Diatom, a microscopic one-celled organism that is enclosed in a protective shell. Diatoms are algae, simple organisms that contain chlorophyll. There are more than 15,000 species. Huge numbers are present in all oceans and in most other waters. Diatoms are an important food source for fish. Some diatoms are found attached to logs, to rocks, or to plants.

Diatoms rarely grow longer than 1/200 of an inch (0.13 mm). The glasslike shell is sculptured in beautiful lacy designs, created by the flow of the diatom's shell-producing secretion. Due to the combination of various pigments within the cell, most diatoms appear golden-brown or yellow. Some, however, are colorless.

Diatoms reproduce rapidly by simple splitting. (The name diatom comes from a Greek word meaning "cut in two.'') After dividing, each new organism keeps one half of the original shell, then secretes the other half.

Diatoms belong to the class Bacillarieae of the phylum Chrysophyta.