As one of the most abundant materials in the world, calcium carbonate, CaCO
, is the main constituent of the skeletons and shells of various marine organisms. It is used in the cement industry and ...plays a crucial role in the global carbon cycle and formation of sedimentary rocks. For more than a century, only three polymorphs of pure CaCO
-calcite, aragonite, and vaterite-were known to exist at ambient conditions, as well as two hydrated crystal phases, monohydrocalcite (CaCO
·1H
O) and ikaite (CaCO
·6H
O). While investigating the role of magnesium ions in crystallization pathways of amorphous calcium carbonate, we unexpectedly discovered an unknown crystalline phase, hemihydrate CaCO
·½H
O, with monoclinic structure. This discovery may have important implications in biomineralization, geology, and industrial processes based on hydration of CaCO
.
Protein‐metal interactions—traditionally regarded for roles in metabolic processes—are now known to enhance the performance of certain biogenic materials, influencing properties such as hardness, ...toughness, adhesion, and self‐healing. Design principles elucidated through thorough study of such materials are yielding vital insights for the design of biomimetic metallopolymers with industrial and biomedical applications. Recent advances in the understanding of the biological structure–function relationships are highlighted here with a specific focus on materials such as arthropod biting parts, mussel byssal threads, and sandcastle worm cement.
Protein–metal interactions were traditionally regarded for their role in metabolic processes. Nowadays, they are also known to enhance the performance of certain biogenic materials, influencing properties such as hardness, toughness, adhesion, and self‐healing. In this Review, recent advances in the understanding of the biological structure–function relationships are highlighted.
Coccoliths are calcitic particles produced inside the cells of unicellular marine algae known as coccolithophores. They are abundant components of sea-floor carbonates, and the stoichiometry of ...calcium to other elements in fossil coccoliths is widely used to infer past environmental conditions. Here we study cryo-preserved cells of the dominant coccolithophore Emiliania huxleyi using state-of-the-art nanoscale imaging and spectroscopy. We identify a compartment, distinct from the coccolith-producing compartment, filled with high concentrations of a disordered form of calcium. Co-localized with calcium are high concentrations of phosphorus and minor concentrations of other cations. The amounts of calcium stored in this reservoir seem to be dynamic and at a certain stage the compartment is in direct contact with the coccolith-producing vesicle, suggesting an active role in coccolith formation. Our findings provide insights into calcium accumulation in this important calcifying organism.
Biomineralization had apparently evolved independently in different phyla, using distinct minerals, organic scaffolds, and gene regulatory networks (GRNs). However, diverse eukaryotes from ...unicellular organisms, through echinoderms to vertebrates, use the actomyosin network during biomineralization. Specifically, the actomyosin remodeling protein, Rho-associated coiled-coil kinase (ROCK) regulates cell differentiation and gene expression in vertebrates' biomineralizing cells, yet, little is known on ROCK's role in invertebrates' biomineralization. Here, we reveal that ROCK controls the formation, growth, and morphology of the calcite spicules in the sea urchin larva. ROCK expression is elevated in the sea urchin skeletogenic cells downstream of the Vascular Endothelial Growth Factor (VEGF) signaling. ROCK inhibition leads to skeletal loss and disrupts skeletogenic gene expression. ROCK inhibition after spicule formation reduces the spicule elongation rate and induces ectopic spicule branching. Similar skeletogenic phenotypes are observed when ROCK is inhibited in a skeletogenic cell culture, indicating that these phenotypes are due to ROCK activity specifically in the skeletogenic cells. Reduced skeletal growth and enhanced branching are also observed under direct perturbations of the actomyosin network. We propose that ROCK and the actomyosin machinery were employed independently, downstream of distinct GRNs, to regulate biomineral growth and morphology in Eukaryotes.
The spider fang is a natural injection needle, hierarchically built from a complex composite material comprising multiscale architectural gradients. Considering its biomechanical function, the spider ...fang has to sustain significant mechanical loads. Here we apply experiment-based structural modelling of the fang, followed by analytical mechanical description and Finite-Element simulations, the results of which indicate that the naturally evolved fang architecture results in highly adapted effective structural stiffness and damage resilience. The analysis methods and physical insights of this work are potentially important for investigating and understanding the architecture and structural motifs of sharp-edge biological elements such as stingers, teeth, claws and more.
