Diatoms and radiolaria, coloured scanning electron micrograph (SEM). Diatoms are unicellular organisms and a major group of algae. They are encased within a hard cell wall made from silica known as a frustule. Radiolarians are amoeboid protozoa that produce intricate mineral skeletons. They are found as zooplankton throughout the ocean. Magnification: x1050 when printed at 10 cm wide.