Algae present in the group Bacillariophyta (class Bacillariophyceae) are known as diatoms. They constitute a very large assemblage of unicellular algae.
Diatoms differ from other algae due to their symmetrical structure and delicately designed cell walls. Thus, diatoms are considered the most beautiful microscopic algae.
Characteristics of Bacillariophyta
- Diatoms are found in freshwater as well as marine habitats. Some members are attached to the substrata, while others are free-floating.
- The plant body is unicellular or sometimes colonial. It is always diploid.
- The thalli are of different shapes. They may be oval, spherical, triangular, rod-shaped, disc-shaped, or boat-shaped.
- The cells are bilaterally or radially symmetrical.
- All vegetative cells are diploid.
- The cell wall of diatoms is called the frustule. It consists of two overlapping valves, or halves, arranged in the form of a box with its lid.
- The wall is made up of pectic substances with a high amount of silica deposit. It shows characteristic secondary structures like spines and bristles.
- Cells usually possess several discoid or two large plate-like chromatophores, which contain many photosynthetic pigments.
- Photosynthetic pigments are chlorophyll-a, chlorophyll-c, and carotenoids, along with xanthophylls like diatoxanthin, diadinoxanthin, and fucoxanthin. Diatomin (xanthophylls + carotenoids) gives the thallus a golden brown colour.
- Pyrenoids are present.
- Reserve food materials are in the form of fats, volutin, and chrysolaminarin.
- Some cells exhibit gliding movement caused by the streaming of cytoplasm.
- The only motile structure formed in this group is the male gamete of the centric diatoms. It has a single pantonematic flagellum, which is apically inserted.
- Reproduction usually occurs through cell division and auxospore formation.
- Sexual reproduction takes place by either anisogamy or oogamy.
Distribution of Bacillariophyta
There are about 250 genera of living diatoms, with around 100,000 species.
Diatoms are found in all possible habitats, especially in freshwater and marine water. They are usually associated together in great abundance in the aquatic ecosystem.
Some diatoms are either strictly freshwater (e.g., Navicula pupula, Melosira variens, Denticula tenuis, Synedra, Asterionella, etc.) or strictly marine (e.g., Fragilaria, Corethron, Sceletonema, Biddulphia, etc.). A few members, such as Frustulia, Pinnularia, and Navicula, are terrestrial.
Cymbella and Gomphonema occur as epiphytes on other algae like Cladophora and Enteromorpha. Licmophora is an endozoic member, growing within living organisms.
Plant body of Bacillariophyta
Diatoms are unicellular. The cells are of different shapes. They may be circular, oval, elongated, triangular, boat-shaped, disc-shaped, wedge-shaped, straight rod-like, etc.
Sometimes diatoms form groups and are embedded in a gelatinous matrix. In colonial form, the cells may be present as a branched body (Licmophora) or in chains (Melosira, Cymbella).