ANAEROBIC ORGANISM
Anaerobic organisms (anaerobes) are organisms, which can live and develop in the absence of free oxygen in the medium. Anaerobic organisms are common mainly among prokaryotes. Among eukaryotes only very few forms are capable of anaerobic life. These are the forms, which secondary accommodated themselves to the absence of oxygen. They are yeast, some protozoa, and multicellular intestinal parasites.
Metabolism of anaerobic organisms is caused by the necessity to use oxidizers other than oxygen. Many anaerobes, which use organic substances, accomplish various kinds of fermentation, where reduced compounds (alcohols, fatty acids) are formed. Others use inorganic oxidizers - nitrates, sulfur compounds, carbon dioxide.