Differential interference contrast micrograph of Cymbella sp. diatom. Diatoms are single-celled photosynthetic algae, of which there are about 100,000 species. They have mineralised cell walls (frustules) that contain silica and provide protection and support. Cymbella diatoms have elongated cells that are typically boat-shaped. They are commonly found in freshwater habitats. Magnification: 372x when printed 10 centimetres wide.