Calcium Carbonate: How Life Rocks
A revolution in materials science will begin as soon as engineers "make like an oyster." Calcium carbonate (CaCO3) is a very abundant mineral on the earth, but its consistency is often weak, like chalk, or brittle, like calcite. Chemical engineers could gain "pearls of wisdom" from a lowly oyster, says the Pacific Northwest National Laboratory. From "How Seashells Get Their Strength":
Seashells and lobster claws are hard to break, but chalk is soft enough to draw on sidewalks. Though all three are made of calcium carbonate crystals, the hard materials include clumps of soft biological matter that make them much stronger. A study today in Nature Communications reveals how soft clumps get into crystals and endow them with remarkable strength. [Emphasis added.]
The incorporation of protein with calcium carbonate makes the weak become strong. How can two flimsy materials pull this off? Here's how it works:
The strength of a material depends on how easy it is to disrupt its underlying crystal matrix. If a material is compressed, then it becomes harder to break the matrix apart. Proteins trapped in calcium carbonate crystals create a compressive force -- or strain -- within the crystal structure.
Unlike the strain that makes muscles sore, this compressive strain is helpful in materials, because it makes it harder to disrupt the underlying crystal structure, thereby adding strength. Scientists understand how forces, stress and strain combine to make strong materials, but they understand less about how to create the materials in the first place.
Life creates such materials with ease, but how they do it leaves engineers baffled. Now, PNNL scientists have a new theory that supplants the old one. "We've found a completely different mechanism," the lead scientist says. It took careful observing with Atomic Force Microscopy (AFM) to hit upon the secret. They seeded a strong solution of calcium carbonate with springy spheres of organic matter. As the solution crystallized into calcite, they watched as the organic balls clung to the edges of "steps" of terraced mineral as the material washed over it. The mineral compressed the organic matter but left a cavity, giving it some compressive strain. The process is explained in detail in their paper in Nature Communications (open access). The paper begins by saying that biology has significantly enhanced their understanding of how to make strong materials.
Soluble additives are widely used to control crystallization processes, providing an experimentally simple and versatile strategy to generate crystals with defined polymorphs, morphologies and sizes. Significant insights into additive-directed crystallization, and the effects of these additives on crystal properties, have come from in vitro studies of biomineralization processes, where the use of advanced characterization methods have demonstrated that the macromolecules active in controlling crystallization become occluded within the crystal structure, modifying crystal texture and lattice strain, causing local disorder and enhancing mechanical properties.
Those macromolecules are typically proteins -- long, folded chains of amino acids whose specific structures are encoded by genes. The scientists, however, used simple micelles (spheres of polar molecules) that only "mimic" some of the properties of the biomacromolecules (proteins) used in vivo. To make their doping agents, they dumped copolymer powder into water, stirred it, and adjusted the pH with sodium hydroxide to get the micelles to form. This is a far cry from the elegance of the proteins used in life. They called their resulting materials "artificial biominerals." The oysters' biomineral construction is much more complex and elaborate, employing molecular machines to put each molecule where it belongs.
Calcium Carbonate and You
Where in your own body is calcium carbonate used to create a strong material? Try bone. Try teeth. Additionally, your ears have tiny crystals of calcium carbonate called otoliths ("ear stones"). Located in the vestibule of the inner ear near the semicircular canals, these little rocks in your head help you keep your balance. Nerves connected to the otoliths sense every tiny inertial change in gravity, so that you know which way is up.
According to NIH Health Information, about 99 percent of the calcium carbonate in your body is stored in bones and teeth. You can ingest calcium carbonate directly, as in Tums and Rolaids, but we most often obtain it from food and water; some dark green vegetables, such as kale, have significant amounts. Bones are continually remodeled by specialized cells that deposit bone (osteoblasts) or dissolve it (osteoclasts). Without the protein ingredients, natural calcium carbonate would make our bones too weak (like chalk) or too brittle (like calcite or aragonite). The proteins embedded with the mineral give bone compressive strength, light weight, and flexibility.
Teeth have a way of growing stronger with age. Strains cause tiny fractures called tufts. The body fills these in with organic matter, making them more durable and stress-resistant (Science Daily). The basket-weave structure of mineral in enamel thus enables teeth to be "self-healing" to a degree. "This type of infilling bonds the opposing crack walls, which increases the amount of force required to extend the crack later on," scientists at George Washington University found back in 2002. In addition, eggshells are made up almost entirely of calcium carbonate, but they can be extremely hard to crack when held upright in the hand.
Artistic Calcium Carbonate
Life's design skills with calcium carbonate go far beyond materials science. Consider "A Day in the Life of an Ammonite," as Sarah Gibson titled an entry at the PLOS Paleo blog. Her experience gathering ammonite specimens from around the world excited her admiration for these squid-like shelled animals.
To me, the ammonites were beautiful fossils, exquisite and intricate, perfectly coiled, some with an iridescent patina increasing their aesthetic beauty. The variation was great from one shell to the next.
Ammonites are among many organisms, including sunflowers and conch shells, that follow the Fibonacci sequence and the Golden Ratio in their proportions -- a remarkable mathematical pattern that appears beautiful to the human mind, but seems superfluous for mere survival. Cristobal Vila's stunning video bears watching again.
Calcium carbonate can precipitate out of water, forming limestone, dolomite, chalk, marble and travertine. Some of the deposits, as in cave speleothems and hot spring terraces, can be quite beautiful. Those formations can be explained by chemical and physical laws. But is that true for the living examples? No -- for the same reasons we gave in a recent post about Buckyballs and BMCs.
We can observe natural laws depositing dissolved calcium carbonate into terraces and other lovely rock formations, but those structures don't do anything. They have no function. By contrast, nothing in the chemistry of calcium carbonate can produce a femur bone, an ammonite shell, or a conch shell. These materials possess beauty and function because informational codes direct the construction process. Molecular machines, directed by the genetic code, take in the materials from the environment they need for the structures. Only intelligence can produce coded information that directs the construction of integrated functional structures.
Geological Intelligent Design
Here's an interesting footnote. Cave formations were long thought to originate solely by abiotic deposition of limestone dripping from groundwater into the cave. That's part of the story, but since the 1990s or so, scientists have found something amazing: microbes play an important role in the process of calcite deposition in stalactites and other speleothems (example from Biogeosciences). Indeed, the eyes of science are opening up to grand vistas of discovery, beginning to see how major earth processes, including rock dissolution, rock formation, and the transformation of soils and sediments involve the smallest forms of life (see International Journal of Speleology).
This means that many "natural" environmental processes are mediated by the presence of factors whose actions are guided by complex specified information encoded in DNA. Or to put it another way, there may be more intelligent design going on in "nonliving" geological processes than we currently realize.
Image: Calcium carbonate from clamshells, by Chemicalinterest (Own work) [Public domain], via Wikimedia Commons.