Fe(II)-Oxidizing Prokaryotes
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FE(II)-OXIDIZING PROKARYOTES Kristina L. Straub University of Vienna, Vienna, Austria
Synonyms Fe-oxidizers; Fe(II)-oxidizing prokaryotes/microorganisms; Ferrous iron-oxidizing prokaryotes/microorganisms; Iron-oxidizers; Iron-oxidizing prokaryotes/ microorganisms.; Iron(II)-oxidizing prokaryotes/ microorganisms Definition Fe(II)-oxidizing prokaryotes. Diverse species of the prokaryotic domains Bacteria and Archaea have the ability to oxidize Fe(II), ferrous iron, to Fe(III), ferric iron. The electrons obtained from the oxidation of Fe(II) are utilized for energy generation in aerobic or anaerobic respiration and/or for assimilative reduction reactions. Introduction Aerobic neutrophilic Fe(II)-oxidizing bacteria were among the first environmentally relevant prokaryotes that were discovered and studied in the nineteenth century. At that time, eye-catching ochre deposits in ponds and slowly running waters had attracted the attention of microbiologists. Microscopic analyses of such ochre deposits revealed the prevalence of twisted stalks and/or sheaths that were covered with iron minerals. Bacteria were suggested to be responsible for the production of these specific iron-covered structures, and subsequently the first aerobic Fe(II)-oxidizing prokaryotes were described: Gallionella ferruginea and Leptothrix ochracea (reviewed by Emerson, 2000; Canfield et al., 2005). In contrast, the anaerobic oxidation of Fe(II) by phototrophic bacteria (Widdel et al., 1993), nitrate-reducing bacteria
(Straub et al., 1996), and archaea (Hafenbradl et al., 1996) was not discovered until the end of the twentieth century. Aerobic and anaerobic Fe(II)-oxidizing prokaryotes play decisive roles in the biogeochemical cycle of iron. In comparison to other physiological groups, only few species of Fe(II)-oxidizing prokaryotes are available in pure culture. The capability to oxidize Fe(II) is apparently phylogenetically widespread because the known species belong to diverse phyla of the Bacteria and the Archaea. The mechanism(s) of prokaryotic Fe(II)oxidation are largely unknown (Canfield et al., 2005).
Geochemical aspects of Fe(II) Iron is the fourth most abundant element in the Earth’s crust, and depending upon the geochemical conditions, it occurs either in soluble forms or in a variety of minerals (Cornell and Schwertmann, 2003). In natural habitats, Fe(III) minerals are reduced to Fe(II) mainly by the metabolic activities of prokaryotes. The concentration of dissolved Fe(II) is controlled by precipitation-dissolution reactions and by adsorption processes: Fe(II) precipitates with carbonates, phosphates, or sulfides and tends to adsorb to soil particles and mineral surfaces (Cornell and Schwertmann, 2003; Canfield et al., 2005). Chemical reoxidation reactions of Fe(II) depend on the pH and on the presence of appropriate oxidants. The reaction rate of Fe(II) with molecular oxygen is very low at acidic pH values, and hence Fe(II) is fairly stable in the presence of oxygen under acidic conditions. With increasing pH, the reaction rate of Fe
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