BACILLARIOPHYCEAE

Bacillariophyceae

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.

Diatoms are found in all possible habitats. Commonly they are found in fresh water (Denticula tenuis, Navicula pupula, Meridion circulare, Cymbella ventricosa, Melosira variens, Amorpha ovalis etc.), sea water (Corethron, Biddulphia, Sceletonema, Fragilaria, Tropido- nensis etc.) and soil (Pinnularia, Navicula, Frustulia etc.).

The terrestrial species (Amorpha, Navicula, Pinnularia etc.) are able to withstand desiccation for a long period.

Plant Body of Diatoms:

Plant body is unicellular, generally moves singly. The cells are of different shapes viz. round, oval, elongated, rod-shaped, triangular, disc-shaped etc. Sometimes they become aggre­gated and get embedded in a gelatinous matrix, but they do not behave like multicellular orga­nisms.

Cell Structure of Diatoms:

The cell consists of cell wall and protoplast (Fig. 3.101 A, B, C). The cells are covered by a siliceous wall, the frustule. It consists of two overlapping halves, the theca. The upper one is epitheca and lower one is hypotheca.

Classification of Bacillariophyta:

Class Bacillariophyceae has been divided into two orders. Pennales and Centrales.

Order: Centrales:

1. Thallus radially symmetrical.

2. Gliding movement absent.

3. Sexual reproduction anisogamous or oogamous.

4. Gametes are motile.

Order: Pennales:

1. Members are bilaterally symmetrical.

2. Cells show gliding movement.

3. Sexual reproduction is amoeboid.

Family: Naviculoideae:

(i) Members are fresh water in habitat.

(ii) Valve view is boat shaped.

(iii) Raphe is present in both the valves.

Reproduction:

Navicula reproduces by two methods: Vegetative and sexually.

Vegetative Reproduction:

`           It takes place by the mitotic cell division or fission. Successive cell division takes place very rapidly at night. Presence of aluminium-silicate in water is essential for cell division to occur. As the cell division starts, the cell protoplast increases in diameter. The cell also increases in size.

Sexual Reproduction:

It takes place by the formation of auxospores. The successive decrease of cell size in vegetative reproduction is prevented by the auxospore formation. The auxospore formation is actually a ‘restorative process’ because the reduction in the original size of the cells, during the cell division is restored. During the process only those cells which have diminished sufficiently in size can act as ‘sex cells’ or conjugating cells.

Oogonium: Single vegetative cell behaves as an oogonium. The protoplast of oogonium undergoes meiotic division and forms four nuclei. Of the four nuclei three degenerate and the remaining one functions as an egg.

Antheridium: The pattern of development of sperms varies in different species. In species like Melosira varians the protoplast undergoes meiotic division and forms four haploid nuclei.

Fertilisation: After coming out of the antheri­dium only one sperm enters inside the oogonium and fertilises the egg. The resultant zygote under­goes mitotic division but one nucleus degene­rates in each division. The remaining nucleus with its protoplast behaves as an auxospore.

The different uses of diatoms are:

1. Diatomite:

After the death of diatom cells the outer coverings i.e., the silicified walls become accumulated at the bottom of water. The accumulation may be thicker during favourable conditions. These deposits are called diatomaceous earth, diatomite or keiselghur.

It is very suitable for use in different industries:

a. As Filter:

It is used as filter in different industries like sugar (to filter microorga­nism), oil and chemical industry. Diato­mite is also used as filter for battery boxes.

b. As Insulator:

It is used as insulator in boilers and blast furnaces for its heat- resistant ability.

c. As Absorbent:

It is used as absorbent of liquid nitroglycerine.

d. Other Uses:

Diatomite is used as abra­sive (i.e., capable of rubbing or grinding down) substance for the manufacture of metal paints, polish, varnish, toothpaste etc. It is also used with bake-lite for elec­trical fuse and switch boxes.

2. Petroleum:

Much of the petroleum is con­sidered to be of diatom origin as they are found in association with large oil deposits.

3. Food:

Due to their great abundance in the different seas and their use as food by marine animals, they are called the ‘grasses of the sea’. Those animals may be con­sumed as food by man and maintain the food chain.

4. Testing of Microscopic Lenses:

Due to the fine markings on shell, the diatom cells are used to test microscopic lenses.