Diatoms photosynthesize. They are categorized as either protists or chromists.
They provide a significant amount of the world's oxygen supply (some say 35%).
There are over a hundred thousand species identified.
Diatoms live anywhere there is water and light, including lakes, streams, estuaries, oceans, puddles and wet rocks or soil.
That brown scum you see on the rocks in the stream is probably diatoms.
Diatomaceous earth is sometimes used in gardening as a pest control.
For more diatom info, check out the Diatom Home Page.