2 mm in size. They are found just about anywhere there is light and moisture. They are far more diverse and abundant in freshwaters, where they are the commonest organism found and make up the base of many freshwater food pyramids. In these regions, they are the most important organisms at the base of the food chain. D iatoms have a highly differentiated cell wall, which is impregnated with opaline silica, so that their growth is subject to the availability of silicon in the water.
Diatom classification is based on the features of the skeleton.
D iatoms are important ecological indicators because they are sensitive to such factors as salinity, temperature, pH and pollution. In limnology, freshwater diatoms are the primary tool for reconstructing lake conditions, especially changes in pH and fertility. They play a key role in monitoring acid rain and the pollution of the world's freshwater. A catalogue of the diatom assemblages is being prepared which will serve as a guideline to the diatom flora of the Sea for use by future students and researchers. Those that live in the plankton float freely about with the water currents.