A diatom is any member of a class of unicellular algae, known formally as Bacillariophyceae, that live in cold waters of relatively low salinity. Diatoms come a wide variety of beautiful, symmetrical shapes. All have shell-like, brittle cell walls made out of silica (glass) and pectin. The walls, which are porous to allow materials in to and out of the cell, consist of two interlocking halves that fit together like a pillbox.
Because they depend on sunlight for photosynthesis, diatoms generally live in the upper 200 m of oceans and bodies of fresh water. Some species simply float in the water currents near the surface; others attach themselves to larger floating objects or to the sea floor.
When diatoms die, they slowly sink to the seabed. The buildup of trillions of these shells forms a crumbly white sediment known as diatomaceous earth or diatomite, which is used in manufacturing pool filters and abrasives, including toothpaste.