2.3. Amorphous Minerals 4354 3. Biological Routes to Controlling Morphology 4354 3.1. General Mechanisms 4356 3.1.1. Soluble and Insoluble Organic Molecules 4356 3.1.2. Control over Crystal Polymorph 4357 3.1.3. Control over Crystal Orientation 4359 3.2. Single-Crystal Biominerals 4359 3.2.1. Organic and Inorganic Soluble Additives 4360 3.2.2. Templating of Single-Crystal Morphologies 4361 3.3. Polycrystalline Biominerals 4366 3.3.1. Nacre Formation in Mollusks 4366 3.3.2. ForaminiferasA Biogenic Mesocrystal 4368 3.4. Amorphous Biominerals 4370 3.4.1. Silicification in Diatoms 4370 3.4.2. In Vitro Studies of Silicification in Diatoms 4371 4. Bioinspired Routes to Controlling Crystal Morphologies 4371

Controlling Mineral Morphologies and Structures in Biological and Synthetic Systems

Molecular recognition is a term used widely in many branches of chemistry and biochemistry. It encompasses a multitude of molecular systems ranging from the small molecule-small molecule interactions of macrocyclic ligands with metal cations, through macromolecular processes involving enyzme-substrate reactions, protein-protein coupling and antibody-antigen specificity to supramolecular events involving cell-cell communication. In each case, the specificity of interaction is determined by complementarity in size, charge and molecular shape, often generated by the interplay of structural, stereochemical and dynamical relationships. The fact that these processes are intrinsic to biochemical reactions in solution and within membrane assemblies suggests that all functional systems evolved from chemical recognition.

Molecular recognition in biomineralization

Coral reefs constitute real oasis sheltering for about one third of the identified fishes, representing a major advantage for the economy and tourism of many tropical countries. However it is paradoxical to notice that their formation at the cellular level or even at the scale of the organism is still poorly known. Effectively, biomineralisation, the process that is at the basis of their edification, is always the subject of numerous researches. Two combined mechanisms lead to the formation of a biomineral, the synthesis/secretion of macromolecules referred to as ‘organic matrix’, and the transport of ions (calcium, bicarbonates and protons in the case of calcification) to the mineralising site. This review shows a view of the works carried out on biomineralisation in scleractinian corals, including some aspects on the control of calcification by environmental parameters. It also gives insights into the biological basis of the use of coral skeletons as environmental archives in palaeo-oceanography. To cite this

http://mgg.rsmas.miami.edu/groups/bio/allemand et al 2004.pdf

General Palaeontology (Palaeobiochemistry) Biomineralisation in reef-building corals: from molecular mechanisms to environmental control

In the freshwater areas of the Everglades, Florida, U.S.A., carbonate is precipitated in dense cyanobacterial mats. Precipitation is linked with photosynthesis in the mats in a quantitative relationship. On ground of field observations and experiments a model for precipitation in the filamentous cyanobacteriaScytonema is proposed, which links precipitation to bicarbonate use in photosynthesis and subsequent release of OH− ions. Besides supersaturation of the water with respect to carbonate and photosynthetic bicarbonate use, precipitation requires a suitable sheath structure and composition. The characteristics of the sheath seem to be responsible for a distinct crystal morphology in the two generaScytonema andSchizothrix, as well as for the restriction of calcification to the outer sheath inScytonema. In the immediate vicinity of the trichom precipitation seems to be inhibited. Comparison of this form of calcifying cyanobacteria with calcification in calcareous algae shows many similarities and rises the question of the biological significance of calcification or precipitation. The precipitated carbonate shows equilibrium precipitation in its δ oxygen values, while it is enriched in13C relative to the ambient water. This agrees with a model of precipitation in which the carbonate derives from the water immediately surrounding the filament. There the water is depleted in12C which is preferably taken up for photosynthesis. No respiratory carbon is involved in precipitation. From measurements of the amount of precipitation in the field and in experiments the annual sedimentation rate is estimated to be 0.024 to 0.24 mm. These values fall within the range of laminae thicknesses in fossil algal laminites.

/pdf/the-biology-of-carbonate-precipitation-by-cyanobacteria-510ojs06z5.pdf

The biology of carbonate precipitation by cyanobacteria

http://mgg.rsmas.miami.edu/groups/bio/Allemand%20et%20al%202004.pdf

Biomineralisation in reef-building corals: from molecular mechanisms to environmental control

The organic matrix of calcium carbonate structures can be involved in the formation of the biomineral by regulating various stages of crystal growth. The preponderance of evidence suggests that matrix can act as an initiator of crystal growth at the mineralizing front. The mechanism for this initiation is not known. However, the matrix may provide a template for the crystal lattice, or more likely, a surface that can simply stabilize the critical nucleus of calcium and carbonate. The degree of specialization of the nucleation sites is uncertain; however, they are hypothesized to be a class of soluble, acidic, calcium-binding polymers which are present in matrix extracted from biomineral. Recent studies have revealed that similar soluble components of extracted matrix have the capability of inhibiting nucleation and crystal growth in vitro and biomineralization in organisms exposed to the extracts. Further, crystals grown in vitro in the presence of extracted matrix have a morphology distinct from those of crystals grown without matrix. These findings generally support the hypotheses generated either a priori or from morphological observations which suggest that matrix must limit crystal growth and may well influence structure of biomineral. Hypotheses which explain a dual function of the soluble matrix have these components periodically released to the mineralization front where they can associate with more insoluble forms of matrix and sequentially initiate or inhibit growth. There also exists evidence to suggest that soluble regulatory components are intra-crystalline in orientation and thus may be released continuously

C. Steven Sikes

Papers

Regulation of Carbonate Calcification by Organic Matrix