Anaerobes

Abstract A diverse world of microorganisms inhabits anaerobic environments on Earth, and these obtain their energy by fermentation or by anaerobic respiration, in which electron acceptors other than molecular oxygen drive the oxidation of organic compounds. We also know phototrophic and chemoautotrophic processes that function in the absence of oxygen. Collaboration of different groups of microorganisms can lead to degradation of complex organic matter also under anaerobic conditions, with carbon dioxide or a mixture of carbon dioxide and methane as the main products, depending on the availability of electron acceptors other than molecular oxygen, such as nitrate or sulfate. The different groups of anaerobes are essential for the functioning of the biogeochemical cycles of carbon, nitrogen, sulfur and other elements, and also play important roles in the production of fermented food products, treatment of organic wastes and other biotechnological processes. Key Concepts: Life developed on planet Earth in the absence of molecular oxygen (O 2 ). The anaerobic way of life is widespread in the prokaryotic world – in Bacteria as well as Archaea. Permanently anaerobic environments include marine and freshwater lake bottom sediments, digestive systems, the deep subsurface and others. Some protozoa can live anaerobically. Recently a community of multicellular meiozoa that lives in permanently anoxic conditions was discovered in L'Atalante basin, 3.5 km below the surface of the Mediterranean Sea. In the absence of molecular oxygen, many prokaryotes can use alternative electron acceptors, such as nitrate, sulfate, Fe(III) and others, for respiration (‘anaerobic respiration’). When no suitable electron acceptor is available, energy can be gained by fermentation processes, in which ATP is gained by substrate‐level phosphorylation. Complete anaerobic breakdown of organic matter is a cooperative process in which fermentative microorganisms, bacteria performing anaerobic respiration and methanogenic Archaea cooperate. Hydrogen and acetate are key intermediates in many anaerobic degradation processes.