Light micrograph of an unidentified diatom. The diatoms are a distinctive group of single-celled algae containing about 10, 000 species. They form an important part of the plankton at the base of the marine & freshwater food chains. The characteristic feature of diatoms is their intricately patterned, glass- like cell wall, or frustule. The frustule consists of 2 halves which fit together like the lid & bottom of a box. The frustule is often decorated with rows of tiny holes, known as striae, which are arranged radially (in centric diatoms) or on the 2 sides of a central axis (in pennate diatoms). Magnification: high power.