Yellow-green algae or the Xanthophyceae (xanthophytes) are an important group of

heterokont The stramenopiles, also called heterokonts, are protists distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, an ...

algae. Most live in fresh water, but some are found in marine and soil habitats. They vary from single-celled

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

s to simple colonial and filamentous forms. Xanthophyte

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

s contain the

photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...

pigments chlorophyll ''a'', chlorophyll ''c'', β-carotene, and the carotenoid diadinoxanthin. Unlike other

Stramenopile The stramenopiles, also called heterokonts, are protists distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, an ...

s (heterokonts), their chloroplasts do not contain fucoxanthin, which accounts for their lighter colour. Their storage

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...

is chrysolaminarin. Xanthophyte cell walls are produced of

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

and

hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls. Cellulose is crystalline, strong, an ...

. They appear to be the closest relatives of the

brown algae Brown algae (: alga) are a large group of multicellular algae comprising the class (biology), class Phaeophyceae. They include many seaweeds located in colder waters of the Northern Hemisphere. Brown algae are the major seaweeds of the temperate ...

Classifications

The species now placed in the Xanthophyceae were formerly included in the Chlorophyceae. In 1899, Lüther created the group Heterokontae for green algae with unequal flagella. Pascher (1914) included the Heterokontae in the

Chrysophyta Chrysophyta or golden algae is a term used to refer to certain heterokonts. It can be used to refer to: * Chrysophyceae (golden algae), Bacillariophyceae (diatoms), and Xanthophyceae (yellow-green algae) together. E.g., Adolf A. Pascher, Pascher ( ...

. In 1930, Allorge renamed the group as Xanthophyceae. The monadoid (unicellular flagellates) and also sometimes the amoeboid species have been included by some authors in the Protozoa or

Protista A protist ( ) or protoctist is any Eukaryote, eukaryotic organism that is not an animal, Embryophyte, land plant, or fungus. Protists do not form a Clade, natural group, or clade, but are a Paraphyly, paraphyletic grouping of all descendants o ...

, as order Heterochloridina (e.g., Doflein and Reichenow, 1927–1929), as class Xanthomonadina, with orders Heterochloridea and Rhizochloridea (e.g., Deflandre, 1956), as order Heterochlorida (e.g., Hall, 1953, Honigberg et al., 1964), as order Heteromonadida (e.g., Leedale, 1983), or as subclass Heterochloridia (e.g., Puytorac et al., 1987). These groups are called ambiregnal protists, as names for these have been published under either or both of the ICZN and the ICN.

AlgaeBase (2020)

Xanthophyceae have been divided into the following five orders in some classification systems: * '' Dictyosphaeriopsis'' * '' Groenlandiella'' * '' Halosphaeropsis'' * '' Pelagocystis'' * '' Polyedrium'' * '' Pseudopleurochloris'' * '' Raphidosphaera'' * '' Sphaerochloris'' * ''

Tiresias In Greek mythology, Tiresias (; ) was a blind prophet of Apollo in Thebes, Greece, Thebes, famous for clairvoyance and for being transformed into a woman for seven years. He was the son of the shepherd Everes (mythology), Everes and the nymph ...

'' * Order Botrydiales Schaffner 1922 ** Family Botrydiaceae Rabenhorst 1863 e.g. '' Botrydium'' * Order Mischococcales Fritsch 1927 ** Family Botrydiopsidaceae Hibberd 1980 e.g. '' Botrydiopsis'' ** Family Botryochloridaceae Pascher 1938 e.g. '' Ilsteria'' ** Family Centritractaceae Pascher 1937 e.g. '' Centritractus'' ** Family Characiopsidaceae Pascher 1938 e.g. '' Characiopsis'', '' Chlorothecium'' ** Family Chloropediaceae Pascher 1931 e.g. '' Chloropedia'' ** Family Gloeobotrydaceae Pascher 1937 e.g. '' Gloeobotrys'' ** Family Gloeopodiaceae Pascher 1938 e.g. '' Gloeopodium'' ** Family Mischococcaceae Pascher 1912 e.g. '' Mischococcus'' ** Family Ophiocytiaceae Lemmermann 1899 e.g. '' Ophiocytium'' ** Family Pleurochloridaceae Pascher 1937 e.g. '' Meringosphaera'', '' Pleurochloris'' ** Family Trypanochloridaceae Geitler ex Pascher 1938 e.g. '' Trypanochloris'' * Order Rhizochloridales Pascher 1925 ** Family Myxochloridaceae Pascher 1937 e.g. '' Myxochloris'' ** Family Rhizochloridaceae Pascher 1925 e.g. '' Rhizochloris'' ** Family Stipitococcaceae Pascher ex Smith 1933 e.g. '' Stipitococcus'' * Order Tribonematales Pascher 1939 ** Family Heterodendraceae Pascher 1939 e.g. '' Heterodendron'' ** Family Heteropediaceae Hibberd 1982 e.g. '' Heterococcus'', '' Heteropedia'' ** Family Neonemataceae Ettl 1977 e.g. '' Neonema'' ** Family Tribonemataceae West 1904 e.g. '' Tribonema'' ** Family Xanthonemataceae Silva 1980 e.g. '' Xanthonema'' * Order Vaucheriales Nägeli ex Bohlin 1901Christensen, T. 1987. ''Seaweeds of the British Isles.'' Volume 4 Tribophyceae (Xanthophyceae). British Museum (Natural History), London ** Family Vaucheriaceae (Gray) Dumortier 1822 e.g. '' Vaucheria''

