According to the University of Colorado, diatom species are very particular about the water chemistry in which they live, and have a distinct range of pH and salinity where they grow. They also have specific tolerances for other environmental variables, which makes them suitable to be used in environmental assessment and monitoring. In addition, fossil diatoms can be used to determine conditions of the past.
Each species of diatom has distinct requirements for the pH, salinity, nutrient concentration, suspended sediment, flow regime and elevation in which it will grow. Therefore, different species are found under different conditions. In addition, the silica cell walls of a diatom do not decompose, thus diatoms in marine and lake sediments can be used to interpret conditions of the past by comparing the preferences of modern species to the fossils of past diatoms. Diatoms are also important global sources of carbon fixation, and serve as a "biofuel" for aquatic food webs. It is estimated that 40 percent of the earth's oxygen is produced by diatoms.
According to the University of California at Berkeley, diatoms have an extensive fossil record going back to the Cretaceous period. Some rocks are formed almost entirely of fossil diatoms and are called diatomite or diatomaceous earth. These deposits are mined commercially as abrasives and filtering aids.