Diatoms: Characteristics, Occurrence and Reproduction

Description of Diatoms:

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.

The important characteristics of the class Bacillariophyceae are:

1. They are commonly unicellular and free- living but some members form colonies of various shapes like filaments, mucilaginous colonies etc.

2. Microscopic cells are of different shapes. They may be oval, spherical, triangular, boat- shaped etc.

3. Plant bodies are either bilateral or radial in symmetry.

4. 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.

5. The cell wall is composed of pectic sub­stances impregnated with high amount of siliceous substance.

6. The wall may have secondary structures like spines, bristles etc.

7. Vegetative cells are diploid (2n).

8. The cells generally have many discoid or two large plate-like chromatophores. Some cells possess stellate chromatophore.

9. The photosynthetic pigments are chlorophyll a, chlorophyll c along with xanthophylls like fucoxanthin, diatoxanthin and diadinoxanthin.

10. Reserve food is oil, volutin and crysolaminarin.

11. Some vegetative cells show gliding move­ment.

12. Motile structure (antherozoid) has single pantonematic flagellum.

13. Vegetative multiplication takes place by cell division, which is very common. Some of the cells become very much reduced in size.

14. They produce characteristic spore, the auxospore which develops to regain the normal size.

15. Sexual reproduction takes place by isogamy and oogamy.