification, most groups of algae (sometimes all) were included in Protista, later also abandoned in favour of Eukaryota. However, as a legacy of the older plant life scheme, some groups that were also treated as protozoans in the past still have duplicated classifications (see ambiregnal protists). Some parasitic algae (e.g., the green algae ''
Prototheca ''Prototheca'' is a genus of algae in the family Chlorellaceae.See the NCBIbr>webpage on Prototheca Data extracted from the All the species within this genus, even though classified as green algae, have forfeited their photosynthetic ability and ...
'' and ''Helicosporidium'', parasites of metazoans, or ''Cephaleuros'', parasites of plants) were originally classified as fungi, sporozoans, or protistans of ''incertae sedis'', while others (e.g., the green algae ''Phyllosiphon'' and ''Rhodochytrium'', parasites of plants, or the red algae ''Pterocladiophila'' and ''Gelidiocolax mammillatus'', parasites of other red algae, or the dinoflagellates ''Oodinium'', parasites of fish) had their relationship with algae conjectured early. In other cases, some groups were originally characterized as parasitic algae (e.g., ''Chlorochytrium''), but later were seen as endophytic algae. Some filamentous bacteria (e.g., ''Beggiatoa'') were originally seen as algae. Furthermore, groups like the apicomplexans are also parasites derived from ancestors that possessed plastids, but are not included in any group traditionally seen as algae.


Relationship to land plants

The first land plants probably evolved from shallow freshwater charophyte algae much like ''Chara (alga), Chara'' almost 500 million years ago. These probably had an isomorphic alternation of generations and were probably filamentous. Fossils of isolated land plant spores suggest land plants may have been around as long as 475 million years ago.


Morphology

A range of algal Morphology (biology), morphologies is exhibited, and Convergent evolution, convergence of features in unrelated groups is common. The only groups to exhibit three-dimensional multicellular Thallus, thalli are the Red algae, reds and Brown algae, browns, and some Chlorophyta, chlorophytes. Apical growth is constrained to subsets of these groups: the Florideophyceae, florideophyte reds, various browns, and the charophytes. The form of charophytes is quite different from those of reds and browns, because they have distinct nodes, separated by internode 'stems'; whorls of branches reminiscent of the horsetails occur at the nodes. Conceptacles are another
polyphyletic 300px, Cladogram of the primates, showing a monophyly (the simians, in yellow), a paraphyly">monophyly.html" style="text-decoration: none;"class="mw-redirect" title="primates, showing a monophyly">primates, showing a monophyly (the simians, ...
trait; they appear in the coralline algae and the Hildenbrandiales, as well as the browns. Most of the simpler algae are unicellular flagellates or amoeboids, but colonial and nonmotile forms have developed independently among several of the groups. Some of the more common organizational levels, more than one of which may occur in the biological life cycle, lifecycle of a species, are * Colony (biology), Colonial: small, regular groups of motile cells * Capsoid: individual non-motile cells embedded in mucilage * Coccoid: individual non-motile cells with cell walls * Palmelloid: nonmotile cells embedded in mucilage * Filamentous: a string of nonmotile cells connected together, sometimes branching * Parenchymatous: cells forming a thallus with partial differentiation of tissues In three lines, even higher levels of organization have been reached, with full tissue differentiation. These are the brown algae,—some of which may reach 50 m in length (kelps)—the red algae, and the green algae. The most complex forms are found among the charophyte algae (see Charales and
Charophyta The Charophyta () or charophytes () is a group of freshwater green algae, sometimes treated as a division, but also as a superdivision, or an unranked clade. The terrestrial plants, the Embryophyta most likely emerged within Charophyta, possibly ...
), in a lineage that eventually led to the higher land plants. The innovation that defines these nonalgal plants is the presence of female reproductive organs with protective cell layers that protect the zygote and developing embryo. Hence, the land plants are referred to as the Embryophytes.


