Diatoms (Bacillariophyta)

Diatoms, the Bacillariophyta, are photosynthetic organisms enclosed in exquisitely beautiful silica shells called frustules. They are among the most important aquatic microorganisms, extremely abundant in the plankton and in sediments of marine and freshwater ecosystems, and an important food source for marine organisms. Some live in soils or on moist mosses. Diatoms have an extensive fossil record going back to the Cretaceous, and some rocks are formed almost entirely of fossil diatoms. These deposits, known as diatomite or diatomaceous earth, are mined commercially as abrasives and filtering aids, and fossil diatom assemblages also record past environmental conditions.