Diatoms are single-celled plants that have transparent cell walls. They are found in both fresh and marine waters. They are very small in size. Each diatom is enclosed in a box-like shell whose edges are held by a girdle. They float over water and attach themselves to rock or large plants with a jelly-like material. They change nutrients in water to organic substances. These organic substances are the main source of food for ocean creatures. The oil produced by diatoms gets stored in fish liver, for example, cod liver oil, which is of commercial importance.