Diatoms and Spirogyra algae. Darkfield illuminated polarised light micrograph of Vorticella sp. diatoms (round) amongst Spirogyra sp. green algae (long, thin) filaments. 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. They form an important part of the plankton at the base of the marine and freshwater food chains. Spirogyra is filamentous green algae named for the spiral arrangement of its chloroplasts. It's cylindrical cells connect end to end in long green filaments. Magnification: x200, when printed 10 centimetres wide.