From particle attachment to space-filling coral skeletons

Correlative geochemical imaging of Desmophyllum dianthus reveals biomineralisation strategy as a key coral vital effect

The chemical and isotopic composition of stony coral skeletons form an important archive of past climate. However, these reconstructions are largely based on empirical relationships often complicated by "vital effects" arising from uncertain physiological processes of the coral holobiont. The skeletons of deep-sea corals, such as Desmophyllum dianthus, are characterised by micron-scale or larger geochemical heterogeneity associated with: (1) centres of calcification (COCs) where nucleation of new skeleton begins, and (2) fibres that thicken the skeleton. These features are difficult to sample cleanly using traditional techniques, resulting in uncertainty surrounding both the causes of geochemical differences and their influence on environmental signals. Here we combine optical, and in-situ chemical and isotopic, imaging tools across a range of spatial resolutions (~ 100 nm to 10 s of μm) in a correlative multimodal imaging (CMI) approach to isolate the microstructural geochemistry of each component. This reveals COCs are characterised by higher organic content, Mg, Li and Sr and lower U, B and δ11B compared to fibres, reflecting the contrasting biomineralisation mechanisms employed to construct each feature. CMI is rarely applied in Environmental/Earth Sciences, but here we illustrate the power of this approach to unpick the "vital effects" in D. dianthus, and by extension, other scleractinian corals.

Spatial variability of and effect of light on the cœlenteron pH of a reef coral

Coral reefs, the largest bioconstruction on Earth, are formed by calcium carbonate skeletons of corals. Coral skeleton formation commonly referred to as calcification occurs in a specific compartment, the extracellular calcifying medium (ECM), located between the aboral ectoderm and the skeleton. Calcification models often assume a direct link between the surrounding seawater and the ECM. However, the ECM is separated from the seawater by several tissue layers and the cœlenteron, which contains the cœlenteric fluid found in both polyps and cœnosarc (tissue connecting the polyps). Symbiotic dinoflagellate-containing cells line the cœlenteron and their photosynthetic activity contributes to changes in the chemistry of the cœlenteric fluid, particularly with respect to pH. The aim of our study is to compare cœlenteron pH between the cœnosarc and polyps and to compare areas of high or low dinoflagellate density based on tissue coloration. To achieve this, we use liquid ion exchange (LIX) pH microsensors to profile pH in the cœlenteron of polyps and the cœnosarc in different regions of the coral colony in light and darkness. We interpret our results in terms of what light and dark exposure means for proton gradients between the ECM and the coelenteron, and how this could affect calcification.

Myriad Mapping of nanoscale minerals reveals calcium carbonate hemihydrate in forming nacre and coral biominerals

Calcium carbonate (CaCO3) is abundant on Earth, is a major component of marine biominerals and thus of sedimentary and metamorphic rocks and it plays a major role in the global carbon cycle by storing atmospheric CO2 into solid biominerals. Six crystalline polymorphs of CaCO3 are known-3 anhydrous: calcite, aragonite, vaterite, and 3 hydrated: ikaite (CaCO3·6H2O), monohydrocalcite (CaCO3·1H2O, MHC), and calcium carbonate hemihydrate (CaCO3·½H2O, CCHH). CCHH was recently discovered and characterized, but exclusively as a synthetic material, not as a naturally occurring mineral. Here, analyzing 200 million spectra with Myriad Mapping (MM) of nanoscale mineral phases, we find CCHH and MHC, along with amorphous precursors, on freshly deposited coral skeleton and nacre surfaces, but not on sea urchin spines. Thus, biomineralization pathways are more complex and diverse than previously understood, opening new questions on isotopes and climate. Crystalline precursors are more accessible than amorphous ones to other spectroscopies and diffraction, in natural and bio-inspired materials.

