Review
Bacterial anaerobic respiration and electron transfer relevant to the biotransformation of pollutants

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Abstract

Bacterial anaerobic respiration is one of the most ancient and essential metabolism processes, possessing the characteristics of both flexibility and high diversity, and a very close relationship with the physiological function in the ecological environment. Under anaerobic conditions, bacteria and anthropogenic substances can form coupling process facilitating terminal electron transfer. Several forms of bacterial anaerobic respiration and electron transfer related to the biotransformation of pollutants, including respiration with humics, sulfonates, halogenated chemicals, azo compounds, TNTs, metallic and non-metallic elements, are reviewed in this paper. These respirations and electron transfers on diverse electron acceptors in the environment have important biotechnological implications because these biochemical reactions have their roles on the transformation/degradation of toxic substances and the cycling of organic carbon as well as many inorganic elements. Furthermore, remediation of sites contaminated with toxic pollutants based on bacterial anaerobic respirations is being recognized widely.

Introduction

Chemical industries have changed the ecosystem landscape on earth through synthesis and purification of a wide range of chemicals to be used as solvents, herbicide/pesticides, intermediates and many other applications. More than 100,000 chemicals are manufactured commercially on a daily basis (Swoboda-Colberg, 1995). Most of these substances belong to xenobiotic compounds synthesized at large quantities and high purity and they resist to biodegradation and may result in severe destruction of the natural ecological balance and human health due to discharge and accumulation in the environments (Harrad, 2000). Elimination of these pollutants from the contaminated environments becomes focal points of many research projects in an effort to protect the environment. Among the available techniques for treatments, microorganisms undoubtedly play an ecologically very important role in bioremediation (Liu and Suflita, 1993).

Anaerobic processes by microorganisms are deeply rooted in the origin and evolution of life (Wolfe, 1999, Hederstedt, 1999) and are involved in degradation of a wide range of pollutants (Harwood et al., 1999). Bacterial anaerobic respiration is capable of using selective priority pollutants as terminal electron acceptor (TEA) and reducing them by accepting electrons from the respiratory electron transfer chain. Moreover, some xenobiotic compounds can serve as electron donors for bacterial respiration, in which the chemical compounds are oxidized by the specific enzyme on the bacterial membrane and/or inside the cells. These findings offered new scientific insights that anaerobic biotransformation of environmental pollutants can be coupled to bacterial anaerobic respiration and electron transport and such biochemical processes can be utilized for the control or elimination of environmental pollution. Several forms of bacterial anaerobic respiration closely related to the biotransformation of pollutants under strictly anaerobic or anoxic conditions are discussed below.

Section snippets

Respiration with humics

Humus is the most abundant natural organics occurring in the environment. Humic substances are highly polymerized natural polymers of high molecular weight and poor biodegradability and they are formed during the decomposition of plant materials by microorganisms in soils and sediments. The remarkable recalcitrance of such materials is reflected by its long residence times in the environment, exceeding 500 years. This material is also highly heterogenous in chemical structures depending on the

Ecological and environmental significance

Respiration is a necessary metabolic activity in the evolution and diversification of life. In common life forms, almost all energy (ATP) is derived from respiration with a few exceptions. The generation of energy in eukaryote is carried our by mitochondria, but in prokaryote the same process is on its cellular membrane. Modern concept of respiration chain has been built upon the results from two different research approaches for more than ten years (Stryer, 1995). The process of electron

Prospectives

Recently, several different novel forms of anaerobic bacterial respiration have been described. From the point of the anaerobic respiration, thermodynamics is the fundamental basis for such substances to serve as TEAs (Zehnder and Stumm, 1998). If bacteria could conserve energy for growth with a selective substance as the sole TEA, the biochemical reaction can be called one form of anaerobic respiration. Based on our current understanding of the anaerobic bacterial respiration, bacteria could

Acknowledgements

The author (Y-GH) gratefully acknowledges the support by Young People Innovation Foundation of SCSIO, CAS (07SC011009), National Natural Science Foundation (30800032), and Foundation of President of the Chinese Academy of Sciences (07YQ091001) and K.C. Wong Education Foundation, Hong Kong.

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