Turfs

The term algal turf is commonly used but poorly defined. Algal turfs are thick, carpet-like beds of seaweed that retain sediment and compete with foundation species like coral reef, corals and kelp forest#kelp, kelps, and they are usually less than 15 cm tall. Such a turf may consist of one or more species, and will generally cover an area in the order of a square metre or more. Some common characteristics are listed: * Algae that form aggregations that have been described as turfs include diatoms, cyanobacteria, chlorophytes, phaeophytes and rhodophytes. Turfs are often composed of numerous species at a wide range of spatial scales, but monospecific turfs are frequently reported. * Turfs can be morphologically highly variable over geographic scales and even within species on local scales and can be difficult to identify in terms of the constituent species. * Turfs have been defined as short algae, but this has been used to describe height ranges from less than 0.5 cm to more than 10 cm. In some regions, the descriptions approached heights which might be described as canopies (20 to 30 cm).


Physiology

Many algae, particularly members of the Characeae species, have served as model experimental organisms to understand the mechanisms of the water permeability of membranes, osmoregulation, turgor regulation, salt tolerance, cytoplasmic streaming, and the generation of action potentials. Phytohormones are found not only in higher plants, but in algae, too.


Symbiotic algae

Some species of algae form symbiosis, symbiotic relationships with other organisms. In these symbioses, the algae supply photosynthates (organic substances) to the host organism providing protection to the algal cells. The host organism derives some or all of its energy requirements from the algae. Examples are:


Lichens

Lichens are defined by the International Association for Lichenology to be "an association of a fungus and a photosynthetic symbiont resulting in a stable vegetative body having a specific structure". The fungi, or mycobionts, are mainly from the Ascomycota with a few from the Basidiomycota. In nature they do not occur separate from lichens. It is unknown when they began to associate. One mycobiont associates with the same phycobiont species, rarely two, from the green algae, except that alternatively, the mycobiont may associate with a species of cyanobacteria (hence "photobiont" is the more accurate term). A photobiont may be associated with many different mycobionts or may live independently; accordingly, lichens are named and classified as fungal species. The association is termed a morphogenesis because the lichen has a form and capabilities not possessed by the symbiont species alone (they can be experimentally isolated). The photobiont possibly triggers otherwise latent genes in the mycobiont. Trentepohlia (alga), Trentepohlia is an example of a common green alga genus worldwide that can grow on its own or be lichenised. Lichen thus share some of the habitat and often similar appearance with specialized species of algae (''aerophytes'') growing on exposed surfaces such as tree trunks and rocks and sometimes discoloring them.


Coral reefs

Coral reefs are accumulated from the calcareous exoskeletons of marine invertebrates of the order Scleractinia (stony corals). These Animal#Food and energy sourcing, animals Metabolism, metabolize Sugar#Chemistry, sugar and oxygen to obtain energy for their cell-building processes, including secretion of the exoskeleton, with water and carbon dioxide as byproducts. Dinoflagellates (algal protists) are often endosymbionts in the cells of the coral-forming marine invertebrates, where they accelerate host-cell metabolism by generating sugar and oxygen immediately available through photosynthesis using incident light and the carbon dioxide produced by the host. Reef-building stony corals (hermatypic corals) require endosymbiotic algae from the genus ''Symbiodinium'' to be in a healthy condition. The loss of ''Symbiodinium'' from the host is known as coral bleaching, a condition which leads to the deterioration of a reef.


Sea sponges

Endosymbiontic green algae live close to the surface of some sponges, for example, breadcrumb sponges (''Halichondria panicea''). The alga is thus protected from predators; the sponge is provided with oxygen and sugars which can account for 50 to 80% of sponge growth in some species.


Lifecycle

Rhodophyta,
Chlorophyta Green algae on coastal rocks at Shihtiping in Taiwan Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular auth ...
, and Heterokontophyta, the three main algal phylum, divisions, have lifecycles which show considerable variation and complexity. In general, an asexual phase exists where the seaweed's cells are diploid, a sexual phase where the cells are haploid, followed by fusion of the male and female gametes. Asexual reproduction permits efficient population increases, but less variation is possible. Commonly, in sexual reproduction of unicellular and colonial algae, two specialized, sexually compatible, haploid gametes make physical contact and fuse to form a zygote. To ensure a successful mating, the development and release of gametes is highly synchronized and regulated; pheromones may play a key role in these processes. Sexual reproduction allows for more variation and provides the benefit of efficient recombinational repair of DNA damages during meiosis, a key stage of the sexual cycle. However, sexual reproduction is more costly than asexual reproduction. Meiosis has been shown to occur in many different species of algae.


