Diatom frustules. The remains of diatom skeletons (frustules). Diatoms are a group of photosynthetic, single-celled algae of about 10,000 species. They form an important part of plankton at the base of the marine and freshwater food chains. Their characteristic feature is the intricately patterned glass-like silica cell-wall, or frustule, which often has rows of tiny holes, known as striae. SEM ( Coloured scanning electron micrograph ) .