Colonial diatom Fragilaria. Scanning electron micrograph (SEM) of the pennate colonial diatom Fragilaria sp. The diatoms are a group of photosynthetic, single-celled algae containing about 100, 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 often has rows of tiny holes, known as striae. Magnification: x1000 when printed at 10 centimetres wide.