Phylum Bacillariophyta

Diatoms have a golden-brown pigment. Some books still place them with the Chrysophyta, the golden-brown algae, but they are now recognized as an entirely separate group. Diatoms have odd little shells made of organic compounds impregnated with silica. The fact that the shells are made of silica is unique to this group. The shells fit over the top of one another like a little box. Diatoms usually reform the lower shell after they divide This means they become smaller and smaller, and when they become too small they leave their shells and fuse through sexual reproduction into a larger size and start over again. They are one of the most important organisms in both freshwater and marine food chains. Diatoms are so abundant that the photosynthesis of diatoms accounts for a large percentage of the oxygen added to the atmosphere each year from natural sources. Their dead shells form huge deposits, that are mined for commercial uses. Diatom shells are sold as diatomacious earth, and used in abrasives, talcs, and chalk. Diatoms are so numerous that their shells form thick deposits all over the world. A single quarry in Lompoc, California, yields over 270,000 metric tons per year. One bed in the Santa Monica Ca. oil fields is over 900 meters thick! Various species of diatoms are also widely used as indicator species of clean or polluted water. The diatoms, like other freshwater organisms react to pollutants. Scientists can see how these protists are doing to determine if the water and the environment is clean. If there is a thriving population of diatoms, the water quality is good. If there are no diatoms or a very small population, the water quality is poor. Using protists and other organisms this way helps scientists. Because they can give information about the quality of the environment, they are called biological indicators. Fish, amphibians, and diatoms are all good biological indicators.