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Enzymology of Electron Transport: Energy Generation With Geochemical Consequences

School of Biology, Georgia Institute of Technology, Environ Sci Technol Building Atlanta, Georgia, 30332, U.S.A., Thomas.Dichristina@biology.gatech.edu

Jim K. Fredrickson and John M. Zachara

Fundamental Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MSIN P7-50, Richland, Washington, 99352, U.S.A., Jim.Fredrickson@pnl.gov, John.Zachara@pnl.gov

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Dissimilatory metal-reducing bacteria (DMRB) are important components of the microbial community residing in redox-stratified freshwater and marine environments. DMRB occupy a central position in the biogeochemical cycles of metals, metalloids and radionuclides, and serve as catalysts for a variety of other environmentally important processes including biomineralization, biocorrosion, bioremediation and mediators of ground water quality. DMRB are presented, however, with a unique physiological challenge: they are required to respire anaerobically on terminal electron acceptors which are either highly insoluble (e.g., Fe(III)- and Mn(IV)-oxides) and reduced to soluble end-products or highly soluble (e.g., U(VI) and Tc(VII)) and reduced to insoluble end-products. To overcome physiological problems associated with metal and radionuclide solubility, DMRB are postulated to employ a variety of novel respiratory strategies not found in other gram-negative bacteria which respire on soluble electron acceptors such as O₂, NO₃^–, SO₄^2–, and CO₂. The novel respiratory strategies include 1) direct enzymatic reduction at the outer membrane, 2) electron shuttling pathways and 3) metal solubilization by exogenous or bacterially-produced organic ligands followed by reduction of soluble organic-metal compounds. The first section of this chapter highlights the latest findings on the enzymatic mechanisms of metal and radionuclide reduction by two of the most extensively studied DMRB (Geobacter and Shewanella), with particular emphasis on electron transport chain enzymology. These advances have drawn significantly upon genomic data for isolated microorganisms from the genera Geobacter and Shewanella (see chapter by Nelson and Methé 2005). The second section emphasizes the geochemical consequences of DMRB activity, including the direct and indirect effects on metal solubility, the reductive transformation of Fe- and Mn-containing minerals, and the biogeochemical cycling of metals at redox interfaces in chemically stratified environments.

	ENZYMATIC BASIS OF IRON AND MANGANESE REDUCTION

 
The electron transport systems of gram-negative bacteria are generally described as inner membrane (IM)-associated electron and proton carriers . . . [Full Text of this Article]

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