Lüther (1899)

Classification according to Lüther (1899): * Class Heterokontae ** Order Chloromonadales ** Order Confervales

Pascher (1912)

Classification according to Pascher (1912): * Heterokontae ** Heterochloridales ** Heterocapsales ** Heterococcales ** Heterotrichales ** Heterosiphonales

Fritsch (1935)

Fritsch (1935) recognizes the following orders in the class Xanthophyceae: *Order Heterochloridales ** Family Heterochloridaceae (e.g., '' Heterochloris'') ** Family Heterocapsaceae (e.g., '' Chlorogloea'') ** Family Mischococcaceae (e.g., '' Mischococcus'') ** Family Heterorhizidaceae (e.g., '' Rhizolekane'') *Order Heterococcales ** Family Halosphaeraceae (e.g., '' Halosphaera'') ** Family Myxochloridaceae (e.g., '' Myxochloris'') ** Family Chlorobotrydaceae(e.g., '' Chlorobotrys'') ** Family Chlorotheciaceae (e.g., '' Chlorothecium'') ** Family Ophiocytiaceae (e.g., '' Ophiocytium'') *Order Heterotrichales ** Family Tribonemataceae (e.g., '' Tribonema'') ** Family Heterocloniaceae (e.g., '' Heterodendron'' *Order Heterosiphonales ** Family Botrydiaceae (e.g., '' Botrydium'')

Smith (1938)

In the classification of Smith (1938), there are six orders in the class Xanthophyceae, placed in the division Chrysophyta: *Order Heterochloridales (e.g., '' Chlorochromonas'') *Order Rhizochloridales (e.g., '' Chlorarachnion'') *Order Heterocapsales (e.g., '' Chlorosaccus'') *Order Heterotrichales (e.g., '' Tribonema'') *Order Heterococcales (e.g., '' Botrydiopsis'') *Order Heterosiphonales (e.g., '' Botrydium'')

Pascher (1939)

Pascher (1939) recognizes 6 classes in Heterokontae: *Class Heterochloridineae *Class Rhizochloridineae *Class Hetcrocapsineae *Class Heterococcincae *Class Hetcrotrichineae *Class Heterosiphonineae

Copeland (1956)

Copeland (1956) treated the group as order Vaucheriacea: * Kingdom Protoctista ** Phylum Phaeophyta *** Class Heterokonta **** Order Vaucheriacea ***** Family Chlorosaccacea ***** Family Mischococcacea ***** Family Chlorotheciacea ***** Family Botryococcacea ***** Family Stipitococcacea ***** Family Chloramoebacea ***** Family Tribonematacea ***** Family Phyllosiphonacea

Ettl (1978), van den Hoek et al. (1995)

In a classification presented by van den Hoek, Mann and Jahns (1995), based on the level of organization of the thallus, there are seven orders: * Order Chloramoebales (e.g., '' Chloromeson'') - flagellate organisms * Order Rhizochloridales (e.g., '' Rhizochloris, Myxochloris'') - ameboid organisms * Order Heterogloeales (e.g., '' Gloeochloris'') - palmelloid (tetrasporal) organisms * Order Mischococcales (e.g., '' Chloridella, Botrydiopsis, Characiopsis, Ophiocytium'') - coccoid organisms * Order Tribonematales (e.g., '' Tribonema, Heterococcus, Heterodendron'') - filamentous organization * Order Botrydiales (e.g., '' Botrydium'') - siphonous organization; sexual reproduction isogamous or anisogamous * Order Vaucheriales (e.g., '' Vaucheria'') - siphonous organization; sexual reproduction oogamous These are the same orders of the classification of Ettl (1978), an updated version of the classic work by Pascher (1939). Ultrastructural and molecular studies shows that the Mischococcales might be paraphyletic, and the Tribonematales and Botrydiales polyphyletic, and suggests two orders at most be used until the relationships within the division are sorted.

Maistro et al. (2009)

Informal groups, according to Maistro et al. (2009): * Botrydiopsalean clade * Chlorellidialean clade * Tribonematalean clade * Vaucherialean clade Unicellular flagellates, amoeboid and palmelloid taxa were not included in this study.

Adl et al. (2005, 2012)

According to Adl et al. (2005, 2012): * Tribonematales (genera ''Botrydium, Bumilleriopsis, Characiopsis, Chloromeson, Heterococcus, Ophiocytium, Sphaerosorus, Tribonema, Xanthonema'') * Vaucheriales (genus ''Vaucheria'') File:Stipitococcus capense.svg, ''Stipitococcus capense'' (Rhizochloridales) File:British fresh-water algae, exclusive of Desmidieae and Diatomaceae (1882-1884) (20231360259).jpg, ''Ophiocytium arbusculum'' (Mischococcales), formerly ''Sciadium arbuscula'' File:British fresh-water algae, exclusive of Desmidieae and Diatomaceae (1882-1884) (19795451234).jpg, '' Vaucheria'' sp. File:Album général des Cryptogames, Pl. 21.jpg, Other genera File:Alger, Botydium granulatum, Nordisk familjebok.png, ''Botrydium granulatum'' ( Botrydiales) File:Vaucheria sp thallus 01.jpg, '' Vaucheria'' sp. (Vaucheriales), thallus File:Vaucheria sp sexial reproductive organ01.jpg, '' Vaucheria'' sp., sexual reproductive organs

References

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