Physical Phenomena Governing Mineral Morphogenesis in Molluscan Nacre

Mollusks, as well as many other living organisms, have the ability to shape mineral crystals into unconventional morphologies and to assemble them into complex functional mineral-organic structures, an observation that inspired tremendous research efforts in scientific and technological domains. Despite these, a biochemical toolkit that accounts for the formation of the vast variety of the observed mineral morphologies cannot be identified yet. Herein, phase-field modeling of molluscan nacre formation, an intensively studied biomineralization process, is used to identify key physical parameters that govern mineral morphogenesis. Manipulating such parameters, various nacre properties ranging from the morphology of a single mineral building block to that of the entire nacreous assembly are reproduced. The results support the hypothesis that the control over mineral morphogenesis in mineralized tissues happens via regulating the physico-chemical environment, in which biomineralization occurs: the organic content manipulates the geometric and thermodynamic boundary conditions, which in turn, determine the process of growth and the form of the biomineral phase. The approach developed here has the potential of providing explicit guidelines for the morphogenetic control of synthetically formed composite materials.

The response of coral skeletal nano structure and hardness to ocean acidification conditions

Ocean acidification typically reduces coral calcification rates and can fundamentally alter skeletal morphology. We use atomic force microscopy (AFM) and microindentation to determine how seawater pCO2 affects skeletal structure and Vickers hardness in a Porites lutea coral. At 400 µatm, the skeletal fasciculi are composed of tightly packed bundles of acicular crystals composed of quadrilateral nanograins, approximately 80-300 nm in dimensions. We interpret high adhesion at the nanograin edges as an organic coating. At 750 µatm the crystals are less regular in width and orientation and composed of either smaller/more rounded nanograins than observed at 400 µatm or of larger areas with little variation in adhesion. Coral aragonite may form via ion-by-ion attachment to the existing skeleton or via conversion of amorphous calcium carbonate precursors. Changes in nanoparticle morphology could reflect variations in the sizes of nanoparticles produced by each crystallization pathway or in the contributions of each pathway to biomineralization. We observe no significant variation in Vickers hardness between skeletons cultured at different seawater pCO2. Either the nanograin size does not affect skeletal hardness or the effect is offset by other changes in the skeleton, e.g. increases in skeletal organic material as reported in previous studies.

Understanding Snail Mucus Biosynthesis and Shell Biomineralisation through Genomic Data Mining of the Reconstructed Carbohydrate and Glycan Metabolic Pathways of the Giant African Snail (Achatina fulica)

The giant African snail (Order Stylommatophora: Family Achatinidae), Achatina fulica (Bowdich, 1822), is the most significant and invasive land snail pest. The ecological adaptability of this snail involves high growth rate, reproductive capacity, and shell and mucus production, driven by several biochemical processes and metabolism. The available genomic information for A. fulica provides excellent opportunities to hinder the underlying processes of adaptation, mainly carbohydrate and glycan metabolic pathways toward the shell and mucus formation. The authors analysed the 1.78 Gb draft genomic contigs of A. fulica to identify enzyme-coding genes and reconstruct biochemical pathways related to the carbohydrate and glycan metabolism using a designed bioinformatic workflow. Three hundred and seventy-seven enzymes involved in the carbohydrate and glycan metabolic pathways were identified based on the KEGG pathway reference in combination with protein sequence comparison, structural analysis, and manual curation. Fourteen complete pathways of carbohydrate metabolism and seven complete pathways of glycan metabolism supported the nutrient acquisition and production of the mucus proteoglycans. Increased copy numbers of amylases, cellulases, and chitinases highlighted the snail advantage in food consumption and fast growth rate. The ascorbate biosynthesis pathway identified from the carbohydrate metabolic pathways of A. fulica was involved in the shell biomineralisation process in association with the collagen protein network, carbonic anhydrases, tyrosinases, and several ion transporters. Thus, our bioinformatic workflow was able to reconstruct carbohydrate metabolism, mucus biosynthesis, and shell biomineralisation pathways from the A. fulica genome and transcriptome data. These findings could reveal several evolutionary advantages of the A. fulica snail, and will benefit the discovery of valuable enzymes for industrial and medical applications.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Nanoscale Pathway of Modern Dolomite Formation in a Shallow, Alkaline Lake

