Biomineralization

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The shells of living beings, like the shell of this nautilus, are self-assembled constructions of microscopic crystals. Unlike mineral crystals with their characteristic straight lines, angular shapes and flat faces, life has been able to build extraordinary pieces of architecture with continuous curvatures. These are biomineral structures, fascinating both for their beauty and for the different functions these organisms carry out: lenses for seeing, teeth for chewing, skeletons for self-protection, sensors to navigate…

Do you know what minerals are most used by living organisms? Have you ever asked yourself how living things make their mineral structures? Did you know that engineers try to imitate life in order to make new materials?

You undoubtedly know what minerals are. You have heard of sapphires and emeralds, of diamonds and rubies. And of silver and gold. And perhaps too of pyrites, quartz, and feldspar. These are all minerals that Mother Nature makes gifts of, formed out of molten magma, hydrothermal vapours in the depths or in the waters of lakes and oceans.

But you might be surprised to know that another of the driving forces generating minerals is life itself. Living organisms from the five kingdoms are capable of making minerals, 80% of which are crystalline, the rest amorphous. Which minerals? Well, the carbonates – the most abundant biominerals – such as mollusc shells, pearls and coral; the phosphates, such as the skeletons of mammals and birds; silica, such as in the beautiful diatoms or in sponges; iron oxides like some bacteria; but also halides, sulphates, sulphurs, citrates, oxalates, and so on up to sixty different mineral species. All of them are referred to as biominerals and the process of creating them is called biomineralization.

 

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In many cases biomineral structures serve as protection, such as with the shells of molluscs, or as defence, such as with the spines of sea urchins, or as support, for example internal and external skeletons. Other common uses that living organisms give to crystals are: making hard tools, such as teeth, or sensors such as the “compasses” of magnetite of some microorganisms, or the calcite otoliths of vertebrates that enable us to keep our balance.

The most peculiar characteristic of biomineral structures is that they do not keep the external symmetry that is normal for crystal forms, in which the straight line dominates. The morphology of biomineral structures is the curve, like those of the splendid shell of the Nautilus that illustrates this poster, or those of the abalone shell or those of the olive; like the quiet beauty of the of skeletons of microorganisms that Ernst Haeckel illustrated one hundred years ago.

 

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Biominerals break crystallographic symmetry because their shells are made not by one but by thousands of small crystallites, whose nucleation, growth and orientation are perfectly controlled by the organism.

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On the right, structure of the nacre of the abalone shell that is on the left. This structure slatted in sheets of aragonite is not only responsible for the beautiful mother-of-pearl colours but also the shell’s great resistance to fracture.

How was such control managed? By making use of proteins, carbohydrates or lipids that act as inductors and controllers of the nucleation and orientation of the crystals, and even of the mineral phase. But we know little more about how they manage to do it with such incredible precision. And we’d like to know how, because biomineral structures abound with mechanical properties thousands of times better than the same minerals manufactured inorganically. And organisms create them far more economically in energy and more cleanly. The studies carried out by researchers who dream of discovering how life is capable of creating such fascinating structures as the Nautilus and of imitating them in the laboratory and in factories are called Biomimetics.

 

Did you know that…

  • It has been proposed to utilise biomineralizing microorganisms in the restoration of monuments.
  • Nature creates nanomaterials for biomineralization. Scientists investigate how organisms produce nanocompasses that use bacteria and other processes that give rise to nanocrystals to produce extremely high-performing ceramic materials.
  • Pearls are the only gems made by living animals. The most ancient known use of pearls in jewellery was found in a sarcophagus of a Persian princess who died in 520 BCE. These days practically all the pearls used in jewellery are farmed.
  • The dissolving of atmospheric CO2 in seawater increases the pH of the oceans. The acidification of the sea by man-generated carbon dioxide has made the pH of the ocean lower by 0.1 units since the beginning of the industrial era and it is expected to reach 0.3 or 0.5 units in coming years. This could have a huge impact on coral reefs and on all living beings with structures mineralized from carbonate, by reducing saturation with respect to aragonite, which makes it more costly for organisms to produce their skeletons.
  • The teeth of the sea urchin are formed by oriented crystals of calcium carbonate, and had already attracted the attention of Aristotle in 343 BCE, when he wrote about them. His name has remained associated with the “jaw” of urchins, which we call “Aristotle’s Lantern”.

To find out more

  • You can learn more about Biomineralization on Wikipedia (here).
  • Wikipedia also has extensive information about seashells.

 

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