Tiny one-celled organisms called diatoms are found by the billions in all the waters on the face of the Earth. The largest of them are barely visible to the unaided eye, and the smallest are less than a thousandth of an inch long. Biologists classify diatoms as golden or golden-brown forms of algae (see algae). Like other algae, diatoms have no leaves, stems, roots, or flowers, but the cell of every diatom contains chlorophyll, the substance that is responsible for the green color of leafy plants.
Just as it does in plants, chlorophyll absorbs sunlight to help diatoms make sugars from carbon dioxide and water (see plant). Unlike diatoms, most plants next change these sugars to cellulose to make their cell walls strong. Diatoms, on the other hand, take up dissolved silica from the water and form it into a pair of glassy shells adorned with intricate patterns. There are thousands of species of diatoms, each with its own shape. The two shells of a diatom fit together like the top and bottom of a box and are held together along the edges by a softer band called a girdle.
Most diatoms float in the water or fasten themselves with a sort of jelly to stones or other water plants. Diatoms reproduce themselves by various methods. Sometimes the cell divides, the shells separate, and each half grows a new shell on its exposed surface. Two diatoms may combine after shedding their old shells. The united cells then separate into two new individuals, each of which grows a new pair of shells. Some species reproduce by means of spores.
In cold waters, where diatoms are most plentiful, the dead and the discarded shells may form thick deposits on the bottom. After long ages, the shell deposits turn into a porous mineral mass called diatomite (also called diatomaceous earth, tripolite, and kieselguhr). Diatomite deposits are found on the sites of many ancient oceans and lakes. Diatomite is used as a chemical filter, particularly in the sugar industry, and in the preparation of heat-insulating materials and polishing compounds and for mixing with concrete. (See also oceanography.)