Diatoms. Polarised light micrograph of Cymbella sp. and Gomphonema sp. diatoms. 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. Their characteristic feature is the intricately patterned glass-like cell wall, or frustule, which often has rows of tiny holes, known as striae. Magnification: x200 when printed 10 centimetres wide.