Dolomite [CaMg(CO₃)₂] formation under Earth surface conditions is considered largely inhibited, yet protodolomite (with a composition similar to dolomite but lacking cation ordering), and in some cases also dolomite, was documented in modern shallow marine and lacustrine, evaporative environments. Authigenic carbonate mud from Lake Neusiedl, a shallow, episodically evaporative lake in Austria consists mainly of Mg-calcite with zoning of Mg-rich and Mg-poor regions in μm-sized crystals. Within the Mg-rich regions, high-resolution transmission electron microscopy revealed < 5-nm-sized domains with dolomitic ordering, i.e., alternating lattice planes of Ca and Mg, in coherent orientation with the surrounding protodolomite. The calcite with less abundant Mg does not show such domains but is characterized by pitted surfaces and voids as a sign of dissolution. These observations suggest that protodolomite may overgrow Mg-calcite as a result of the changing chemistry of the lake water. During this process, oscillating concentrations (in particular of Mg and Ca) at the recrystallization front may have induced dissolution of Mg-calcite and growth of nanoscale domains of dolomite, which subsequently became incorporated as ordered domains in coherent orientation within less ordered regions. It is suggested that this crystallization pathway is capable of overcoming, at least at the nanoscale, the kinetic barrier to dolomite formation.

Rapid grain boundary diffusion in foraminifera tests biases paleotemperature records

The oxygen isotopic compositions of fossil foraminifera tests constitute a continuous proxy record of deep-ocean and sea-surface temperatures spanning the last 120 million years. Here, by incubating foraminifera tests in 18O-enriched artificial seawater analogues, we demonstrate that the oxygen isotopic composition of optically translucent, i.e., glassy, fossil foraminifera calcite tests can be measurably altered at low temperatures through rapid oxygen grain-boundary diffusion without any visible ultrastructural changes. Oxygen grain boundary diffusion occurs sufficiently fast in foraminifera tests that, under normal upper oceanic sediment conditions, their grain boundaries will be in oxygen isotopic equilibrium with the surrounding pore fluids on a time scale of <100 years, resulting in a notable but correctable bias of the paleotemperature record. When applied to paleotemperatures from 38,400 foraminifera tests used in paleoclimate reconstructions, grain boundary diffusion can be shown to bias prior paleotemperature estimates by as much as +0.86 to -0.46 °C. The process is general and grain boundary diffusion corrections can be applied to other polycrystalline biocarbonates composed of small nanocrystallites (<100 nm), such as those produced by corals, brachiopods, belemnites, and molluscs, the fossils of which are all highly susceptible to the effects of grain boundary diffusion.

Synchrotron X-ray Studies of the Structural and Functional Hierarchies in Mineralised Human Dental Enamel: A State-of-the-Art Review

Hard dental tissues possess a complex hierarchical structure that is particularly evident in enamel, the most mineralised substance in the human body. Its complex and interlinked organisation at the Ångstrom (crystal lattice), nano-, micro-, and macro-scales is the result of evolutionary optimisation for mechanical and functional performance: hardness and stiffness, fracture toughness, thermal, and chemical resistance. Understanding the physical-chemical-structural relationships at each scale requires the application of appropriately sensitive and resolving probes. Synchrotron X-ray techniques offer the possibility to progress significantly beyond the capabilities of conventional laboratory instruments, i.e., X-ray diffractometers, and electron and atomic force microscopes. The last few decades have witnessed the accumulation of results obtained from X-ray scattering (diffraction), spectroscopy (including polarisation analysis), and imaging (including ptychography and tomography). The current article presents a multi-disciplinary review of nearly 40 years of discoveries and advancements, primarily pertaining to the study of enamel and its demineralisation (caries), but also linked to the investigations of other mineralised tissues such as dentine, bone, etc. The modelling approaches informed by these observations are also overviewed. The strategic aim of the present review was to identify and evaluate prospective avenues for analysing dental tissues and developing treatments and prophylaxis for improved dental health.

