Pacific Northwest physicists show insects using heavy metals to strengthen appendages

Sophisticated physical measurements show how insects and other invertebrates make use of heavy metals to strengthen and sharpen their appendages in a way that is different from the biomineralization process used to form the teeth, bones and other organs in a wide variety of animals. 

The new work by a team of 19 physicists from the University of Oregon, the Pacific Northwest National Laboratory in Richland, Wash., and Oregon State University, appeared online Sept. 1 in the peer-reviewed journal Scientific Reports.

The study examined the mineral content of worm jaws, ant mandibles, spider fangs, centipede pincers and scorpion stings. The heavy metals zinc, manganese and copper as well as the halogen bromine were found in concentrations ranging from about 1% to 25% of dry mass. 

The authors call the new biological minerals heavy element biomaterials (HEBs). 

Extensive examination of the mechanical properties of these HEB tissues showed that they approach the hardness of non-HEB biomineralized tissues such as salmon teeth and crab claws, and exceed them in resistance to abrasion and fracture. The HEB tissues also showed greater resistance to fracture. These are important considerations for small animals that must overcome their limited force by employing sharp cutting and puncturing tools which cannot be re-sharpened once damaged.

“Compared with other hard materials grown by these animals, the wear-resistant zinc material enables heavily used tools to puncture stiff substances using only one-fifth of the force,” lead author and University of Oregon physicist Robert Schofield wrote in a Sept. 1 research brief in The Conversation, an online academic newsletter. 

How the study evolved 

Earlier studies of crab claws had helped define the problem. The claws of certain crab species contain both non-HEB calcium-enriched tissue and HEB tissue containing bromine, offering the opportunity to compare the two types of biomaterials in a single organism. 

The studies of crab claws had shown that the calcium-enriched cuticle was three times harder and nearly five times stiffer than the bromine-enriched portion. 

This study extended the measurements of crab cuticle to smaller organs of a species of nereid worms, spider, ant, and scorpion, containing manganese and zinc. For the first time, the authors measured new properties, not previously reported for any manganese- or zinc-containing heavy element biomaterials, namely loss tangent, energy of fracture, abrasion resistance, and impact resistance.

Instrumentation and techniques used included an atomic force microscope, atom probe tomography, time-of-flight–secondary ion mass spectrometry and extended X-ray analysis of fine structure.

Difference of HEB to non-HEB biomineralization

The heavy element biomaterials (HEB) are distinct from the earlier known form of biomineralized tissue (non-HEB). In the earlier known biomineralization, there is a distinct organic and inorganic phase. For example when a tooth grows it forms a matrix of proteins and other living material that allows for inclusion of the minerals calcium and phosphorus that give it hardness. 

But these distinct phases did not show up in X-ray or electron diffraction examinations of the zinc-rich “tools” of ant and scorpion. Nor did they appear in the examination by extended X-ray analysis of fine structure (EXAFS) of worm jaws, ant mandibles, spider fangs, centipede pincers, and scorpion stings. 

It remains unclear how the zinc and other heavy metals are incorporated into the living tissue. 

“One possibility is that a small fraction of the zinc, for example, forms bridges between proteins, and these cross-links stiffen the material – like crossbeams stiffen a building,” Schofield wrote in The Conversation. “We also think that when a fang bangs into something hard, these zinc cross-links may break first, absorbing energy to keep the fang itself from chipping.  

“We speculate that the abundance of extra zinc is a ready supply for healing the material by quickly reestablishing the broken zinc-histidine cross-links between proteins,” Schofield added. 

Discovery of 'non-HEB' biomineralization

Biomineralization was a remarkable discovery made in the late 1950s by the geochemist Heinz Lowenstam, while visiting a beach in Bermuda. Standing in a tidal pool he noticed a chiton, a type of marine mollusk, gnawing indentations into the smooth limestone bottom of a tidal pool. The prevailing knowledge at the time was that chiton teeth were soft like insect wings, but how could soft material cut into limestone? X-ray diffraction studies revealed that the teeth were capped with magnetite, a tough iron oxide crystal with magnetic properties.

Subsequent studies established the existence of magnetite in bacteria that could orient to the Earth’s field, and soon biomineralized magnetite was found in a variety of other organisms, including the human brain. Lowenstam’s work emphasized that the magnetite was formed by a biologic process, a view that was not widely accepted at first, as the prevailing view was that of strictly chemical origin of minerals.

It took 30 years before the concept of biomineralization became widely accepted. Lowenstam wrote a book on the evolution of calcified skeletons with biologist Lynn Margulis in 1980, at a time when both his view of biomineralization, and Margulis’ discovery of the endosymbiotic incorporation of the mitochondria and chloroplast into eukaryotic cells, were still meeting considerable resistance. 

The acceptance of the heavy element biomaterials appears to be moving more swiftly.