ReviewNovel forms of anaerobic respiration of environmental relevance
Introduction
The well-known forms of microbial respiration in which oxygen, nitrate, sulfate, or carbon dioxide serve as electron acceptors for the oxidation of organic carbon and/or hydrogen have been studied in great detail. However, there are other less-studied types of microbial respiration, many discovered within the past decade, which may also have important environmental and biotechnological impacts. The purpose of this article is to review recent (1998–2000) advances in the understanding of these novel forms of microbial respiration. Because of space considerations, one extensively studied form of anaerobic respiration, reductive dechlorination, has not been reviewed here (see 1, 2 for more details).
Section snippets
Dissimilatory Fe(III) reduction
The most abundant alternative electron acceptor for anaerobic respiration in many sedimentary environments is Fe(III) [3]. Fe(III) reduction is known to play an important role in the degradation of organic matter naturally present in aquatic sediments and the subsurface 3, 4, as well as in the degradation of organic contaminants in polluted aquifers 5•, 6••, 7. Fe(III)-reducing microorganisms may also have a major impact on the fate of metals in the environment by using these metals instead of
Humics reduction
All Fe(III)-reducing microorganisms that have been evaluated also have the ability to transfer electrons to humic substances (HS) and other extracellular quinones 30, 31. This includes the Fe(III)-reducing hyperthermophilic microorganisms [32]. Current evidence suggests that although Fe(III) chelated with HS may be one electron-accepting group in HS 33, 34, quinones are the primary electron-accepting moieties 34, 35. Once the quinones are reduced the hydroquinones can abiotically reduce Fe(III)
Reduction of inorganic toxic compounds
Microorganisms have been found to conserve energy to support growth from electron transport to a variety of toxic metals and metalloids 3, 38. Many dissimilatory metal-reducing microorganisms have the potential to reductively precipitate radioactive metals such as uranium, technetium, and cobalt in contaminated subsurface environments [8]. Shewanella alga strain BrY was shown to enzymatically reduce radioactive Co(III) complexed with EDTA to Co(II) [39], while S. saccharophilia was found to
Novel respirations involving sulfur and nitrate
A big development in the understanding of anaerobic respiration in anaerobic sediments was the discovery of a giant sulfide-oxidizing, nitrate-reducing microorganism in marine shelf sediments near Namibia [54••]. This organism known as ‘Thiomargarita namibiensis’ has not been isolated in pure culture but has been found to be closely related to Thioploca species, which have a similar form of respiration. Cells of T. namibiensis had diameters up to 0.3 mm. Most of the cell comprises a large
Conclusions
This summary emphasizes that novel forms of anaerobic respiration continue to be discovered. Much is still to be learned about the physiology, biochemistry and ecology of these forms of respiration and their potential biotechnological applications. Furthermore, it seems likely that there are still important forms of anaerobic respiration yet to be found.
Update
A membrane-bound Fe(III) reductase complex that contains a c-type cytochrome has recently been recovered from Geobacter sulfurreducens. This is the first description of a purified NADH-dependent Fe(III) reductase from a microorganism capable of coupling Fe(III) reduction to growth [65].
Acknowledgements
The authors’ research in this area is supported by grants from the Department of Energy and the National Science Foundation.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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