Diatoms (Bacillariophyta): Structure, Reproduction and Economic Importance

Diatoms are microscopic, unicellular algae that belong to the phylum Bacillariophyta. They are widely distributed in nature and are found in both freshwater and marine habitats. Diatoms form a major part of phytoplankton and play an important role in aquatic ecosystems. The branch of biology that deals with the study of diatoms is known as diatomology.

Occurrence of Diatoms

Diatoms are found in oceans, rivers, lakes, ponds, and even moist soil. Most diatoms are marine, while some are freshwater forms. They float freely in water and constitute a major portion of the primary producers in aquatic food chains. Due to the presence of oil droplets in their cytoplasm, they can float easily on the surface of water.Structure of Diatoms

Diatoms are unicellular organisms and show great variation in shape. They may be spindle-shaped, rod-shaped, circular, oval, polygonal, or elongated. The most characteristic feature of diatoms is their cell wall, which is made up of cellulose impregnated with silica. This makes the cell wall hard and glass-like.

The diatom cell wall is known as a frustule and consists of two overlapping halves:

• Epitheca – the larger upper half

• Hypotheca – the smaller lower half

These two halves fit together like a soap box. The frustule shows fine and characteristic designs such as striations, pores, and nodules. Due to the presence of silica, the cell wall does not decompose easily.

The cytoplasm contains a nucleus, mitochondria, chloroplasts, and food reserves. The chloroplasts contain chlorophyll a, c, and carotenoid pigments, which give diatoms their golden-brown colour. Food is stored mainly in the form of oil droplets and leucosin.

Classification of Diatoms

Based on symmetry and shape of the valves, diatoms are classified into two orders:

1. Centrales

• Found mainly in marine water

• Valves are circular

• Show radial symmetry

2. Pennales

• Found mainly in freshwater

• Valves are elongated

• Show bilateral symmetry

Reproduction in Diatoms

Diatoms reproduce by both asexual and sexual methods.

Asexual Reproduction

Asexual reproduction occurs mainly by binary fission. During cell division, the nucleus divides mitotically. The two halves of the frustule separate slightly. Each daughter cell receives one half of the parent frustule (either epitheca or hypotheca) and forms the other half by secretion.

As a result, one daughter cell remains of the original size, while the other becomes slightly smaller. Repeated asexual reproduction leads to a gradual reduction in the size of diatom cells.

Sexual Reproduction

Sexual reproduction occurs when the size of the diatom cell becomes too small due to repeated asexual divisions. The protoplast of each cell forms haploid gametes by meiosis.

Gametes of two different cells fuse to form a diploid zygote (2x). The zygote enlarges to form an auxospore, which restores the original size of the diatom. The auxospore then divides mitotically to produce normal-sized diploid diatom cells.

Auxospores are larger in size than vegetative cells and do not have a rigid siliceous wall initially.

Diatomaceous Earth

When diatoms die, their siliceous cell walls accumulate at the bottom of water bodies. Over millions of years, these deposits form thick layers known as diatomaceous earth. These deposits may be thousands of metres thick and are commercially very important.

Economic Importance of Diatoms

 As Food

Diatoms constitute a major portion of marine phytoplankton. They are the primary producers in aquatic ecosystems and serve as food for many marine animals, including fish and whales.

 Industrial Uses of Diatomaceous Earth

Due to its porous and abrasive nature, diatomaceous earth has several industrial applications:

1. It is used in making filters for sugar industries and brewing industries.

2. It is used in the manufacture of toothpaste.

3. It is fire-resistant and used in making high-temperature furnaces.

4. It is used for polishing metals.

5. It is used in making temperature-resistant bricks.

6. It is used for packing corrosive chemicals.

7. It is used in insulation of pipes.

8. It is used in dynamite manufacture as an absorbent material.

9. It is used in insulation of refrigerators.

10. It is used in making cosmetics.

Importance of Diatoms in Nature

Diatoms play a key role in maintaining ecological balance. They contribute significantly to oxygen production through photosynthesis. Diatoms also help in carbon fixation and form the base of aquatic food chains.

Conclusion

Diatoms are one of the most important groups of algae due to their unique structure, mode of reproduction, and economic importance. Their siliceous cell wall, method of size restoration through auxospores, and role as primary producers make them biologically and industrially significant. Understanding diatoms is essential for students preparing for board exams and competitive exams like NEET.