A phosphoporin homolog plays a critical role in the dissimilatory iron respirattion linked to iron (III) reduction by a cold adapted bacterium, Shewanella livingstonensis Ac10
Microbial metal reduction is a key reaction for the bioconversion of soluble and insoluble minerals. Respirationlinked bioreduction of metal compounds is useful for bacterial leaching and bacterial mining, and is also gaining attention as an efficient method for the conversion of toxic metal contaminants to non toxic compounds1) Recent studies on the metal reducing microorganisms revealed that several bacteria are capable of dissimilatory metal reduction and respiratory growth with various metals such as iron, copper, manganese, cobalt, chromium, uranium, selenium, and technetium as the terminal electron acceptor. Two such microorganisms that have received much attention are the metal reducing bacteria, Shewanella oneidensis2) and Geobacter sulfurreducens3) Both these strains are classified as mesophilic bacteria, for which the optimum growth temperatures are reported to be around 30degC. Shewanella species have evolved mechanisms for utilizing insoluble metals as electron acceptors, and can survive in various aerobic and anaerobic habitats, such as deep sea, intestinal tract of fish, freshwater, and coastal sediments. Regarding anaerobic metal reduction by S. oneidensis MR 1, a well studied model for bacterial metal respiration, three possible mechanisms are suggested: (i) direct electron transfer following contact between minerals and multihaem c type cytochromes on the cell surface4 6) or extracellular nanowires7) (ii) delivery of the reducing equivalents from the cytoplasmic membrane to the extracellular matrix by the electron shuttles such as flavin8, 9) (iii) solubilization of metal oxides with metal chelators10, 11) such as menaquinone so that they can contact with the membrane associated electron transport components. Although these electron transport mechanisms are supposed to contribute to the survivability of Shewanella species at various anaerobic environments, the exact mechanism for respiration linked metal reduction at cold environments widely distributed across the Earth is not well understood.