Diatoms. Light micrograph of pennate diatoms, a group of single-celled algae. Pennate diatoms are symmetrical about a line. The other major type are centric diatoms, which are symmetrical around a single point. A characteristic feature of diatoms is their intricately patterned, glass-like cell wall, the frustule. The frustule consists of two halves which fit together like the lid and bottom of a box. Diatoms form an important part of the plankton in the marine and freshwater food chains.