SEM of the fractured shell of the diatom Aulacodiscus. Diatoms are single-celled photosynthetic algae, of which there are about 100,000 species. They have mineralized cell walls (frustules) that contain silica and provide protection and support. The diatoms form an important part of the plankton at the base of the marine and freshwater food chains. Magnification x14,000