Iron metabolism in anoxic environments at near neutral pH

Anaerobic dissimilatory ferric iron-reducing and ferrous iron-oxidizing bac teria gain energy through reduction or oxidation of iron minerals and presu mably play an important role in catalyzing iron transformations in anoxic e nvironments. Numerous ferric iron-reducing bacteria have been isolated from a great diversity of anoxic environments, including sediments, soils, deep terrestrial subsurfaces. and hot springs. In contrast, only Few ferrous ir on-oxidizing bacteria are known so far. At neutral pH, iron minerals are ba rely soluble, and the mechanisms of electron transfer to or from iron miner als are still only poorly understood. In natural habitats, humic substances may act as electron carriers for ferric iron-reducing bacteria. Also ferme nting bacteria were shown to channel electrons to ferric iron via humic aci ds. Whether quinones or cytochromes released from cells act as electron tra nsfer components in ferric iron reduction is still a matter of debate. Anae robic ferrous iron-oxidizing phototrophic bacteria, on the other hand, appe ar to excrete complexing agents to prevent precipitation of ferric iron oxi des at their cell surfaces. The present review evaluates recent findings on the physiology of ferric iron-reducing and ferrous iron-oxidizing bacteria with respect to their relevance to microbial iron transformations in natur e. (C) 2001 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.