2. Diatoms are a major group of algae, and are among the most common types of phytoplankton.
Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons (e.g. Fragilaria), zigzags (e.g. Tabellaria), or stars (e.g. Asterionella).
Diatoms generally range in size from 2-200μm. The skeleton of a diatom, or frustule, is made of very pure silica (hydrated silicon dioxide) coated with a layer of organic material. These frustules show a wide diversity in form, but are usually almost bilaterally symmetrical.
3. The skeleton or frustuleis divided into two parts, one of which (the epitheca) overlaps the other (the hypotheca) like the lid of a box or petridish.
4.
5. Diatoms are of two types –
Centric diatoms (Centrales), which are radiallysymmetrical
Pennatediatoms (Pennales), which are bilaterally symmetrical
◦Pennatediatoms without a raphe, araphids
◦Pennatediatoms with a raphe, raphids
6. Both parts of a frustulemay be highly perforated. Pennatediatoms show a long slit, the raphe, along the long axis.
Through the raphe, the living diatom secretes mucilage, with which it may attach to a substrate or move by gliding over the substrate.
7. Major pigments of diatoms are –
◦Chlorophylls a and c
◦Beta-carotene
◦Fucoxanthin
◦Diatoxanthin
◦Diadinoxanthin
Major storage products are –
◦Chrysolaminarin
◦Lipids
8. Reproduction
Reproduction of diatom is primarily asexual by binary fission, with each daughter cell receiving one of the parent cell's two frustules (or theca).
9. This is used by each daughter cell as the larger frustule(or epitheca) into which a second, small frustule(or hypotheca) is constructed.
This form of division results in a size reduction of the daughter cell that received the smaller frustulefrom the parent and therefore the average cell size of a diatom population decreases.
Nonetheless, in order to restore the cell size of a diatom population for those that do endure size reduction, sexual reproduction and auxosporeformation must occur.
10.
11. Vegetative cells of diatoms are diploid (2N) and so meiosis can take place, producing male and female gametes which then fuse to form the zygote.
The zygote sheds its silica theca and grows into a large sphere covered by an organic membrane, the auxospore.
A new diatom cell of maximum size, the initial cell, forms within the auxosporethus beginning a new generation.