DIVERSITY OF BACTERIAL IRON MINERALIZATION

Bacterial cells, growing naturally in freshwater and marine environmen ts or experimentally in culture, can precipitate a variety of authigen ic iron minerals. With the vast majority of bacteria biomineralization is a two-step process: initially metals are electrostatically bound t o the anionic surfaces of the cell wall and surrounding organic polyme rs, where they subsequently serve as nucleation sites for crystal grow th. The biogenic minerals have crystal habits and chemical composition s similar to those produced by precipitation from inorganic solutions because they are governed by the same equilibrium principles that cont rol mineralization of their inorganic counterparts. As the latter stag es of mineralization are inorganically driven, the type of biomineral formed is inevitably dependent on the available counter-ions, and henc e, the chemical composition of the waters in which the microorganisms are growing. In oxygenated waters, iron hydroxides are a common precip itate and can form passively through the binding of dissolved ferric s pecies to negatively charged polymers or when soluble ferrous iron spo ntaneously reacts with dissolved oxygen to precipitate as ferric hydro xide on available nucleation sites (e.g. bacteria). Alternatively, the metabolic activity of Fe(II)-oxidizing bacteria can induce ferric hyd roxide precipitation as a secondary by-product. Ferric hydroxide may t hen serve as a precursor for more stable iron oxides, such as goethite and hematite via dissolution-reprecipitation or dehydration, respecti vely, or it may react with dissolved silica, phosphate or sulphate to form other authigenic mineral phases. Under suboxic to anoxic conditio ns, ferric hydroxide may be converted to magnetite, siderite, and iron sulphides through various reductive processes associated with organic matter mineralization. Under biologically controlled conditions, wher e mineralization is completely regulated, magnetotactic bacteria form magnetite and greigite as navigational tools to guide themselves into their preferred habitat. In general, the formation of iron biominerals is not difficult to achieve, bacteria simply provide charged surfaces that bind metals and they excrete metabolic waste products into the s urrounding environment that induce mineralization. The ubiquitous pres ence of bacteria in aquatic systems and their inherent ability to biom ineralize, therefore, makes them extremely important agents in driving both modern and ancient geochemical cycles. (C) 1998 Elsevier Science B.V. All rights reserved.