Numbers

The ''Algal Collection of the US National Herbarium'' (located in the National Museum of Natural History) consists of approximately 320,500 dried specimens, which, although not exhaustive (no exhaustive collection exists), gives an idea of the order of magnitude of the number of algal species (that number remains unknown). Estimates vary widely. For example, according to one standard textbook, in the British Isles the ''UK Biodiversity Steering Group Report'' estimated there to be 20,000 algal species in the UK. Another checklist reports only about 5,000 species. Regarding the difference of about 15,000 species, the text concludes: "It will require many detailed field surveys before it is possible to provide a reliable estimate of the total number of species ..." Regional and group estimates have been made, as well: * 5,000–5,500 species of red algae worldwide * "some 1,300 in Australian Seas" * 400 seaweed species for the western coastline of South Africa, and 212 species from the coast of KwaZulu-Natal. Some of these are duplicates, as the range extends across both coasts, and the total recorded is probably about 500 species. Most of these are listed in List of seaweeds of South Africa. These exclude phytoplankton and crustose corallines. * 669 marine species from California (US) * 642 in the check-list of Britain and Ireland and so on, but lacking any scientific basis or reliable sources, these numbers have no more credibility than the British ones mentioned above. Most estimates also omit microscopic algae, such as phytoplankton. The most recent estimate suggests 72,500 algal species worldwide.


Distribution

The distribution of algal species has been fairly well studied since the founding of phytogeography in the mid-19th century. Algae spread mainly by the dispersal of spores analogously to the dispersal of Plantae by seeds and spores. This dispersal can be accomplished by air, water, or other organisms. Due to this, spores can be found in a variety of environments: fresh and marine waters, air, soil, and in or on other organisms. Whether a spore is to grow into an organism depends on the combination of the species and the environmental conditions where the spore lands. The spores of freshwater algae are dispersed mainly by running water and wind, as well as by living carriers. However, not all bodies of water can carry all species of algae, as the chemical composition of certain water bodies limits the algae that can survive within them. Marine spores are often spread by ocean currents. Ocean water presents many vastly different habitats based on temperature and nutrient availability, resulting in phytogeographic zones, regions, and provinces. To some degree, the distribution of algae is subject to floristic discontinuities caused by geographical features, such as Antarctica, long distances of ocean or general land masses. It is, therefore, possible to identify species occurring by locality, such as "Pacific algae" or "North Sea algae". When they occur out of their localities, hypothesizing a transport mechanism is usually possible, such as the hulls of ships. For example, ''Ulva reticulata'' and ''U. fasciata'' travelled from the mainland to Hawaii in this manner. Mapping is possible for select species only: "there are many valid examples of confined distribution patterns." For example, ''Clathromorphum'' is an arctic genus and is not mapped far south of there. However, scientists regard the overall data as insufficient due to the "difficulties of undertaking such studies."


Ecology

Algae are prominent in bodies of water, common in terrestrial environments, and are found in unusual environments, such as on Snow algae, snow and Ice algae, ice. Seaweeds grow mostly in shallow marine waters, under deep; however, some such as ''Navicula pennata'' have been recorded to a depth of . A type of algae, ''Ancylonema nordenskioeldii'', was found in Greenland in areas known as the 'Dark Zone', which caused an increase in the rate of melting ice sheet. Same algae was found in the Italian Alps, after pink ice appeared on parts of the Presena glacier. The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column (phytoplankton) provide the food base for most marine food chains. In very high densities (algal blooms), these algae may discolor the water and outcompete, poison, or asphyxiate other life forms. Algae can be used as indicator organisms to monitor pollution in various aquatic systems. In many cases, algal metabolism is sensitive to various pollutants. Due to this, the species composition of algal populations may shift in the presence of chemical pollutants. To detect these changes, algae can be sampled from the environment and maintained in laboratories with relative ease. On the basis of their habitat, algae can be categorized as: Aquatic plant, aquatic (planktonic, benthic, Marine biology, marine, freshwater, lentic, lotic), Terrestrial plant, terrestrial, Aerobiology, aerial (subaerial), lithophytic, halophytic (or euryhaline), psammon, thermophilic, Psychrophile, cryophilic, epibiont (epiphytic, epizoic), endosymbiont (endophytic, endozoic), parasitic, calcareous, calcifilic or lichenic (phycobiont).


