It is a large group of algae consisting of 200 genera and over 10,000 species, out of which 92 genera and about 569 species are reported from India. They are commonly known as Diatoms. The diatoms are the most beautiful microscopic algae due to their structure and sculpturing of their walls.
They occur in various habitats like fresh water, saline water and also in terrestrial condition on or within the soil. Sometimes they also occur as epiphytes along with algae, on the leaf of forest trees, mostly in tropical rain forests. Depending on the mode of nutrition they may be photosynthetic autotrophs or photosynthetic symbionts or heterotrophs.
Characteristics of Diatoms:
The important characteristics of the class Bacillariophyceae are:
- They are commonly unicellular and free- living but some members form colonies of various shapes like filaments, mucilaginous colonies etc.
- Microscopic cells are of different shapes. They may be oval, spherical, triangular, boat- shaped etc.
- Plant bodies are either bilateral or radial in symmetry.
- The cells are surrounded by a rigid cell wall, called frustule, consisting of upper epitheca and lower hypotheca; arranged in the form of a box with its lid.
- The cell wall is composed of pectic substances impregnated with high amount of siliceous substance.
- The wall may have secondary structures like spines, bristles etc.
- Vegetative cells are diploid (2n).
- The cells generally have many discoid or two large plate-like chromatophores. Some cells possess stellate chromatophore.
- The photosynthetic pigments are chlorophyll a, chlorophyll c along with xanthophylls like fucoxanthin, diatoxanthin and diadinoxanthin.
- Reserve food is oil, volutin and crysolaminarin.
- Some vegetative cells show gliding movement.
- Motile structure (antherozoid) has single pantonematic flagellum.
- Vegetative multiplication takes place by cell division, which is very common. Some of the cells become very much reduced in size.
- They produce characteristic spore, the auxospore which develops to regain the normal size.
- Sexual reproduction takes place by isogamy and oogamy.
Reproduction of Diatoms:
Diatom reproduces by vegetative and sexual means.
1. Vegetative Reproduction:
Vegetative reproduction performs with the help of cell division (Fig. 3.102). It takes place usually at midnight or in the early morning.
During cell division the protoplast of the cell enlarges slightly, thus the cell increases in volume and slightly separates both the theca (epitheca and hypotheca). Then the protoplast undergoes mitotic division and gets separated along the longitudinal axis through the median line.
Thus one half of protoplast remains in epitheca and the other one in hypotheca. One side of the protoplast thus remains naked. Now both the theca i.e., epitheca and hypotheca of mother cell behave as epitheca of the daughter cells.
Thus new silicious valves are deposited towards the naked sides of the protoplast and always behave as hypotheca of the daughter cells. Connecting bands are developed between the theca. Later on, the daughter cells get separated.
During cell division, both the theca i.e., epitheca and hypotheca of the mother cell behave as epitheca of the daughter cells. So at the side where the hypotheca behaves as epitheca, the cell becomes reduced in size. Thus with continuous cell division some cells gradually become reduced in size.
2. Sexual Reproduction:
The pattern of sexual reproduction differs in both orders — Pennales and Centrales. During this process, auxospore is formed in both the groups. During cell division, those cells become reduced in size, are able to regain their normal size through the formation of auxospore, so it is a “restorative process” rather than multiplication.
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