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Selenium is widely but variably distributed on earth, having an average abundance in the lithosphere of about 0.05 ppm. Both organic and inorganic selenium compounds are present in the oceans. Oceanic selenium concentration increases with depth, and concentrations as high as 0.2 p~g/1 are found in deep ocean waters. The aquatic chemistry of selenium is complicated since it can exist in four different oxidation states and in a variety of inorganic and organic compounds. Microorganisms play an important role in controlling the chemical speciation of selenium in the aquatic environment.