Diatoms (pronounced "DIE-uh-toms") are a type of single-celled golden-brown algae. They have a cell wall made up of silica, the main ingredient of sand and glass. They can live singly or in colonies, both types can be seen in this photo. Some diatoms are motile, they move about by oozing slime through tiny pores in their shells, sliding over the substrate like microscopic slugs. Rocks in lakes and rivers are often slippery due to large numbers of diatoms.
The shells sink to the bottom of the lake or ocean when the organism dies and loses its "bouyancy compensating" oil droplet. Over millions of years the compressed shells solidified and turned to chalk. The shells of dead diatoms are also used as the abrasive substance in toothpaste (look for "hydrated silica" on the ingredient label). I get quite a reaction from my students when they learn that fact! :)
Diatoms are photosynthetic and produce large amounts of oxygen (due to the astronomical numbers of them in the lakes and oceans). There are over 100,000 different species of diatoms worldwide. It is very easy to find diatoms in pond, lake, river and ocean water samples. These specimens are magnified at 100X, I wish the resolution of the camera was better so the beautiful detail of the cells would be visible in the photos.