diatom

Microscopic alga found in all parts of the world in either fresh or marine waters. Diatoms consist of single cells that secrete a hard cell wall made of silica. There are approximately 10,000 species of diatom. (Division Bacillariophyta.)

The cell wall of a diatom is made up of two overlapping valves known as frustules, which are impregnated with silica, and which fit together like the lid and body of a pillbox. Diatomaceous earths (diatomite) are made up of the valves of fossil diatoms, and are used in the manufacture of dynamite and in the rubber and plastics industries.

Diatoms are not just found in water, but can be found in microhabitats as diverse as bird's feathers and the leaves of trees. They are the dominant organisms in Antarctic and Arctic sea ice, where they grow at temperatures around −5°C.