Testing hypotheses on the calcification in scleractinian corals using a spatio-temporal model that shows a high degree of robustness

Calcification in photosynthetic scleractinian corals is a complicated process that involves many different biological, chemical, and physical sub-processes that happen within and around the coral tissue. Identifying and quantifying the role of separate processes in vivo or in vitro is difficult or not possible. A computational model can facilitate this research by simulating the sub-processes independently. This study presents a spatio-temporal model of the calcification physiology, which is based on processes that are considered essential for calcification: respiration, photosynthesis, Ca^2＋-ATPase, carbonic anhydrase. The model is used to test different hypotheses considering ion transport across the calicoblastic cells and Light Enhanced Calcification (LEC). It is also used to quantify the effect of ocean acidification (OA) on the Extracellular Calcifying Medium (ECM) and ATP-consumption of Ca^2＋-ATPase. It was able to reproduce the experimental data of three separate studies and finds that paracellular transport plays a minor role compared to transcellular transport. In the model, LEC results from increased Ca^2＋-ATPase activity in combination with increased metabolism. Implementing OA increases the concentration of CO₂ throughout the entire tissue, thereby increasing the availability of CO₃^- in the ECM. As a result, the model finds that calcification becomes more energy-demanding and the calcification rate increases.

Investigating calcification-related candidates in a non-symbiotic scleractinian coral, Tubastraea spp

In hermatypic scleractinian corals, photosynthetic fixation of CO2 and the production of CaCO3 are intimately linked due to their symbiotic relationship with dinoflagellates of the Symbiodiniaceae family. This makes it difficult to study ion transport mechanisms involved in the different pathways. In contrast, most ahermatypic scleractinian corals do not share this symbiotic relationship and thus offer an advantage when studying the ion transport mechanisms involved in the calcification process. Despite this advantage, non-symbiotic scleractinian corals have been systematically neglected in calcification studies, resulting in a lack of data especially at the molecular level. Here, we combined a tissue micro-dissection technique and RNA-sequencing to identify calcification-related ion transporters, and other candidates, in the ahermatypic non-symbiotic scleractinian coral Tubastraea spp. Our results show that Tubastraea spp. possesses several calcification-related candidates previously identified in symbiotic scleractinian corals (such as SLC4-γ, AMT-1like, CARP, etc.). Furthermore, we identify and describe a role in scleractinian calcification for several ion transporter candidates (such as SLC13, -16, -23, etc.) identified for the first time in this study. Taken together, our results provide not only insights about the molecular mechanisms underlying non-symbiotic scleractinian calcification, but also valuable tools for the development of biotechnological solutions to better control the extreme invasiveness of corals belonging to this particular genus.

Study on the Development and Growth of Coral Larvae

Studies on the early development of corals are required for academic research on coral reefs and applied reef conservation, but the interval between observations is usually weeks or months. Thus, no study has comprehensively explored the development of coral larvae after settlement. This study observed Galaxea fascicularis, Mycedium elephantotus, Pocillopora verrucosa, and Seriatopora caliendrum larvae after settlement, including their growth process and the formation of tentacles, skeletons, and polyps. The G. fascicularis and M. elephantotus polyps exhibited the skeleton-over-polyp mechanism, whereas the P. verrucosa and S. caliendrum polyps exhibited the polyp-over-skeleton mechanism. During asexual reproduction, the Symbiodiniaceae species clustered on the coenosarc, resulting in polyp development and skeletal growth. M. Elephantotus was unique in that its tentacles were umbrella-shaped, and its polyp growth and Symbiodiniaceae species performance during asexual reproduction differed from those of the other three corals. Although both P. verrucosa and S. caliendrum have branching morphologies, their vertical development stages were dissimilar. S. caliendrum relied on the mutual pushing of individuals in the colony to extend upward, whereas P. verrucosa had a center individual that developed vertically. The findings of this study can serve as a reference for future research on coral breeding, growth, and health assessments.

