Diatoms (Bacillariophyta) are one of the most distinctive and successful groups of unicellular algae, occurring throughout the world in marine,
brackish and fresh waters, as well as in damp subaerial habitats. They are usually estimated to contribute about 25-35 % of the world’s
productivity (in terms of carbon fixation) and to be represented by 50 – 200 thousand species depending upon one’s species concept. Their
most distinctive characteristic is the possession of elaborate, siliceous cell walls, features of which are used to define and classify species.
Because silica resists degradation, diatoms are regularly preserved in both freshwater and marine sediments, and in conjunction with
knowledge of their ecological specificity, such records have been used to infer lake or ocean histories. In particular, diatoms in lake sediments
have been used to deduce changes in pH and nutrient status, as well as climate change. They are also used extensively to infer water
quality in contemporary aquatic systems.