Light micrograph of assorted diatoms. 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 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 & bottom of a box. The frustule 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: x200 at 35mm size.