Bacterial Production of Iron Sulfides
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DENNIS A. BAZYLINSKI Department of Anaerobic Microbiology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
ABSTRACT Iron sulfide production by bacteria can be classified as extracellular or intracellular. Extracellular iron sulfide production is mediated by anaerobic, dissimilatory sulfate-reducing bacteria which produce sulfide as a product of their respiration. Released sulfide reacts with iron (and other metals) in the extracellular environment producing a variety of iron sulfide minerals including "amorphous iron sulfide", mackinawite, greigite, pyrrhotite, marcasite, and pyrite. The type of minerals formed is dependent upon pH, Eh, and other physical and chemical factors. Extracellular production of these minerals are examples of biologically-induced mineralization in which mineral formation occurs from chemical and/or physical changes in the surrounding environment by the organism. Intracellular iron sulfides exist in bacteria in several forms including: iron-sulfur centers in iron-sulfur proteins, amorphous iron sulfide deposits, and crystalline iron sulfides. Iron-sulfur proteins are the most important class of electron transfer proteins and are found in all bacteria. Several different structural types of iron-sulfur centers are found in these proteins. Amorphous, intracellular iron sulfide "particles" have been reported in some dissimilatory sulfate-reducing bacteria when grown in high concentrations of iron. The "par~ticles" are not separable from lysed cells and have no known function. Crystalline iron sulfide particles have recently been reported in magnetotactic bacteria collected from sulfide-rich aquatic habitats. Ferrimagnetic greigite (Fe 3 S4 ), pyrrhotite (Fel-,S), and non-magnetic pyrite (FeS 2 ) have been identified as the mineral phases of their magnetosomes. These particles have narrow size distributions and often have well-defined morphologies, characteristics associated with biologically controlled mineralization. Greigite and pyrrhotite can function in magnetotaxis but the function of pyrite is unknown. The production of crystalline iron sulfides by bacteria have significant biogeochemical and palaeomagnetic implications.
INTRODUCTION Bacteria mediate the production of both extracellular and intracellular iron sulfides. These compounds exist as a number of different forms. Some have well-recognized functions while the production of others appears to be fortuitous. The purpose of this paper is to describe the numerous types of iron sulfides produced by bacteria, the possible mechanisms for their formation, their functions in the overall physiology of the cell, and their biogeochemical implications.
EXTRACELLULA
IRON SULFIDES
Extracellular metal (including iron) sulfide formation has been most extensively studied in the sulfate-reducing bacteria. These microorganisms are strictly anaerobic and respire with sulfate as the terminal electron Mat. Res. Soc. Symp. Proc. Vol. 218. @1991 Materials Research Society
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acceptor reducing it to sulfide as the final pro
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