Amorphous-to-crystal transition in the layer-by-layer growth of bivalve shell prisms

Biomineralization integrates complex physical and chemical processes bio-controlled by the living organisms through ionic concentration regulation and organic molecules production. It allows tuning the structural, optical and mechanical properties of hard tissues during ambient-condition crystallisation, motivating a deeper understanding of the underlying processes. By combining state-of-the-art optical and X-ray microscopy methods, we investigated early-mineralized calcareous units from two bivalve species, Pinctada margaritifera and Pinna nobilis, revealing chemical and crystallographic structural insights. In these calcite units, we observed ring-like structural features correlated with a lack of calcite and an increase of amorphous calcium carbonate and proteins contents. The rings also correspond to a larger crystalline disorder and a larger strain level. Based on these observations, we propose a temporal biomineralization cycle, initiated by the production of an amorphous precursor layer, which further crystallizes with a transition front progressing radially from the unit centre, while the organics are expelled towards the prism edge. Simultaneously, along the shell thickness, the growth occurs following a layer-by-layer mode. These findings open biomimetic perspectives for the design of refined crystalline materials. STATEMENT OF SIGNIFICANCE: Calcareous biominerals are amongst the most present forms of biominerals. They exhibit astonishing structural, optical and mechanical properties while being formed at ambient synthesis conditions from ubiquitous ions, motivating the deep understanding of biomineralization. Here, we unveil the first formation steps involved in the biomineralization cycle of prismatic units of two bivalve species by applying a new multi-modal non-destructive characterization approach, sensitive to chemical and crystalline properties. The observations of structural features in mineralized units of different ages allowed the derivation of a temporal sequence for prism biomineralization, involving an amorphous precursor, a radial crystallisation front and a layer-by-layer sequence. Beyond these chemical and physical findings, the herein introduced multi-modal approach is highly relevant to other biominerals and bio-inspired studies.

Distinct fine-scale variations in calcification control revealed by high-resolution 2D boron laser images in the cold-water coral Lophelia pertusa

Coral calcification is a complex biologically controlled process of hard skeleton formation, and it is influenced by environmental conditions. The chemical composition of coral skeletons responds to calcification conditions and can be used to gain insights into both the control asserted by the organism and the environment. Boron and its isotopic composition have been of particular interest because of links to carbon chemistry and pH. In this study, we acquired high-resolution boron images (concentration and isotopes) in a skeleton sample of the azooxanthellate cold-water coral Lophelia pertusa. We observed high boron variability at a small spatial scale related to skeletal structure. This implies differences in calcification control during different stages of skeleton formation. Our data point to bicarbonate active transport as a critical pathway during early skeletal growth, and the variable activity rates explain the majority of the observed boron systematic.

Biomineralization: Integrating mechanism and evolutionary history

Calcium carbonate (CaCO₃) biomineralizing organisms have played major roles in the history of life and the global carbon cycle during the past 541 Ma. Both marine diversification and mass extinctions reflect physiological responses to environmental changes through time. An integrated understanding of carbonate biomineralization is necessary to illuminate this evolutionary record and to understand how modern organisms will respond to 21st century global change. Biomineralization evolved independently but convergently across phyla, suggesting a unity of mechanism that transcends biological differences. In this review, we combine CaCO₃ skeleton formation mechanisms with constraints from evolutionary history, omics, and a meta-analysis of isotopic data to develop a plausible model for CaCO₃ biomineralization applicable to all phyla. The model provides a framework for understanding the environmental sensitivity of marine calcifiers, past mass extinctions, and resilience in 21st century acidifying oceans. Thus, it frames questions about the past, present, and future of CaCO₃ biomineralizing organisms.

Colloid assembly and transformation (CAT): The relationship of PILP to biomineralization

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Comparative analysis of biominerals to PILP minerals.

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Role of intrinsically disordered proteins in modulating biomineralization.

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Discussion on the viscoelastic nature of the PILP phase and biominerals.

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New pathway terminology proposed- Colloid Assembly & Transformation (CAT).