Cultural associations

In classical Chinese, the word is used both for "algae" and (in the modest tradition of the scholar-official, imperial scholars) for "literary talent". The third island in Kunming Lake beside the Summer Palace in Beijing is known as the Zaojian Tang Dao, which thus simultaneously means "Island of the Algae-Viewing Hall" and "Island of the Hall for Reflecting on Literary Talent".


Cultivation


Seaweed farming


Bioreactors


Uses


Agar

Agar, a gelatinous substance derived from red algae, has a number of commercial uses. It is a good medium on which to grow bacteria and fungi, as most microorganisms cannot digest agar.


Alginates

Alginic acid, or alginate, is extracted from brown algae. Its uses range from gelling agents in food, to medical dressings. Alginic acid also has been used in the field of biotechnology as a Biocompatibility, biocompatible medium for cell encapsulation and cell immobilization. Molecular cuisine is also a user of the substance for its gelling properties, by which it becomes a delivery vehicle for flavours. Between 100,000 and 170,000 wet tons of ''Macrocystis'' are harvested annually in New Mexico for Alginic acid, alginate extraction and abalone feed.


Energy source

To be competitive and independent from fluctuating support from (local) policy on the long run, biofuels should equal or beat the cost level of fossil fuels. Here, algae-based fuels hold great promise, directly related to the potential to produce more biomass per unit area in a year than any other form of biomass. The break-even point for algae-based biofuels is estimated to occur by 2025.


Fertilizer

For centuries, seaweed has been used as a fertilizer; George Owen of Henllys writing in the 16th century referring to drift weed in South Wales: Today, algae are used by humans in many ways; for example, as fertilizers, soil conditioners, and livestock feed. Aquatic and microscopic species are cultured in clear tanks or ponds and are either harvested or used to treat effluents pumped through the ponds. Algaculture on a large scale is an important type of aquaculture in some places. Maerl is commonly used as a soil conditioner.


Nutrition

Naturally growing seaweeds are an important source of food, especially in Asia leading some to label it as a superfood. They provide many vitamins including: A, Thiamine, B1, Riboflavin, B2, Vitamin B6, B6, niacin, and Vitamin C, C, and are rich in iodine, potassium, iron, magnesium, and calcium. In addition, commercially cultivated microalgae, including both algae and cyanobacteria, are marketed as nutritional supplements, such as Spirulina (dietary supplement), spirulina, ''
Chlorella ''Chlorella'' is a genus of about thirteen species of single-celled green algae belonging to the division Chlorophyta. The cells are spherical in shape, about 2 to 10 μm in diameter, and are without flagella. Their chloroplasts contain the gre ...
'' and the vitamin-C supplement from ''Dunaliella'', high in beta-carotene. Algae are national foods of many nations: China consumes more than 70 species, including ''fat choy (vegetable), fat choy'', a cyanobacterium considered a vegetable; Japan, over 20 species such as ''nori'' and ''aonori''; Ireland, dulse; Chile, cochayuyo. laver (seaweed), Laver is used to make laver bread in Wales, where it is known as ; in Korea, . It is also used along the west coast of North America from California to British Columbia, in Hawaii and by the Māori people, Māori of New Zealand. Sea lettuce and Alaria esculenta, badderlocks are salad ingredients in Scotland, Ireland, Greenland, and Iceland. Algae is being considered a potential solution for world hunger problem. There exist 2 popular forms of Algae which are used in cuisine: *
Chlorella ''Chlorella'' is a genus of about thirteen species of single-celled green algae belonging to the division Chlorophyta. The cells are spherical in shape, about 2 to 10 μm in diameter, and are without flagella. Their chloroplasts contain the gre ...
: This form of Algae is found in freshwater and contains photosynthetic pigments in its
chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water ...

