Yellow-green algae or xanthophytes are an important group of heterokont

Heterokont

The heterokonts or stramenopiles are a major line of eukaryotes currently containing more than 100,000 known species. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton...

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

Flagellate

Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...

s to simple colonial and filamentous forms. Xanthophyte chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s contain the photosynthetic

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 pigments Chlorophyll a

Chlorophyll a

Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary...

, Chlorophyll b

Chlorophyll b

Chlorophyll b is a form of chlorophyll. Chlorophyll b helps in photosynthesis by absorbing light energy. It is more soluble than chlorophyll a in polar solvents because of its carbonyl group. Its color is yellow, and it primarily absorbs blue light....

, β-Carotene

Beta-carotene

β-Carotene is a strongly-coloured red-orange pigment abundant in plants and fruits. It is an organic compound and chemically is classified as a hydrocarbon and specifically as a terpenoid , reflecting its derivation from isoprene units...

, and the carotenoid

Carotenoid

Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

 diadinoxanthin. Unlike other heterokonts, their chloroplasts do not contain fucoxanthin

Fucoxanthin

Fucoxanthin is a xanthophyll, with formula C42H58O6. It is found as an accessory pigment in the chloroplasts of brown algae and most other heterokonts, giving them a brown or olive-green color...

, which accounts for their lighter colour. Its storage polysaccharide

Polysaccharide

Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...

 is chrysolaminarin

Chrysolaminarin

Chrysolaminarin is a linear polymer of β and β linked glucose units in a ratio of 11:1. It used to be known as leucosin. Chrysolaminarin is arguably one of the most common biopolymers in the world with cellulose being the other.-Function:...

. Xanthophyte cell walls are produced of cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 and hemicellulose

Hemicellulose

A hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...

. They appear to be the closest relatives of the brown algae

Brown algae

The Phaeophyceae or brown algae , is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments, both as food and for the habitats they form...

.

Orders

The Xanthophyceae have been divided into the following four orders in some classification systems:

• Order Botrydiales
  • Asterosiphon
  • Botrydium
    Botrydium
    Botrydium is a genus of thalloid alga comprising approximately 11 species. Specimens can reach around 2 mm in size and produce tetraspores.- Species :The species currently recognised are:*B. argillaceum*B. becherianum...
    
    

• Order Mischococcales
  • Botrydiopsis
  • Botryochloris
  • Bumilleriopsis
  • Chlorellidium
  • Mischococcus
  • Monodus
  • Ophiocytium
  • Pleurochloris
  • Pseudobumilleriopsis
  • Sphaerosorus

• Order Tribonematales Pascher
  • Bumilleria
  • Heterococcus
  • Heterothrix
  • Tribonema
  • Xanthonema

• Order Vaucheriales Nägeli ex Bohlin
  • Vaucheria
    Vaucheria
    Vaucheria is a genus of Xanthophyceae or yellow-green algae. It is one of only two genera in the family Vaucheriaceae. The type species of the genus is Vaucheria disperma....
    
    

Recent ultrastructural and molecular phylogenetic DNA (nuclear and plastid) research 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.