Tiny organisms at the bottom of the food chain, diatoms are a type of single celled algae that live in seas, lakes, rivers, soils, and even on other plants and animals. They are so abundant in the oceans that scientists believe they help to control carbon dioxide levels in the atmosphere. Diatoms are also studied to monitor the environmental histories of lakes and seas. The shells are constructed like little glass pill boxes, with one half fitting into the other to protect the living cell within. (Diatom = far north). SEM x 8000.