Respiration-linked Metal Reduction in Bacteria

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Microbes that can couple their anaerobic growth to the reduction of manganese(IV) and iron(III) have only recently been isolated and make it possible to investigate the physiology and biochemistry of the microbial processes responsible for the use of manganese(IV) and iron(III) as terminal electron acceptor for anaerobic respiration. These microbes may act as important biogeochemical agents for the cycling of these metals in aquatic and soil environments. The more immediate objectives of this project include the study of one such microbe, Shewanella (formerly Alteromonas) putrefaciens strain MR-1, including: (1) determination of the growth energetics when growth is coupled to the reduction of manganese or iron, and comparison of these energetics to those involving the use of other electron acceptors(O2, nitrate, and fumarate); (2) determination and preliminary characterization of the electron transport components involved in the respiratory reduction of these metals; and (3) purification and characterization the terminal reductases responsible for the reduction of these metals. Several aspects of this work form the foundation for its scientific rationale. First, the ability of certain bacteria to couple their growth to the dissolution of manganese and iron oxides represents a novel form of bacterial metabolism; this project uncovers aspects of bacterial physiology and biochemistry heretofore unknown. Gaining insight into these mechanisms broadens the current state of knowledge concerning anaerobic (i.e. oxygen-free) respiration, knowledge which is currently limited to studies of only a few bacteria. Such versatility makes this bacterium a valuable model for studying the components and regulation of oxygen-free respiration. Thirdly, as this bacterium could make a significant contribution to the cycling of manganese and iron in aquatic sediments and flooded soils, knowledge of its mechanisms of metal reduction enhances our understanding of the roles of metal-reducing bacteria in the environment. Overall, the studies lead to new ways of thinking about oxygen-free respiration, as well as the environmental cycling of certain metals.