Diatom frustule. Interference contrast light micrograph of the glass-like cell wall, or frustule, of a living diatom. Diatoms are a group of photosynthetic, 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. Seen here are the internal structures including the nucleus (central granular area), oil droplets (brown) and chloroplasts (green). The characteristic feature of diatoms is their intricately patterned frustule, which often has rows of tiny holes, known as striae.