Light micrograph of a Navicula sp. marine 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 and freshwater food chains. The characteristic feature of diatoms is their intricately patterned, 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 either radially or, as here, on the two sides of a central axis. Magnification: x300 at 35 millimetre size.