Diatoms. Coloured scanning electron micrograph of the diatom Campylodiscus hibernicus. The 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, which is impregnated with silica. The frustule consists of 2 halves which fit together like the lid & bottom of a box. It is often decorated with rows of tiny holes, known as striae, which are arranged either radially or on the 2 sides of a central axis. Magnification: x160 at 35mm size.