Differential interference contrast (DIC) light micrograph of Campylodiscus Sp. diatoms. Diatoms are a distinctive group of single-celled algae containing about 10,000 species. They form a large part of the plankton at the base of the marine and freshwater food chains. A characteristic feature of diatoms is their intricately patterned, silica-impregnated glass-like cell wall, or frustule. The frustule consists of two halves which fit together like the lid and bottom of a box. The frustule is often decorated with rows of tiny holes, known as striae, which are arranged radially in centric diatoms and on the two sides of a central axis in pennate diatoms. Magnification: x200 at 35 millimetre size.