Polarised light micrograph of a Heliozoan (centre) surrounded by diatoms and Spirogyra sp. algae (green). Heliozoans are single-celled freshwater organisms that have many pseudopodia (false feet) radiating outwards from their cell body that are used for capturing prey and locomotion. 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 sp. are long filamentous green algae named for the helical arrangement of their chloroplasts. Magnification: x186 when printed 10 centimetres wide.