Darkfield illuminated polarised light micrograph of a group of Ophrydium sp. protozoa (green, across bottom) and diatoms (rods). Ophrydium are colonial microorganisms that contain green algae inside their bodies that have a symbiotic relationship with the protozoa. They photosynthesise inside the protozoan's body providing it with sugars and oxygen, while deriving protection from their host. 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. Magnification: x186 when printed 10 centimetres wide.