chloroplast
. It is high in iron, zinc, magnesium, vitamin B2 and Omega-3 Fatty acids. Furthermore, it contains all nine of the essential amino acids the body does not produce on its own * Spirulina (genus), Spirulina: Known otherwise as
cyanobacteria Cyanobacteria , also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' comes from their color ( el, κυανός, translit=kyanós, translation=blue), giving them their ot ...
, this is a form of blue-green algae that is filled with nutrients and contains 10% more protein than Chlorella as well as more thiamine and copper. The oils from some algae have high levels of unsaturated fatty acids. For example, ''Parietochloris incisa'' is very high in arachidonic acid, where it reaches up to 47% of the triglyceride pool. Some varieties of algae favored by vegetarianism and veganism contain the long-chain, essential omega-3 fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Fish oil contains the omega-3 fatty acids, but the original source is algae (microalgae in particular), which are eaten by marine life such as copepods and are passed up the food chain. Algae have emerged in recent years as a popular source of omega-3 fatty acids for vegetarians who cannot get long-chain EPA and DHA from other vegetarian sources such as flaxseed oil, which only contains the short-chain alpha-linolenic acid (ALA).


Pollution control

* Sewage can be treated with algae, reducing the use of large amounts of toxic chemicals that would otherwise be needed. * Algae can be used to capture fertilizers in runoff from farms. When subsequently harvested, the enriched algae can be used as fertilizer. * Aquaria and ponds can be filtered using algae, which absorb nutrients from the water in a device called an algae scrubber, also known as an algae turf scrubber. Agricultural Research Service scientists found that 60–90% of nitrogen runoff and 70–100% of phosphorus runoff can be captured from manure effluents using a horizontal algae scrubber, also called an algal turf scrubber (ATS). Scientists developed the ATS, which consists of shallow, 100-foot raceways of nylon netting where algae colonies can form, and studied its efficacy for three years. They found that algae can readily be used to reduce the nutrient runoff from agricultural fields and increase the quality of water flowing into rivers, streams, and oceans. Researchers collected and dried the nutrient-rich algae from the ATS and studied its potential as an organic fertilizer. They found that cucumber and corn seedlings grew just as well using ATS organic fertilizer as they did with commercial fertilizers. Algae scrubbers, using bubbling upflow or vertical waterfall versions, are now also being used to filter aquaria and ponds.


Polymers

Various polymers can be created from algae, which can be especially useful in the creation of bioplastics. These include hybrid plastics, cellulose based plastics, poly-lactic acid, and bio-polyethylene. Several companies have begun to produce algae polymers commercially, including for use in flip-flops and in surf boards.


Bioremediation

The alga ''Stichococcus bacillaris'' has been seen to colonize silicone resins used at archaeological sites; Biodegradation, biodegrading the synthetic substance.


Pigments

The natural pigments (carotenoids and
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll is ...
s) produced by algae can be used as alternatives to chemical dyes and coloring agents. The presence of some individual algal pigments, together with specific pigment concentration ratios, are taxon-specific: analysis of their concentrations with various analytical methods, particularly high-performance liquid chromatography, can therefore offer deep insight into the taxonomic composition and relative abundance of natural algae populations in sea water samples.


Stabilizing substances

Carrageenan, from the red alga ''Chondrus crispus'', is used as a stabilizer in milk products.


Additional images

File:Algae bladder 4290.jpg, Algae bladder


See also

* AlgaeBase * AlgaePARC * Eutrophication * Iron fertilization * ''Marimo'' algae * Microbiofuels * Microphyte * Photobioreactor * Plant#Algae, Plant * Toxoid – anatoxin * Phycotechnology


References


Bibliography


General

* * * * * * . * * *


Regional


Britain and Ireland

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Australia

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New Zealand

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Europe

* * * * * *


Arctic

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Greenland

*


Faroe Islands

* .


Canary Islands

*


Morocco

*


South Africa

*


North America

* * * *


External links

* – a database of all algal names including images, nomenclature, taxonomy, distribution, bibliography, uses, extracts * * * * * * * * * * *
EnAlgae
{{Authority control Algae, Endosymbiotic events Polyphyletic groups