Diatoms are microscopic photosynthetic algae having a cell size range from 2 to 200 micrometers. One of the important characteristic features of diatoms is the presence of a siliceous skeleton or the frustule. Diatoms are found in every aquatic environment such as freshwater as well as marine waters.
The cell walls of diatoms are made of silica. The silica is mainly composed of hydrated silicon dioxide. When diatoms die, the organic material or the living part decomposes, and the siliceous skeleton (diatomite or diatomaceous earth) settles to the bottom of that aquatic environment.