Timelapse light microscopy of diatoms. Diatoms are a group of photosynthetic, single_celled algae containing about 10,000 species. The green structures inside the diatoms are chloroplasts, the site of photosynthesis. 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.