Biomineralization

Biomineralization is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; all six taxonomic kingdoms contain members that are able to form minerals, and over 60 different minerals have been identified in organisms. Examples include silicates in algae and diatoms, carbonates in invertebrates, and calcium phosphates and carbonates in vertebrates. These minerals often form structural features such as sea shells and the bone in mammals and birds. Organisms have been producing mineralised skeletons for the past 550 million years. Ca carbonates and Ca phosphates are usually crystalline, but silica organisms (sponges, diatoms...) are always non crystalline minerals. Other examples include copper, iron and gold deposits involving bacteria. Biologically-formed minerals often have special uses such as magnetic sensors in magnetotactic bacteria (Fe3O4), gravity sensing devices (CaCO3, CaSO4, BaSO4) and iron storage and mobilization (Fe2O3•H2O in the protein ferritin). Biomineralization is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; all six taxonomic kingdoms contain members that are able to form minerals, and over 60 different minerals have been identified in organisms. Examples include silicates in algae and diatoms, carbonates in invertebrates, and calcium phosphates and carbonates in vertebrates. These minerals often form structural features such as sea shells and the bone in mammals and birds. Organisms have been producing mineralised skeletons for the past 550 million years. Ca carbonates and Ca phosphates are usually crystalline, but silica organisms (sponges, diatoms...) are always non crystalline minerals. Other examples include copper, iron and gold deposits involving bacteria. Biologically-formed minerals often have special uses such as magnetic sensors in magnetotactic bacteria (Fe3O4), gravity sensing devices (CaCO3, CaSO4, BaSO4) and iron storage and mobilization (Fe2O3•H2O in the protein ferritin). In terms of taxonomic distribution, the most common biominerals are the phosphate and carbonate salts of calcium that are used in conjunction with organic polymers such as collagen and chitin to give structural support to bones and shells. The structures of these biocomposite materials are highly controlled from the nanometer to the macroscopic level, resulting in complex architectures that provide multifunctional properties. Because this range of control over mineral growth is desirable for materials engineering applications, there is significant interest in understanding and elucidating the mechanisms of biologically controlled biomineralization. Among metazoans, biominerals composed of calcium carbonate, calcium phosphate or silica perform a variety of roles such as support, defense and feeding. It is less clear what purpose biominerals serve in bacteria. One hypothesis is that cells create them to avoid entombment by their own metabolic byproducts. Iron oxide particles may also enhance their metabolism. If present on a super-cellular scale, biominerals are usually deposited by a dedicated organ, which is often defined very early in the embryological development. This organ will contain an organic matrix that facilitates and directs the deposition of crystals. The matrix may be collagen, as in deuterostomes, or based on chitin or other polysaccharides, as in molluscs. The mollusc shell is a biogenic composite material that has been the subject of much interest in materials science because of its unusual properties and its model character for biomineralization. Molluscan shells consist of 95–99% calcium carbonate by weight, while an organic component makes up the remaining 1–5%. The resulting composite has a fracture toughness ≈3000 times greater than that of the crystals themselves. In the biomineralization of the mollusc shell, specialized proteins are responsible for directing crystal nucleation, phase, morphology, and growths dynamics and ultimately give the shell its remarkable mechanical strength. The application of biomimetic principles elucidated from mollusc shell assembly and structure may help in fabricating new composite materials with enhanced optical, electronic, or structural properties.The most described arrangement in mollusc shells is the nacre - prismatic shells, known in large shells as Pinna or the pearl oyster (Pinctada). Not only the structure of the layers differ, but their mineralogy and chemical composition also differ. Both contain organic components (proteins, sugars and lipids) and the organic components are characteristic of the layer, and of the species. The structures and arrangements of mollusc shells are diverse, but they share some features: the main part of the shell is a crystalline Ca carbonate (aragonite, calcite), despite some amorphous Ca carbonate occurs; and despite they react as crystals, they never show angles and facets. The examination of the inner structure of the prismatic units, nacreous tablets, foliated laths... shows irregular rounded granules. Fungi are a diverse group of organisms that belong to the eukaryotic domain. Studies of their significant roles in geological processes, 'geomycology', has shown that fungi are involved with biomineralization, biodegradation, and metal-fungal interactions. In studying fungi's roles in biomineralization, it has been found that fungi deposit minerals with the help of an organic matrix, such as a protein, that provides a nucleation site for the growth of biominerals. Fungal growth may produce a copper-containing mineral precipitate, such as copper carbonate produced from a mixture of (NH4)2CO3 and CuCl2. The production of the copper carbonate is produced in the presence of proteins made and secreted by the fungi. These fungal proteins that are found extracellularly aid in the size and morphology of the carbonate minerals precipitated by the fungi. In addition to precipitating carbonate minerals, fungi can also precipitate uranium-containing phosphate biominerals in the presence of organic phosphorus that acts a substrate for the process. The fungi produce a hyphal matrix, also known as mycelium, that localizes and accumulates the uranium minerals that have been precipitated. Although uranium is often deemed as toxic towards living organisms, certain fungi such as Aspergillus niger and Paecilomyces javanicus can tolerate it. Though minerals can be produced by fungi, they can also be degraded; mainly by oxalic-acid producing strains of fungi. Oxalic acid production is increased in the presence of glucose for three organic acid producing fungi – Aspergillus niger, Serpula himantioides, and Trametes versicolor. These fungi have been found to corrode apatite and galena minerals. Degradation of minerals by fungi is carried out through a process known as neogenesis. The order of most to least oxalic acid secreted by the fungi studied are Aspergillus niger, followed by Serpula himantioides, and finally Trametes versicolor. These capabilities of certain groups of fungi have a major impact on corrosion, a costly problem for many industries and the economy.