Protein Complex Guides Nacre Formation

a macro image of a group of oyster brown, white, and black oyster shells

Biological Strategy

Black-lip pearl shell

AskNature Team

Image: Alex Hurst / Flickr / Some rights reserved

Chemically Assemble Mineral Crystals

The vast majority of biochemical assembly and break down processes––even by the most complex organisms––occur within cells. In fact, cells are able to perform hundreds, even thousands, of chemical transformations at the same time under life-friendly conditions (ambient temperature and pressure in an aqueous environment). Within cells, organisms utilize organic compounds to facilitate the assembly of mineral crystals. For example, oysters utilize acidic proteins and sulfated polysaccharides to assemble layers of aragonite crystals.

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Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Bivalves

Class Bivalvia (“two-valved”): Clams, mussels, oysters

The bivalve name says it all: these animals are characterized by a two-sided shell that is connected at the hinge with a muscle. The 15,000 species in 90 families have flattened bodies, no head, and generally live a sedentary lifestyle. Instead of having a circular band of teeth like other molluscs, many are filter feeders. They use cilia, or hair-like extensions, to move water into their bodies and pluck out plankton to eat. Consequently, there are no terrestrial bivalves, and only 15% live in freshwater ecosystems. The rest live in marine environments.

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The protein, Pif, secreted by pearl oysters contributes to the strength and shape of the shell due to key amino acids that guide mineral crystal growth.

Behind every hard, mineral crystalline material in nature, there’s a bevy of soft s guiding its formation and contributing to its strength and resilience. The hard calcium carbonate shells of oysters rely on a set of proteins that perform several key functions. They trigger dissolved calcium carbonate to crystallize out of solution, direct crystal growth into an ordered arrangement, and attach themselves to the crystals and each other to form a strongly bonded, properly shaped material. Although these proteins are made up of hundreds of building blocks, several amino acids appear to play major roles. Acidic amino acids, particularly aspartate, play a major role in crystal formation and binding protein and crystals together. Sulfur-containing amino acids, particularly cysteine, play a major role in binding proteins to each other to add strength and hold its shape.