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Processes in Microbial Ecology$
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David L. Kirchman

Print publication date: 2011

Print ISBN-13: 9780199586936

Published to Oxford Scholarship Online: December 2013

DOI: 10.1093/acprof:oso/9780199586936.001.0001

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Processes in anoxic environments

Processes in anoxic environments

(p.195) Chapter 11 Processes in anoxic environments
Processes in Microbial Ecology

David L. Kirchman

Oxford University Press

During organic material degradation in oxic environments, electrons from organic material (the electron donor) are transferred to oxygen (the electron acceptor) in the process of aerobic respiration. Other compounds, such as nitrate, iron, sulphate, and carbon dioxide, take the place of oxygen during anaerobic respiration in anoxic environments. The order in which these compounds are used by bacteria and archaea is set by thermodynamics. However, concentrations and chemical state also determine the relative importance of electron acceptors in organic carbon oxidation. Oxygen is most important in the biosphere, while sulphate dominates in marine systems, and carbon dioxide in environments with low sulphate concentrations. Nitrate respiration is important in the nitrogen cycle but not in organic material degradation, because of low nitrate concentrations. Organic material is degraded and oxidized by a complex consortium of organisms – the anaerobic food chain – in which the byproducts from physiological type of organisms becomes the starting material of another. The consortium consists of biopolymer hydrolysis, fermentation, hydrogen gas production, and the reduction of either sulphate or carbon dioxide. The byproduct of sulphate reduction – sulphide and other reduced sulphur compounds – is oxidized back eventually to sulphate by either non-phototrophic, chemolithotrophic organisms or by phototrophic microbes. The byproduct of another main form of anaerobic respiration – carbon dioxide reduction – is methane, which is produced only by specific archaea. Methane is degraded aerobically by bacteria and anaerobically by some archaea, sometimes in a consortium with sulphate-reducing bacteria.

Keywords:   electron tower, syntrophy, dissimilatory sulfate reduction, methanogenesis, anaerobic anoxygenic photosynthesis, biocorrrosion, ANME, amitochondriate protozoa

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