Light micrograph of diatoms Pleurosigma angulatum. The blue colour is a diffraction effect. 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. 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 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) or on the 2 sides of a central axis (in pennate diatoms). Magnification: x200 at 35mm size.