The members of the Xanthophyta or Heterokontae (class Xanthophyceae) are characterized by their yellow-green pigments, including xanthophylls, which distinguish them from other algae.
They 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.
Image Source: Plantscience4u
Reproduction in Xanthophyta
Members of Xanthophyta reproduce by vegetative, asexual, and sexual methods.
Vegetative Reproduction
The vegetative reproduction in Xanthophyta mainly takes place by fragmentation.
The thallus can break into small fragments due to accidental breakage. Each fragment grows independently to form a new thallus.
Vegetative reproduction also occurs by cell division. Algal cells divide mitotically into two daughter cells. These cells may develop a new independent organism.
Asexual Reproduction
Xanthophyta reproduces asexually by the formation of zoospores and aplanospores.
Zoospores
Zoospores are large multiflagellate, ovoid structure and develop singly or in numbers within a club-shaped zoosporangium.
In some members of Xanthophyta (e.g., Vaucheria), the zoospore contains many nuclei and chloroplasts in addition to numerous pairs of heterokont flagella, almost equal in length.
After maturation, zoospores are liberated. Then the resting period started, and the zoospores withdrew their flagella.
Each zoospore germinates, which forms tubular structures. One of them develops into a new thallus.
Aplanospores
Aplanospores are thin-walled, non-flagellated zoospores formed inside the aplanosporangium. After liberation, the aplanospore may germinate into a new plant.
Cysts Formation
It is formed during unfavourable conditions.
The entire protoplast, segmented by transverse division and then rounding off, secretes a thick wall. Such thick-walled segments are known as akinetes or hypnospores.
The akinetes may divide further into a number of thin-walled bodies called cysts. Under favorable conditions, the akinetes and cysts germinate into new thalli.
Sexual Reproduction
Sexual reproduction is very rare in Xanthophyta.
In Botrydium, an isogamous type of reproduction is found.
Gametes are formed during the rainy season. The protoplast of a vesicle is divided into uninucleate segments, which form biflagellate, pear-shaped gametes. These gametes have one to four chromatophores, with or without an eye spot.
Later, any two gametes come close to each other, flagella disappear, and fusion occurs. As a result, a diploid zygote is formed.
The nucleus of the zygote undergoes meiosis and forms 4–8 motile meiospores. These haploid meiospores develop new plants.
In Vaucheria, an advanced oogamous type of sexual reproduction takes place.
The species of Vaucheria may be homothallic or heterothallic. In homothallic species, the formation of antheridium and oogonium is always associated with the formation of transverse septa.
The antheridium is a slender, curved, hook-like structure, opened by a terminal pore. It is formed at the end of a short lateral branch, slightly before the development of oogonium.
The tip of the branch becomes densely filled with cytoplasm, containing many nuclei and a few chloroplasts. This portion bends like a horn and gets cut off from the rest of the filament by a cross wall. The protoplast divides into uninucleate biflagellate antherozoids, or sperm.
The oogonium development begins simultaneously with the accumulation of a colorless multinucleate mass of cytoplasm called wonderplasm in the main filament near the base of the antheridial branch. The mature oogonium contains a large nucleus at its center. The nucleus of an oogonium is filled with protoplasm to form a single egg or ovum.
After fertilisation, a diploid zygote or oospore is formed. During favorable conditions, the oogonial wall disintegrates, releasing the oospore. The oospore germinates into a new thallus.
Classification of Xanthophyta
The division Xanthophyta includes only one class Xanthophyceae, which contains only one order Vaucheriales (Heterosiphonales).
The order Vaucheriales is divided into two families. One is Botrydiaceae and the other is Vaucheriaceae.
Phyllogeny and Interrelationships
Three major evolutionary lines may be traced among Xanthophyta, like Chlorophyta. These are-
- Development of non-motile solitary or colonial forms from an unicellular motile ancestor.
- Development of tubular or siphonaceous forms.
- Development of multicellular filamentous types.
Due to morphological similarities with the chlorophyceae, many phycologists advocate the theory of parallel evolution and similarity in the origin of the group from the flagellate. Since the members of Xanthophyta lack elaborate thalli as found in Chlorophyceae, this sequence of parallel evolution does not go too far.
Fritsch (1935) believes that the group is still in the process of evolution, while some consider it a reduced stock.
Due to similarities in food reserves, the composition of the cell wall, and other physiological and biological features, members of Xanthophyta are compared to those of Chrysophyta.
But the presence of chlorophyll-e and other features led modern phycologists to propose an independent status for the division Xanthophyta.
