Diatoms ("cut in two", from the Latin tém-n-ō "I cut") are a major group of microalgae found in the oceans, waterways and soils of the world. They are the most common type of phytoplankton, and generate about 20 percent of the oxygen produced on the planet each year.[6] Living diatoms number in the trillions: they contribute nearly half of the organic material found in oceans, and the shells of dead diatoms can reach as much as a half mile deep on the ocean floor. Diatoms are unicellular, although they can form colonies. They are producers within the food chain.

A unique feature of diatoms is that they are surrounded by a cell wall made of silica (hydrated silicon dioxide) called a frustule.[7] These frustules show a wide diversity in form, but usually approach bilateral symmetry, hence the group name. The symmetry is not perfect since one of the valves is slightly larger than the other, allowing one valve to fit inside the edge of the other. Only male gametes of centric diatoms have flagella, making them capable of movement beyond that exerted on diatoms by water currents.