Phase contrast light micrograph of an Asterionella formosa (star-shaped) diatom with attached Monosiga (choanoflagellates). 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. Choanoflagellates are single-celled flagellate protozoa. They may be free-swimming or attached to a substrate (sessile). They are the closest relatives of animals (metazoans). Magnification: 186x when printed 10 centimetres wide.