The field of biomineralization has undergone a revolution in the past 25 years, which paralleled the discovery by Gower of a polymer-induced liquid-precursor (PILP) mineralization process. She proposed this in vitro model system might be useful for studying the role biopolymers play in biomineralization; however, the ramifications of this pivotal discovery were slow to be recognized. This was presumably because it utilized simple polypeptide additives, and at that time it was not recognized that the charged proteins intimately associated with biominerals are often intrinsically disordered proteins (IDPs). Over the years, many enigmatic biomineral features have been emulated with this model system, too many to be mere coincidence. Yet the PILP system continues to be underacknowledged, probably because of its namesake, which indicates a “liquid precursor”, while we now know the phase appears to have viscoelastic character. Another factor is the confusing semantics that arose from the discovery of multiple “non-classical crystallization” pathways. This review suggests a more relevant terminology for the polymer-modulated reactions is “colloid assembly and transformation (CAT)”, which we believe more accurately captures the key stages involved in both biomineralization and the PILP process. The PILP model system has helped to decipher the key role that biopolymers, namely the IDPs, play in modulating biomineralization processes, which was not readily accomplished in living biological systems. Some remaining challenges in understanding the organic–inorganic interactions involved in biomineralization are discussed, which further highlight how the PILP model system may prove invaluable for studying the simple, yet complex, CAT crystallization pathway.

Faster Crystallization during Coral Skeleton Formation Correlates with Resilience to Ocean Acidification

The mature skeletons of hard corals, termed stony or scleractinian corals, are made of aragonite (CaCO₃). During their formation, particles attaching to the skeleton’s growing surface are calcium carbonate, transiently amorphous. Here we show that amorphous particles are observed frequently and reproducibly just outside the skeleton, where a calicoblastic cell layer envelops and deposits the forming skeleton. The observation of particles in these locations, therefore, is consistent with nucleation and growth of particles in intracellular vesicles. The observed extraskeletal particles range in size between 0.2 and 1.0 μm and contain more of the amorphous precursor phases than the skeleton surface or bulk, where they gradually crystallize to aragonite. This observation was repeated in three diverse genera of corals, Acropora sp., Stylophora pistillata—differently sensitive to ocean acidification (OA)—and Turbinaria peltata, demonstrating that intracellular particles are a major source of material during the additive manufacturing of coral skeletons. Thus, particles are formed away from seawater, in a presumed intracellular calcifying fluid (ICF) in closed vesicles and not, as previously assumed, in the extracellular calcifying fluid (ECF), which, unlike ICF, is partly open to seawater. After particle attachment, the growing skeleton surface remains exposed to ECF, and, remarkably, its crystallization rate varies significantly across genera. The skeleton surface layers containing amorphous pixels vary in thickness across genera: ∼2.1 μm in Acropora, 1.1 μm in Stylophora, and 0.9 μm in Turbinaria. Thus, the slow-crystallizing Acropora skeleton surface remains amorphous and soluble longer, including overnight, when the pH in the ECF drops. Increased skeleton surface solubility is consistent with Acropora’s vulnerability to OA, whereas the Stylophora skeleton surface layer crystallizes faster, consistent with Stylophora’s resilience to OA. Turbinaria, whose response to OA has not yet been tested, is expected to be even more resilient than Stylophora, based on the present data.

Effect of poly(acrylic acid) on crystallization of calcium carbonate in a hydrogel

As carbonate ions are diffused into an agarose hydrogel containing calcium ions and poly(acrylic acid), elliptical and spherical calcites are controllably formed depending on the concentration of poly(acrylic acid) and the position of the hydrogel.

The calcifying interface in a stony coral primary polyp: An interplay between seawater and an extracellular calcifying space

Stony coral exoskeletons build the foundation for the most biologically diverse marine ecosystems on Earth, coral reefs, which face major threats due to many anthropogenic-related stressors. Therefore, understanding coral biomineralization mechanisms is crucial for coral reef management in the coming decades and for using coral skeletons in geochemical studies. This study combines in-vivo imaging with cryo-electron microscopy and cryo-elemental mapping to gain novel insights into the biological microenvironment and the ion pathways that facilitate biomineralization in primary polyps of the stony coral Stylophora pistillata. We document increased tissue permeability in the primary polyp and a highly dispersed cell packing in the tissue directly responsible for producing the coral skeleton. This tissue arrangement may facilitate the intimate involvement of seawater at the mineralization site, also documented here. We further observe an extensive filopodial network containing carbon-rich vesicles extruding from some of the calicoblastic cells. Single-cell RNA-Sequencing data interrogation supports these morphological observations by showing higher expression of genes involved in filopodia and vesicle structure and function in the calicoblastic cells. These observations provide a new conceptual framework for resolving the ion pathway from the external seawater to the tissue-mineral interface in stony coral biomineralization processes.

