Diversity of bacterial iron mineralization

Abstract

Bacterial cells, growing naturally in freshwater and marine environments or experimentally in culture, can precipitate a variety of authigenic iron minerals. With the vast majority of bacteria biomineralization is a two-step process: initially metals are electrostatically bound to the anionic surfaces of the cell wall and surrounding organic polymers, where they subsequently serve as nucleation sites for crystal growth. The biogenic minerals have crystal habits and chemical compositions similar to those produced by precipitation from inorganic solutions because they are governed by the same equilibrium principles that control mineralization of their inorganic counterparts. As the latter stages of mineralization are inorganically driven, the type of biomineral formed is inevitably dependent on the available counter-ions, and hence, the chemical composition of the waters in which the microorganisms are growing. In oxygenated waters, iron hydroxides are a common precipitate and can form passively through the binding of dissolved ferric species to negatively charged polymers or when soluble ferrous iron spontaneously reacts with dissolved oxygen to precipitate as ferric hydroxide on available nucleation sites (e.g. bacteria). Alternatively, the metabolic activity of Fe(II)-oxidizing bacteria can induce ferric hydroxide precipitation as a secondary by-product. Ferric hydroxide may then serve as a precursor for more stable iron oxides, such as goethite and hematite via dissolution–reprecipitation or dehydration, respectively, or it may react with dissolved silica, phosphate or sulphate to form other authigenic mineral phases. Under suboxic to anoxic conditions, ferric hydroxide may be converted to magnetite, siderite, and iron sulphides through various reductive processes associated with organic matter mineralization. Under biologically controlled conditions, where 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 surrounding environment that induce mineralization. The ubiquitous presence of bacteria in aquatic systems and their inherent ability to biomineralize, therefore, makes them extremely important agents in driving both modern and ancient geochemical cycles.