The members of the Xanthophyta or Heterokontae (class Xanthophyceae) are commonly known as yellow-green algae.
Characteristics of Xanthophyta:
Xanthophyta show the following general characteristics:
- Members of the Xanthophyta are mostly freshwater algae.
- The plant body may be unicellular or multicellular (colonial, palmelloid, or coccoid).
- The cell wall is often absent, but when present, it contains a higher content of pectic substances.
- The chromatophores are discoid, with many in each cell.
- Pyrenoids are usually absent.
- Plastids are yellow-green in colour. They contain chlorophyll-a, chlorophyll-c, β-carotene, and xanthophyll (diadinoxanthin, violaxanthin, and lutein).
- Food reserves are oil, lipids, and leucosin.
- Motile bodies often bear two flagella. The flagella are unequal and inserted at the anterior end.
- Asexual reproduction takes place by zoospores, aplanospores, or akinetes.
- Sexual reproduction is rare. It’s mostly isogamous.
Distribution of Xanthophyta:
There are about 100 genera and 600 species in the division Xanthophyta.
Most of the species are found in freshwater ponds and lakes (e.g., Tribonema). Some species can grow on drying mud (e.g., Botrydium) and also on damp walls and tree trunks (e.g., Ophiocytium).
A few members are marine.
Thallus Structure of Xanthophyta:
There are varied forms of vegetative thalli, ranging from unicellular motile (e.g., Chlorochromonas), palmelloid siphonaceous (e.g., Botrydium), to multicellular filamentous form (e.g., Tribonema).
Vegetative cells are uninucleate, except in siphonaceous forms. The cell wall is composed mainly of pectic substances (either pectic acid or pectose) with a smaller amount of cellulose.
In Ophiocytium, the wall is made up of two halves, and as the cell grows, the tabular position elongated, with its smaller portions overlapping each other. In filamentous genera like Tribonema, H-shaped walls are present.
Each cell contains one or more discoid chromatophores in the protoplast. Due to the presence of excess xanthophyll (β-carotene, diadinoxanthin, violaxanthin, and lutein), the colour of chromatophores is yellow-green. There are also chlorophyll-a and chlorophyll-e present.
Chromatophores usually lack pyrenoids; in some species (e.g., Botrydium), pyrenoids are present.
In the form of a photosensitive organ, the eye spot can be seen, and near the eyespot, flagella arise.
Oil is the main food reserve accumulated in the cytoplasm.