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By Design — How Pearls Get Their Luster

Evolution News
Photo credit: Keith Pomakis, CC BY-SA 2.5 , via Wikimedia Commons.

Pearls are born, not made. They have a mother: mother-of-pearl, or nacre, a shiny, gem-like substance reflecting a shimmering rainbow of colors. It is the envy of materials scientists. And the mother of mother-of-pearl is the lowly oyster that deposits nacre on tiny nuclei with such precision that scientists have marveled at the beautiful results for centuries.

We call our teeth “pearly whites” when they shine like pearls. Pearls are not always white, though; they exhibit subtle overtones of color, with green often bringing the highest price. Five Japanese scientists investigated the iridescence or luster of pearls, to help oyster farmers understand how the most desirable colors in the pearl industry are achieved, and to help bioengineers mimic their optical properties.

Pearls are gifts of nature and one of the most fascinating gems. Most gems such as gold, diamonds, sapphires, and rubies are mined from the ground, but pearls are produced by mollusk shells that live in seas or lakes. Among these gems, pearls are the only gems that humans can create from nature. The luster and color of pearls are known as structural colors, which originate from multiple reflections in a nanolayered structure of nacre (also called mother of pearl). Nacre is composed of aragonite crystal layers separated by conchiolin, which is a secreted by mollusk shells. The thickness of these layers determines the color of reflection from the nacre.  [Emphasis added.]

Building on the work of Brewster, Rayleigh, and Raman, the team first measured the reflection, transmission and scattering properties of pearls with high precision. They measured the thicknesses of the layers of aragonite and protein within pearls down to micrometer resolution. Then they designed a model that could visualize the color reflectivity of computer-generated spheres set to various thicknesses of aragonite. The results were “in good agreement with the measured spectra.” Their work is published in Nature’s open-access journal Scientific Reports.

Pearl Purpose

Do pearls have a function? The Natural History Museum says that “Pearls are made by marine oysters and freshwater mussels as a natural defence against an irritant such as a parasite entering their shell or damage to their fragile body.” This is interesting but does not explain why pearls have such amazing optical properties, or why they keep growing once the irritant is sufficiently covered. It would seem that a layer of mud would do the trick. 

Abiotic Pearls

Spherical gems can be made abiotically. Cave pearls are formed when calcite drips onto particles of sand under dripstone. The steady drip tends to coat them evenly, resulting sometimes in beautiful sets of near-spherical crystals. These cave pearls, however, though shiny when wet, lack the iridescence or luster of mollusk pearls. They look more like marbles, reflecting light only at the surface. Oyster pearls reflect light from multiple layers within the pearl, creating the sparkling sheen that gemologists treasure. As a structural color, the iridescence accentuates certain wavelengths while interfering with others. Scattering within the pearl makes white predominate, but overtones of rose, green, or gold are often highly prized (see “How to Tell if a Pearl Is Real” on WikiHow). Color shades can include yellow, orange, pink, purple, blue, green, and black.

Cultured Pearls

One thing that made pearls so valuable to the ancients was the difficulty of diving to get them. To keep pearl oysters from going extinct, pearl farmers now “grow” pearls in oyster farms. Cultured pearls are still 100 percent real but less rare now. The Pearl Source Blog tells about a pearl found by a Filipino fisherman “measuring no less than 26 inches. The gigantic pearl is now valued at $100 million.” That oyster must have really been irritated!

Pearl Construction

The real wonder of pearls is found at the microscopic scale. An electron micrograph in the Ozaki et al. paper, Figure 1(c), shows some of the flat sheets of aragonite as if they had been laid down by a brick mason. Each layer is less than a few hundred millionths of a meter thick. “The thickness of these layers determines the color of reflection from the nacre,” they explain. The “mortar” between the layers is a protein called conchiolin. The aragonite layers are usually 30 to 50 times as thick as the protein layers.

Conchiolin is a protein secreted by the oyster. One species of conchiolin is made up of 21 amino acids (UniProt). This means that the instructions to make this protein, and the know-how to deposit it, are encoded in the nucleus. Conchiolin acts as a nucleating agent for crystallization of calcium carbonate, but the oyster must be controlling its volume, distribution and uniformity for a resulting pearl to be so finely constructed. A figure on Science Direct about conchiolin (Figure 3) shows the complex structure of the oyster’s mantle that secretes the material. Cave pearls, lacking such coding, resemble real pearls only as much as a statue resembles a person.

Biomineralization as Engineering

Pearls are masterpieces of biomineralization. Other instances of organisms using minerals to construct things are just as fascinating. As mentioned, teeth are another prime example. One instance not often considered are otoliths and statoconia. These are crystals in the inner ears of fish and mammals that help give a body balance. We have them in our ears, near the semicircular canals, in tiny sacks called saccules. These crystals are manufactured to critical specifications during development. They connect to sensors that can inform the brain of the body’s position relative to gravity and the amount of acceleration being experienced.

In PNAS, Chang and 9 other researchers in Germany investigated the process of building otoliths in sea urchins. They identified a protein member of the otopetrin family, called otop2l, that liberates protons generated by mineralization process. 

Using the sea urchin larva, we examined the otopetrin ortholog otop2l, which is exclusively expressed in the calcifying primary mesenchymal cells (PMCs) that generate the calcitic larval skeleton. otop2l expression is stimulated during skeletogenesis, and knockdown of otop2l impairs spicule formation. Intracellular pH measurements demonstrated Zn2+-sensitive H+ fluxes in PMCs that regulate intracellular pH in a Na+/HCO3-independent manner, while Otop2l knockdown reduced membrane proton permeability. Furthermore, Otop2l displays unique features, including strong activation by high extracellular pH (>8.0) and check-valve–like outwardly rectifying H+ flux properties, making it into a cellular proton extrusion machine adapted to oceanic living conditions. Our results provide evidence that otopetrin family proton channels are a central component of the cellular pH regulatory machinery in biomineralizing cells. Their ubiquitous occurrence in calcifying systems across the animal kingdom suggest a conserved physiological function by mediating pH at the site of mineralization.

A functional machine that didn’t evolve; that’s the gist of the quote. “Otopetrins are an evolutionary conserved family of proton channels found in animals ranging from basal metazoans to vertebrates and humans.” If it is evolutionary conserved, it is not evolutionary at all. Conservation is the opposite of evolution.

When building otoliths and statoconia, cells could have an acid reflux problem: “1.6 moles of protons are liberated per mole of CaCO3 precipitated, leading to a substantial cellular acid load,” the authors say. Thanks to the protein machine they found, there is an exit door for the excess protons so that the sea urchin larva does not die of intracellular acidosis. Knowing this is important for humans:

These results demonstrated that sea urchin otop2llike its vertebrate homologs, is an essential component of the cellular mineralization machinery. The deep phylogenetic origin of otopetrins and its frequent association with calcifying systems suggests an evolutionary conserved function in the biomineralization process with the underlying mechanisms being largely unknown.

Biomineralization is hard to evolve. When we find machinery at work under precision control, guided by coded instructions, yielding functional gems, we can be confident that Darwin offered nothing to help understand how it originated. Engineers get that.