The Evolution of Calcification in Reef-Building Corals

Corals build the structural foundation of coral reefs, one of the most diverse and productive ecosystems on our planet. Although the process of coral calcification that allows corals to build these immense structures has been extensively investigated, we still know little about the evolutionary processes that allowed the soft-bodied ancestor of corals to become the ecosystem builders they are today. Using a combination of phylogenomics, proteomics, and immunohistochemistry, we show that scleractinian corals likely acquired the ability to calcify sometime between ∼308 and ∼265 Ma through a combination of lineage-specific gene duplications and the co-option of existing genes to the calcification process. Our results suggest that coral calcification did not require extensive evolutionary changes, but rather few coral-specific gene duplications and a series of small, gradual optimizations of ancestral proteins and their co-option to the calcification process.

Phase-Field Modeling of Biomineralization in Mollusks and Corals: Microstructure vs Formation Mechanism

While biological crystallization processes have been studied on the microscale extensively, there is a general lack of models addressing the mesoscale aspects of such phenomena. In this work, we investigate whether the phase-field theory developed in materials' science for describing complex polycrystalline structures on the mesoscale can be meaningfully adapted to model crystallization in biological systems. We demonstrate the abilities of the phase-field technique by modeling a range of microstructures observed in mollusk shells and coral skeletons, including granular, prismatic, sheet/columnar nacre, and sprinkled spherulitic structures. We also compare two possible micromechanisms of calcification: the classical route, via ion-by-ion addition from a fluid state, and a nonclassical route, crystallization of an amorphous precursor deposited at the solidification front. We show that with an appropriate choice of the model parameters, microstructures similar to those found in biomineralized systems can be obtained along both routes, though the time-scale of the nonclassical route appears to be more realistic. The resemblance of the simulated and natural biominerals suggests that, underneath the immense biological complexity observed in living organisms, the underlying design principles for biological structures may be understood with simple math and simulated by phase-field theory.

Pressure-driven fusion of amorphous particles into integrated monoliths

Amorphous calcium carbonate is a hard material that is difficult to make into large, clear blocks. It is also sensitive to heating, and compacting the starting nanoparticles too much tends to lead to crystallization. Mu et al. determined the optimal amount of water in amorphous calcium carbonate to create clear, solid monoliths through compression. The key is to regulate the amount of diffusion in the system so that particle boundaries fuse without triggering sample-wide crystallization. This fusion strategy may also work for similar amorphous inorganic ionic compounds.

Science , abg1915, this issue p. 1466

X-ray linear dichroic ptychography

Biominerals such as seashells, coral skeletons, bone, and tooth enamel are optically anisotropic crystalline materials with unique nanoscale and microscale organization that translates into exceptional macroscopic mechanical properties, providing inspiration for engineering new and superior biomimetic structures. Using Seriatopora aculeata coral skeleton as a model, here, we experimentally demonstrate X-ray linear dichroic ptychography and map the c-axis orientations of the aragonite (CaCO₃) crystals. Linear dichroic phase imaging at the oxygen K-edge energy shows strong polarization-dependent contrast and reveals the presence of both narrow (<35°) and wide (>35°) c-axis angular spread in the coral samples. These X-ray ptychography results are corroborated by four-dimensional (4D) scanning transmission electron microscopy (STEM) on the same samples. Evidence of co-oriented, but disconnected, corallite subdomains indicates jagged crystal boundaries consistent with formation by amorphous nanoparticle attachment. We expect that the combination of X-ray linear dichroic ptychography and 4D STEM could be an important multimodal tool to study nano-crystallites, interfaces, nucleation, and mineral growth of optically anisotropic materials at multiple length scales.

Morphological study of fibrous aragonite in the skeletal framework of a stony coral

The overall calcareous skeletons, including a low-crystalline core and surrounding fibrous crystals, of juvenile stony corals were characterized to clarify the entire calcic architecture and the contribution of abiotic processes.