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The necessity for a third English edition of Professor Rheinheimer's authoritative and comprehensive introduction to micro-organisms in aquatic environments reflects the volume of applied and academic research work in this area over the last five years. The text has therefore been substantially updated throughout, some 180 papers reviewed and quoted and many more inserted in the extensive reference section at the end of the book. New emphasis (chapter 9) is placed on the distribution of micro-organisms, their role in the cycling of the elements, their functions in aquatic ecosystems and their importance for water pollution. Both text and illustrations incorporate all revisions and amendments made by the author to the current, fourth, German edition of the book."