Modes of Biomineralization and Biomimetics

Two fundamentally different modes of biomineralization are summarized by Lowenstam and Weiner [1989]. One is called biologically induced mineralization (BIM), in which an organism modifies its local microenvironment creating conditions suitable for the chemical precipitation of extracellular mineral phases. The second mode is called boundary organized biomineralization (BOB), in which inorganic particles are grown within or on some organic matrix produced by the organism [ Mann et al., 1989].

Bacteria that produce mineral phases by BIM do not strictly control the crystallization process, resulting in particles with no unique morphology and a broad particle size distribution. Non-magnetotactic dissimilatory iron-reducing and sulfate-reducing bacteria produce magnetite, siderite, vivianite, and iron-sulfides by BIM processes [ Lovley, 1990; Bazylinski and Frankel, 1992]. For example, the iron-reducing bacterium Geobacter metallireducens (formerly GS-15) is a non-magnetotactic anaerobe that couples the oxidation of organic matter to the reduction of ferric iron, inducing the extracellular precipitation of fine grained magnetite as a byproduct [ Lovley, 1990]. In laboratory culture, GS-15 can produce 5000 times more magnetite by weight than an equivalent biomass of magnetotactic bacteria. Nevertheless, magnetic measurements show that most of the particles GS-15 produces are within the magnetically unstable, superparamagnetic (SPM) size range for magnetite (<20 nm) at room temperature [ Moskowitz et al., 1993].

In contrast to BIM, bacteria that produce mineral phases by a BOB process exert strict control over size, morphology, composition, position, and crystallographic orientation of the particles [ Mann et al., 1990b; Frankel and Mann, 1994]. The archetypical example of microorganisms using BOB processes to produce iron biominerals are magnetotactic bacteria. These bacteria synthesize intracellular, membrane-bounded FeO, FeS (possible FeS), and FeS particles called magnetosomes. Various arrangements of magnetosomes within cells impart a permanent magnetic dipole moment to the cell, which effectively makes each cell a self-propelled biomagnetic compass. The study of the biomineralization of magnetite magnetosomes has been aided by the isolation and axenic culture of several different magnetotactic bacteria [ Bazylinski, 1990; Meldrum et al., 1993a,b; Schleiffer et al., 1991; Sakaguchi et al., 1993]. Unfortunately, iron sulfide MTB have yet to be isolated and grown in pure culture.

Much of the current research in biomineralization is directed towards identifying, mimicking, or duplicating BOB-type processes in order to produce tailor-made inorganic materials [ Mann, 1993]. In several species of MTB, the magnetite particles are enveloped in a membrane structure that anchors the mineral particles at particular locations in the cell and provides an enclosed microenvironment for precise biological control of magnetosome size and morphology [ Mann et al., 1990; Frankel and Mann, 1994]. The most common magnetosome arrangement is one or more linear chains traversing the long axis of the cell [ Mann, 1993; Frankel and Bazylinski, 1994]. How the bacteria accomplish this is not presently understood, but the bioarchitectural framework of assemblies of aligned magnetic particles in MTB clearly has artificial counterparts in the manufacture of permanent magnets [ Frankel and Bazylinski, 1994]. Biomimetics is a new interdisciplinary field that seeks to understand relationships between structures and functions of biological composites in order to design and synthesize new materials, perhaps without the toxic residues characteristic of non-biological modes of industrial mass production [ Sarikaya, 1994; Mann, 1993]. This research may lead to the synthesis of novel magnetic, electronic, or magnetopharmaceutical materials on a nanometer scale.