20 March 2021 ESRFnews
Of course, it is the nacre inside mollusc shells that fas- cinates most for its beauty and, from a scientific point of view, its amazingly regular and uniform architecture. How can such an intricate structure emerge from a multitude of isolated cells, all secreting material at different loca- tions at the same time? In January this year, the B CUBE team and its ESRF colleagues discovered that the answer lies in the behaviour of helical structural defects, which propagate through growing nacre, like microscale spiral staircases. Using the ESRF nanoprobe beamline ID16A in conjunction with a cryogenic system for extra resolving power, and with help from neural-network algorithms to interpret the data, the researchers were able to observe an attraction between distant, oppositely handed defects. When left-handed defects met their right-handed coun- terparts, the pairs cancelled themselves out, ultimately leaving a structure that was perfectly regular and defect- free (Nat. Phys. doi:0.1038/s41567-020-01069-z). Zlotnikov s is not the only group to study biomineral-
isation at the European synchrotron (see Pearls of wis- dom: more ESRF takes on biomineralisation , top), and he believes it is essential to combine the knowledge of scientists from different backgrounds. Given the multiple scales and complexities of the structures involved, it is also a topic that benefits from new state-of-the-art techniques such as dark-field microscopy, which has been pioneered at ID06 and will be offered on a dedicated ESRF EBS beamline in 2023. But Zlotnikov is confident that basic physics plays a strong role. Cells are not magical, they are not amazing black boxes that can simply 3D-print any- thing, he says. Our question has to be what are the basic mechanisms used by the organisms to induce the forma- tion of these mineralised architectures.
synthetic versions. We re not the only materials science group to study biomineralisation; there are quite a few others, says Zlotnikov. But I think what we re doing that is new is trying to link classical physics and the chemis- try of materials to the most fundamental notions in the theory of evolution. Then based at the Max Planck Institute of Colloids and
Interfaces in Potsdam, Germany, Zlotnikov first explored biomineralisation in 2014, when he and his colleagues employed high-resolution synchrotron-based microto- mography at the ESRF s ID19 beamline to image a mol- lusc shell s outer layer. Like nacre, this substance is also formed by a living organism, although comprising the calcium-carbonate mineral calcite, packed in elongated prisms. By taking images at different depths, the research- ers were able to build a record of the structure at different stages of growth deeper layers being newer than those closer to the surface. On analysis, the layers appeared to follow textbook theories of how grains grow in, for example, engineering materials when heated. The results also implied that the evolution in shape of the prisms is dictated by thermodynamics and the kinetics of crystal growth rather than by the organism itself, except in set- ting the boundary conditions (Nat. Mat. 13 1102).
Textbook theory A few years later, Zlotnikov s group at B CUBE as well as co-workers at the Institute for Solid State Physics and Optics in Hungary and the ESRF was able to put that thermodynamics to the test. Returning to ID19, but also drawing on diffraction data taken at the ID06 beam- line, the researchers were able to visualise the 3D spatial arrangement and the crystallographic properties of the prisms in the shell of another species of mollusc. They also created a computer simulation in which the growth of the shell occurs under a gradual decrease of the super- saturation of the mineral phase, a changing boundary condition similar to that seen in well-known directional solidification in materials science. The experimental data supported the model, suggesting again that the shell assembly is guided by the organism only indirectly, by setting a specific physical and chemical environment at the growth front (Adv. Mater. 30 1803855).
PEARLS OF WISDOM: MORE ESRF TAKES ON BIOMINERALISATION This year, performing X-ray absorption spectroscopy at the ID26 beamline, Alain Manceau at Grenoble Alpes University and the CNRS in France and colleagues discovered a new biomineralisation mechanism in birds that detoxifies mercury (see News, p6). Last year, Boaz Pokroy of the Technion Israel Institute of Technology and colleagues from the Charité University in Berlin, Germany, employed nanotomography, microtomography and powder diffraction at the ID16B, ID19 and ID22 beamlines to reveal that the helical mineral structure grown within a red alga makes the organism highly resistant to mechanical stress (Adv. Sci. doi:10.1002/advs.202000108). In 2019, some of the same researchers collaborated with a group led by Peter Fratzl at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, and used high-resolution powder diffraction at the ID22 beamline to identify an entirely new structure of hydrated calcium carbonate, which they believe could be important for biomineralisation (Science 363 396). In 2018, Yannicke Dauphin of the Pierre and Marie Curie University in Paris, France, and colleagues drew on data taken at ID21 to argue that the detailed structure of eggshells is species-specific, and therefore cannot be explained by competition in crystal growth alone (Connect. Tissue Res. 59 67).
Cells are not amazing black boxes that can simply 3D-print anything
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Igor Zlotnikov studying nacre at the ID16A beamline.
ESRFMar21_Short feature_v7.indd 20 26/02/2021 10:43