Polarised light micrograph of a Fragilaria crotonensis diatom (filamentous, blue) and a Cosmarium sp. desmid (rounded, green). 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. Desmids are a common group of freshwater single-celled algae that have intricate cell walls. They are highly symmetrical and come in a diversity of forms.