ScienceWeek

GEOCHEMISTRY: ON BIOGENIC MINERALS

The following points are made by Danielle Fortin (Science 2004 303:1618):

1) Biogenic minerals are generally those formed in the presence of biological cells (mainly bacteria) and structures outside cells (1). These minerals, which come in a variety of types and shapes, are often small (on the order of nanometers) and occur in close association with the bacterial cell wall. Several studies (2-4) have shown such an association in natural samples taken from a wide range of environments, as well as in synthetic samples produced under laboratory conditions that mimic natural conditions. Some studies have also reported the formation of biogenic minerals inside microbial cells. Although the occurrence of biogenic minerals in natural environments is well documented, the exact formation mechanisms are still poorly understood. A clear understanding of these mechanisms is essential in order to assess how bacteria interact with metals in present and ancient environments. In addition, a clear demonstration that bacteria can template mineral crystallization is also crucial because it might lead to the development of new tools in the search for evidence of past life on Earth and other planets.

2) Many researchers accept that bacteria can trigger mineral formation under saturation conditions through active reactions (from physiological and metabolic activity) and passive reactions (from surface reactivity of the cell wall or extracellular structures such as exopolymers) (1), but the reasons why bacteria favor or promote mineral nucleation are still unclear. One general explanation is that bacteria do so to prevent cell entombment and death by mineral metabolic by-products. Even though the survival of microbial cells is a logical explanation, an alternative is reported by Chan et al (5). These authors propose that neutrophilic iron-oxidizing bacteria promote the formation of elongated iron oxide minerals (identified as akaganeite) onto extracellular polymers (polysaccharides) in order to enhance metabolic energy generation.

3) Chan et al (5) analyzed natural biominerals in an iron oxide-encrusted biofilm collected in a flooded mine. With the help of high-resolution synchrotron spectromicroscopy [x-ray photoemission electron microscopy (X-PEEM) and x-ray absorption near-edge structure (XANES)] and high-resolution transmission electron microscopy (HRTEM), they were able to show that microbially produced polysaccharides can template the nucleation of pseudo-single crystals (that is, having the appearance of single crystal structure) of akaganeite (with aspect ratios of about 1000:1). Unlike previous electron microscopy studies that showed bacteria-mineral associations and concluded that bacteria were likely involved in mineral formation (2,3), the study by Chan et al (5) clearly shows the presence of a microbially derived organic-rich template for iron oxide formation. The clever use of carbon K-edge XANES analysis indicates a strong similarity between the spectra of synthetic acidic polysaccharides and the natural mineralized filaments.

References (abridged):

1. D. Fortin et al., Rev. Mineral. 35, 161 (1997)

2. D. Fortin et al., Am. Mineral. 83, 1399 (1998)

3. F. G. Ferris et al., Nature 320, 609 (1986)

4. J. F. Banfield, S. A. Welch, H. Zhang, T. T. Ebert, R. L. Penn, Science 289, 751 (2000)

5. C. S. Chan et al., Science 303, 1656 (2004)

Science http://www.sciencemag.org

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ON MINERALIZATION IN BIOLOGICAL SYSTEMS

The following points are made by W.L. Murphy and D.J. Mooney (J. Am. Chem. Soc. 2002 124:1910):

1) Mineralization in biological systems is an elegant and structurally complex process involving ionic, stereochemical, and structural interactions at the biomacromolecule-mineral interface.(1,2) A wide variety of organisms utilize diverse schemes to grow biominerals with functions ranging from magnetic sensing(3) (magnetite in magnetotactic bacteria) to structural support(4) (dahllite in vertebrate skeletons). The paradigm linking each biomineralization strategy is a supreme level of control over the physical chemistry of mineral growth.

2) Mineralization during vertebrate bone growth is a classic example in which hydrophobic collagen fibrils are organized into parallel sheets with periodically staggered "hole zones". These spaces are rich in phospho- and glycoproteins, creating a local charge accumulation.(5) The anionic nature of the hole zone, along with structural and stereochemical interactions, are thought to lead to attraction of calcium-rich mineral nuclei and initiation of mineral growth. A similar mechanism drives mollusk shell development, with hydrophobic beta-chitin providing a substrate for deposition of acidic, anionic proteins that drive aragonite nucleation. In each case, an organic, hydrophobic material acts as a framework for deposition of an anionic, hydrophilic mineral nucleator that, in turn, drives mineral nucleation. Although the understanding of complex biological mineralization systems is incomplete and continues to grow, the fundamental mechanisms outlined above can be mimicked in synthetic systems to direct ex vivo biomineralization.

