Phase contrast light micrograph of Fragilaria crotonensis (rectangle-shaped) and Asterionella formosa (star-shaped) diatoms. Diatoms are a type of single-celled photosynthetic algae that form a significant portion of the Earths biomass. They generate about 20 to 50 percent of the oxygen produced on the planet each year. Magnification: 186x when printed 10 centimetres wide.