Polarised light micrograph of Melosira sp. diatoms attached to red algae from the Baltic Sea. Diatoms are a type of single-celled photosynthetic algae. They form a significant portion of the Earths biomass, generating about 20 to 50 percent of the oxygen produced on the planet each year. Melosira diatoms are found in both freshwater and marine habitats. They have uniformly structured valve walls. Algae is usually differentiated based on pigments; red algae contains the pigments phycoerythrin and phycocyanin. Magnification: x186 when printed at 10 centimetres wide.