Animated coloured scanning electron micrograph (SEM) of fresh water centric diatom frustule (skeleton), Cyclotella sp. This freshwater centric diatom is found in the brackish water interface of polluted streams and lakes. Diatoms are a type of algae (Chromophyta, Bacillariophyceae). They consist of two valves, made of silica (hydrated silicon dioxide), which fit together like the top and bottom of a box. This SEM shows the surface of one valve and the broad girdle connecting it to the other valve. The exterior of the valves are often highly decorative, with pores, spikes or other types of projections. It is an important alga in the fresh water food chain.