Diatom (Cymbella helvetica), light micrograph. Diatoms are microscopic, unicellular algae that grow in aquatic environments, both marine and freshwater, and are involved in the formation of phytoplankton. They have intricate cells walls (frustules) composed of silica. Diatoms can be found in practically every aquatic environment on the planet, whether marine, brackish or freshwater, in rivers, lakes, ponds, puddles, but also on the damp surface of rocks, in some soils, between the leaves of mosses, in biofilms that develop on the surface of aquatic plants, etc. They exist both in plankton (the surface of open water) and in benthos (the substratum of aquatic environments). Ecologically, they play a fundamental role in food webs, phytoplankton being the first link in the food chain in the marine environment. Magnification: x450 when printed at 10 centimetres wide.