Biopolymer composites in the exterior shell of the black coral are strong and hard due to weakly bonded chitin strands with strongly crosslinked proteins.

Black corals produce the hard structural framework of their colonies by making strands of chitin – a carbohydrate polymer. Each strand is attached to adjacent strands via numerous weak bonds (hydrogen bonds) producing a strong foundation onto which proteins attach. These proteins crosslink with each other via very strong (covalent) bonds which hardens the composite material. Different species of black coral are able to tailor the strength, density, and flexibility of this chitin-protein composite material to their own specific needs by varying the degree of inter-strand hydrogen bonding and (presumably) the protein crosslinking compounds.

References

"Antipatharians [black corals] have a rigid, erect chitin skeleton that forms a branched, tree-like colony or a long unbranched whip-like coil (wire coral). The structure of the black coral skeleton is that of a laminated composite, constituted primarily of chitin fibrils and non-fibrillar protein...Other components that are also present in the skeleton are lipid, carbohydrate, phenols and sterols. Iodine and bromine appear to be the dominant single elements." (Juárez-de la Rosa et al. 2007:880)

"The OH stretching vibration mode for A. [Antipathes] pennacea exhibits a broad band indicating intermolecular hydrogen bonding of chitin molecules within the chain and the presence of free hydroxyl from hydroxymethyl groups CH2OH...In A. caribbeana, this OH band is not present, therefore, the chitin chains are organized in sheets where they are bonded to adjacent chains by hydrogen bonds, originating a tight network, dominated by a strong CO–NH hydrogen bonds...The crystallite size of chitin after a deproteinization was larger for A. caribbeana than A. pennacea and the absence of free hydroxyl from hydroxymethyl groups CH2OH allows stronger intramolecular bonds within the adjacent chains, favoring a higher fibers packing of chitin, therefore, natural coral of A. caribbeana was harder to pulverize and require longer period of time to be deproteinized, than A. pennacea." (Juárez-de la Rosa et al. 2007:884)

"Mechanically, the skeleton of antipatharians can be polished, bent and molded, and its rigidity and density have been compared advantageously with those of other biomaterials such as wood, bone, mollusk shell, and some insect cuticle." (Juárez-de la Rosa et al. 2007:880)

Journal article
Optical, thermal, and structural characterization of the sclerotized skeleton of two antipatharian coral species

No link available.

Living System/s

Organism
Bottle Brush CoralAntipathesSpecies