3) The highly controlled morphology, physical properties, and nanostructure of biological minerals have led to the development of biomimetic systems for controlled mineral formation ex vivo. The development of ex vivo systems is motivated both by the desire to more completely understand biomineralization processes and by the potential utility of biominerals in industrial and biomedical applications. Prevalent strategies involve pragmatic presentation of polar functional groups that both increase local ion accumulation via electrostatic effects(1) and decrease the energy at the organic substrate-mineral nucleus interface.(1) In each case, the basic, biomimetic premise is that functional groups present in large quantities at the mineralization front in biological systems are capable of inducing mineral nucleation if presented in the appropriate fashion. Select model systems have been extended to biomedical applications, such as hydroxyapatite mineral growth on functionalized titanium and bioactive, ion-exchange glasses. Carbonated hydroxyapatite is the major mineral component in human bone extracellular matrix, and bonelike mineral coatings appear to have pronounced effects on proper bone tissue development. More specifically, a bonelike mineral has been shown to be a prerequisite to bonding of orthopedic implant materials to native bone tissue (osteoconductivity), and may drive osteogenic differentiation of adult human stem cells (osteoinductivity). Despite the potential benefits of biominerals in regenerative medicine, few studies to date have been aimed at mineral formation on degradable biomaterials for use in orthopedic tissue regeneration.

References (abridged):

1. Mann, S.; Archibald, D. D.; Didymus, J. M.; et al. Science 1993, 261, 1286

2. Sarikaya, M. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 14183

3. Chasteen, N. D.; Harrison, P. M. J. Struct. Biol. 1999, 126, 182

4. Weiner, S.; Traub, W.; Wagner, H. D. J. Struct. Biol. 1999, 126, 241

5. Lee, S.; Veis, A. J. Peptide Protein Res. 1980, 16, 231

J. Am. Chem. Soc. http://pubs.acs.org/JACS

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A COPPER BIOMINERAL IN WORM JAWS

The following points are made by H.C. Lichtenegger et al (Science 2002 298:389):

1) Most living organisms rely on hard tissues for support, protection, nutrition, and defense. Biomineralization is a major strategy for tissue hardening and manifests an astonishing diversity of bioceramic structures with exquisite microarchitectures that have specially adapted physical properties (1-5). Although the variety of architectures seems to be almost infinite, Ca-, Si-, and Fe-based minerals are most common. As a basic principle, the hardness of these is largely governed by the type of mineral and the degree of mineralization.

2) In 1980, Gibbs and Bryan first reported copper levels up to 13% w/w in the jaws of the marine polychaete worm Glycera sp. Although it was initially suspected that these levels reflected heavy metal pollution at the collection site, jaw composition was found to be remarkably consistent and independent of copper concentration in the seawater. Copper was the most abundant inorganic component; protein, however, was the most predominant constituent (measured as percent of dry weight). The authors concluded that the copper might play a role in mechanically hardening the proteinaceous material. However, no attempt to determine the actual hardness was made nor was the form of the copper further explored.

3) In summary: Biominerals are widely exploited to harden or stiffen tissues in living organisms, with calcium-, silicon-, and iron-based minerals being most common. In notable contrast, the jaws of the marine bloodworm Glycera dibranchiata contain the copper-based biomineral atacamite [Cu2(OH)3Cl]. Polycrystalline fibers are oriented with the outer contour of the jaw. Using nanoindentation, the authors demonstrate that the mineral has a structural role and enhances hardness and stiffness. Despite the low degree of mineralization, bloodworm jaws exhibit an extraordinary resistance to abrasion, significantly exceeding that of vertebrate dentin and approaching that of tooth enamel.

References (abridged):

1. J. Aizenberg, A. Tkachenko, S. Weiner, L. Addadi, G. Hendler, Nature 412, 819 (2001)

2. L. Addadi and S. Weiner, Nature 411, 753 (2001)

3. S. Kamat, X. Su, R. Ballarini, A. H. Heuer, Nature 405, 1036 (2000)

4. S. Mann, Nature 365, 499 (1993)

5. J. N. Cha, et al., Proc. Natl. Acad. Sci. U.S.. 96, 361 (1999)

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