X-rays reveal why sea urchins are no easy prey

February 14, 2012
Inside a sea urchin spine. Credit: Marina Krumova, University of Konstanz

(PhysOrg.com) -- The spine of a sea urchin is 99.9% chalk, a very common material forming tiny crystals that are very hard but easy to break apart. Scientists have now discovered how these marine animals use chalk or lime to grow spines combining this hardness with shock-absorbing flexibility. Tiny calcite crystals are embedded, like bricks in a wall, into a mortar of amorphous lime mixed with minute amounts of biological proteins. This points the way to the design and synthesis of new hi-tech composite materials, and a project has already begun involving a major concrete manufacturer. The results are published in PNAS (Proceedings of the National Academy of Sciences) dated 13-17 February 2012.

The team of scientists was led by Helmut Cölfen from the University of Konstanz (Germany) and comprised scientists from the universities of Beijing, Bristol, Leeds, Potsdam, the German Federal Centre for Materials Research (BAM) in Berlin, the CNRS in Orsay, the Max-Planck Institute of Colloids and Interfaces in Potsdam and the European Synchrotron Radiation Facility (ESRF) in Grenoble.

The team found the answer to a well-known problem: the hard-to-break spines of sea urchins consist of lime (calcium carbonate), a material which in crystalline form is hard but brittle. In geological deposits, lime usually forms that have very different properties to sea urchin spines as they break easily along their cleavage planes. However, it is known from X-ray analysis that the spines consist of calcite crystals. When they are broken, on the other hand, they do not produce the plane cleavage surfaces of single crystals but a rough fracture surface corresponding more to a glass or a ceramic material.

Helmut Cölfen built an international network of institutes specialising in materials characterisation to tackle this problem with electron microscopy, X-ray diffraction, nano-analysis and other methods.

The use of different X-ray scattering techniques at the ESRF was instrumental to reveal that sea urchin spines are actually built like walls of nanometre-sized bricks of calcite crystals which are aligned in parallel. The bricks are glued together with a mortar of non-crystalline lime. Such a composite arrangement efficiently absorbs shocks and collisions, as it confers elasticity to the material. “It was a real challenge to separately characterise the crystalline and non-crystalline parts of the spines, because the individual structures are extremely small. We had to combine two very different techniques using thin X-ray beams, one optimised for nanocrystals and the other for amorphous structures”, says Aurélien Gourrier of the CNRS and ESRF.

The researchers determined that 92% of the spines consist of crystalline calcite and 8% of amorphous lime. The disordered lime is in turn made of 99.9% calcium carbonate into which a tiny amount of is mixed (0.1%). At a disordered layer thickness of one or two nanometres around the calcite crystals, the amorphous lime ensures that the sting can only be broken with difficulty. This work is the first detailed structural proof of biological mesocrystals. The newly discovered structure solves a decades-long debate on the nature of the sea urchin – thanks to the mesocrystalline structure, it combines the properties of thin calcite nanocrystals and of the disordered layer surrounding them.

The large internal surface area of the nature-made mesocrystals can inspire the design of, for example, new materials that are thin and hardly breakable and at the same time environmentally friendly in production and use. "It is fascinating that nature can turn fragile materials through structuring into high-performance , that manufacturing has not managed to produce so far," says Helmut Cölfen on the global quest to learn from biominerals. His group at the University of Konstanz is already in collaboration with two major international companies on projects dedicated to the manufacture of future high performance concrete.

Explore further: Crystal clear research

More information: Jong Seto et al., Structure-property relationships of a biological mesocrystal in the adult sea urchin spine, PNAS 13-17 February 2012, www.pnas.org/cgi/doi/10.1073/pnas.1109243109

Related Stories

Crystal clear research

September 6, 2011

(PhysOrg.com) -- Scientists have successfully created synthetic crystals whose structures and properties mimic those of naturally occurring biominerals such as seashells.

A crystal clear view of chalk formation

January 12, 2009

(PhysOrg.com) -- It has a beautiful, but also an unpleasant side: crystallization determines the shape of precious stones, but also causes the lime scale in washing machines. How this comes about, has been known for a long ...

A crystal clear view of chalk formation

January 23, 2009

(PhysOrg.com) -- It has a beautiful, but also an unpleasant side: crystallization determines the shape of precious stones, but also causes the lime scale in washing machines. How this comes about, has been known for a long ...

Microscopy reveals structure of calcite shells

November 30, 2009

(PhysOrg.com) -- Lara Estroff and colleagues have taken a deep, detailed look at the way lab-created calcite crystals, similar to those found in nature, grow in tandem with proteins and other large molecules.

Calcium carbonate and climate change

August 30, 2010

(PhysOrg.com) -- What links sea urchins, limestone and climate change? The common thread is calcium carbonate, one of the most widespread minerals on Earth. UC Davis researchers have now measured the energy changes among ...

Recommended for you

Fluorescent probe could light up cancer

March 28, 2017

A fluorescent probe developed by Michigan Tech chemist Haiying Liu illuminates the enzyme beta-galactosidase in a cell culture, which could help cancer surgeons.

How does oxygen get into a fuel cell?

March 28, 2017

In order for a fuel cell to work, it needs an oxidizing agent. TU Wien has now found a way to explain why oxygen does not always enter fuel cells effectively, rendering them unusable.

Zika virus protein mapped to speed search for cure

March 27, 2017

A study published today shows how Indiana University scientists are speeding the path to new treatments for the Zika virus, an infectious disease linked to birth defects in infants in South and Central America and the United ...

Researchers link orphan receptor to opioid-induced itching

March 27, 2017

Opioids have long been an important tool in the world of pain management, but the side effects of these drugs - from addiction and respiratory failure to severe itching and dizziness, can be overwhelming. Scientists have ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.