Polarised light micrograph of Cocconeis sp. diatoms and mineral grains settled on filamentous algae. Cocconeis is a genus of diatoms, single-celled photosynthetic algae, of which there are about 100,000 species. They have mineralised cell walls 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: 186x when printed 10 centimetres wide.