Diatoms are a major group of algae that are commonly found as phytoplankton. They have skeletons made of silica coated with organic material that come in a wide diversity of shapes, either radially or bilaterally symmetrical. Diatoms reproduce asexually through binary fission, where each daughter cell receives one of the parent's frustules and builds a new smaller one, causing population size reduction over time. To counter this, sexual reproduction through meiosis and fusion of gametes occurs, forming a large auxospore that grows into a maximum-sized new initial cell to start the next generation.

1. DIATOMS
General Features, Morphology and Reproduction

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.

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.

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.