The skeletons of adult echinoderms comprise large single crystals of calcite with smooth convoluted fenestrated morphologies, raising many questions about how they form. By using water etching, ...infrared spectroscopy, electron diffraction, and environmental scanning electron microscopy, we show that sea urchin spine regeneration proceeds via the initial deposition of amorphous calcium carbonate. Because most echinoderms produce the same type of skeletal material, they probably all use this same mechanism. Deposition of transient amorphous phases as a strategy for producing single crystals with complex morphology may have interesting implications for the development of sophisticated materials.
Many organisms use amorphous calcium carbonate (ACC) and control its stability by various additives and water; however, the underlying mechanisms are yet unclear. Here, the effect of water and ...inorganic additives commonly found in biology on the dynamics of the structure of ACC during crystallization and on the energetics of this process is studied. Total X‐ray scattering and pair distribution function analysis show that the short‐ and medium‐range order of all studied ACC samples are similar; however, the use of in situ methodologies allow the observation of small structural modifications that are otherwise easily overlooked. Isothermal calorimetric coupled with microgravimetric measurements show that the presence of Mg2+ and of PO43− in ACC retards the crystallization whereas increased water content accelerates the transformation. The enthalpy of ACC with respect to calcite appears, however, independent of the additive concentration but decreases with water content. Surprisingly, the enthalpic contribution of water is compensated for by an equal and opposite entropic term leading to a net independence of ACC thermodynamic stability on its hydration level. Together, these results point toward a kinetic stabilization effect of inorganic additives and water, and may contribute to the understanding of the biological control of mineral stability.
Amorphous calcium carbonate (ACC) is a versatile biomineral tuned by additives and water to form long lasting phases or transient precursors. However, the mechanisms of this stabilization are yet unknown. The effects of additives on the structure dynamics during crystallization and the energetic landscape of ACC are studied. It is shown that kinetics governs the transformation.
Sea urchin larval spicules transform amorphous calcium carbonate (ACC) into calcite single crystals. The mechanism of transformation is enigmatic: the transforming spicule displays both amorphous and ...crystalline properties, with no defined crystallization front. Here, we use X-ray photoelectron emission spectromicroscopy with probing size of 40-200 nm. We resolve 3 distinct mineral phases: An initial short-lived, presumably hydrated ACC phase, followed by an intermediate transient form of ACC, and finally the biogenic crystalline calcite phase. The amorphous and crystalline phases are juxtaposed, often appearing in adjacent sites at a scale of tens of nanometers. We propose that the amorphous-crystal transformation propagates in a tortuous path through preexisting 40- to 100-nm amorphous units, via a secondary nucleation mechanism.
Magnesium is a key component used by many organisms in biomineralization. One role for magnesium is in stabilizing an otherwise unstable amorphous calcium carbonate (ACC) phase. The way in which this ...stabilization is achieved is unknown. Here, we address this question by studying the chemical environment around magnesium in biogenic and synthetic ACCs using Mg K-edge X-ray absorption spectroscopy (XAS). We show that although the short-range structure around the Mg ion is different in the various minerals studied, they all involve a shortening of the Mg−O bond length compared to crystalline anhydrous MgCO3 minerals. We propose that the compact structure around magnesium introduces distortion in the CaCO3 host mineral, thus inhibiting its crystallization. This study also shows that despite technical challenges in the soft X-ray energy regime, Mg K-edge XAS is a valuable tool for structural analysis of Mg containing amorphous materials, in biology and materials science.
Starting from disorder: Anhydrous guanine crystals compose the photonic arrays responsible for the skin and scale iridescence found in Japanese Koi fish. These guanine crystals were found to form in ...intracellular vesicles (see picture) through an amorphous precursor phase. A combined cryo‐SEM and synchrotron radiation X‐ray diffraction study showed the evolution of the crystals in great detail.
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