Phase transitions in biogenic amorphous calcium carbonate.

Crystalline biominerals do not resemble faceted crystals. Current explanations for this property involve formation via amorphous phases. Using X-ray absorption near-edge structure (XANES) spectroscopy and photoelectron emission microscopy (PEEM), here we examine forming spicules in embryos of Strongylocentrotus purpuratus sea urchins, and observe a sequence of three mineral phases: hydrated amorphous calcium carbonate (ACC · H(2)O) → dehydrated amorphous calcium carbonate (ACC) → calcite. Unexpectedly, we find ACC · H(2)O-rich nanoparticles that persist after the surrounding mineral has dehydrated and crystallized. Protein matrix components occluded within the mineral must inhibit ACC · H(2)O dehydration. We devised an in vitro, also using XANES-PEEM, assay to identify spicule proteins that may play a role in stabilizing various mineral phases, and found that the most abundant occluded matrix protein in the sea urchin spicules, SM50, stabilizes ACC · H(2)O in vitro.

Crystal lattice tilting in prismatic calcite.

We analyzed the calcitic prismatic layers in Atrina rigida (Ar), Haliotis iris (Hi), Haliotis laevigata (HL), Haliotis rufescens (Hrf), Mytilus californianus (Mc), Pinctada fucata (Pf), Pinctada margaritifera (Pm) shells, and the aragonitic prismatic layer in the Nautilus pompilius (Np) shell. Dramatic structural differences were observed across species, with 100-µm wide single-crystalline prisms in Hi, HL and Hrf, 1-µm wide needle-shaped calcite prisms in Mc, 1-µm wide spherulitic aragonite prisms in Np, 20-µm wide single-crystalline calcite prisms in Ar, and 20-µm wide polycrystalline calcite prisms in Pf and Pm. The calcite prisms in Pf and Pm are subdivided into sub-prismatic domains of orientations, and within each of these domains the calcite crystal lattice tilts gradually over long distances, on the order of 100 µm, with an angle spread of crystal orientation of 10-20°. Furthermore, prisms in Pf and Pm are harder than in any other calcite prisms analyzed, their nanoparticles are smaller, and the angle spread is strongly correlated with hardness in all shells that form calcitic prismatic layers. One can hypothesize a causal relationship of these correlated parameters: greater angle spread may confer greater hardness and resistance to wear, thus providing Pf and Pm with a structural advantage in their environment. This is the first structure-property relationship thus far hypothesized in mollusk shell prisms.

Crystal nucleation and near-epitaxial growth in nacre.

Nacre is the iridescent inner lining of many mollusk shells, with a unique lamellar structure at the sub-micron scale, and remarkable resistance to fracture. Despite extensive studies, nacre formation mechanisms remain incompletely understood. Here we present 20-nm, 2°-resolution polarization-dependent imaging contrast (PIC) images of shells from 15 mollusk species, mapping nacre tablets and their orientation patterns. These data show where new crystal orientations appear and how similar orientations propagate as nacre grows. In all shells we found stacks of co-oriented aragonite (CaCO3) tablets arranged into vertical columns or staggered diagonally. Near the nacre-prismatic (NP) boundary highly disordered spherulitic aragonite is nucleated. Overgrowing nacre tablet crystals are most frequently co-oriented with the underlying aragonite spherulites, or with another tablet. Away from the NP-boundary all tablets are nearly co-oriented in all species, with crystal lattice tilting, abrupt or gradual, always observed and always small (plus or minus 10°). Therefore aragonite crystal growth in nacre is near-epitaxial. Based on these data, we propose that there is one mineral bridge per tablet, and that "bridge tilting" may occur without fracturing the bridge, hence providing the seed from which the next tablet grows near-epitaxially.

Mollusk shell nacre ultrastructure correlates with environmental temperature and pressure.

Nacre, or mother-of-pearl, the tough, iridescent biomineral lining the inner side of some mollusk shells, has alternating biogenic aragonite (calcium carbonate, CaCO(3)) tablet layers and organic sheets. Nacre has been common in the shells of mollusks since the Ordovician (450 million years ago) and is abundant and well-preserved in the fossil record, e.g., in ammonites. Therefore, if any measurable physical aspect of the nacre structure was correlated with environmental temperatures, one could obtain a structural paleothermometer of ancient climates. Using X-ray absorption near-edge structure (XANES) spectroscopy, Photoelectron emission spectromicroscopy (PEEM), and X-ray linear dichroism we acquired polarization-dependent imaging contrast (PIC) maps of pristine nacre in cross-section. The new PIC-map data reveal that the nacre ultrastructure (nacre tablet width, thickness, and angle spread) is species-specific in at least eight mollusk species from completely different environments: Nautilus pompilius, Haliotis iris, Haliotis rufescens, Bathymodiolus azoricus, Atrina rigida, Lasmigona complanata, Pinctada margaritifera, and Mytilus californianus. Nacre species-specificity is interpreted as a result of adaptation to diverging environments. We found strong correlation between nacre crystal misorientations and environmental temperature, further supported by secondary ion mass spectrometry measurements of in situ δ(18)O in the nacre of one shell. This has far-reaching implications: nacre texture may be used as a paleothermometer of ancient climate, spanning 450 million years of Earth's history.

Validation of a Retrospective Computing Model for Mortality Risk in the Intensive Care Unit.

OBJECTIVE: To compare the predictive performance of Epic Systems Corporation's proprietary intensive care unit (ICU) mortality risk model (IMRM) with that of the Acute Physiology and Chronic Health Evaluation (APACHE) IV score. METHODS: This is a retrospective cohort study of patients treated from January 1, 2008, through January 1, 2018. This single-center study was performed at Mayo Clinic (Rochester, MN), a tertiary care teaching and referral center. The primary outcome was death in the ICU. Discrimination of each risk model for hospital mortality was assessed by comparing area under the receiver operating characteristic curve (AUROC). RESULTS: The cohort mostly comprised older patients (median age, 64 years) and men (56.7%). The mortality rate of the cohort was 3.5% (2251 of 63,775 patients). The AUROC for mortality prediction was 89.7% (95% CI, 89.5% to 89.9%) for the IMRM, which was significantly greater than the AUROC of 88.2% (95% CI, 87.9% to 88.4%) for APACHE IV (P<.001). CONCLUSION: The IMRM was superior to the commonly used APACHE IV score and may be easily integrated into electronic health records at any hospital using Epic software.

Mechanism of calcite co-orientation in the sea urchin tooth.

Sea urchin teeth are remarkable and complex calcite structures, continuously growing at the forming end and self-sharpening at the mature grinding tip. The calcite (CaCO(3)) crystals of tooth components, plates, fibers, and a high-Mg polycrystalline matrix, have highly co-oriented crystallographic axes. This ability to co-orient calcite in a mineralized structure is shared by all echinoderms. However, the physico-chemical mechanism by which calcite crystals become co-oriented in echinoderms remains enigmatic. Here, we show differences in calcite c-axis orientations in the tooth of the purple sea urchin ( Strongylocentrotus purpuratus ), using high-resolution X-ray photoelectron emission spectromicroscopy (X-PEEM) and microbeam X-ray diffraction (muXRD). All plates share one crystal orientation, propagated through pillar bridges, while fibers and polycrystalline matrix share another orientation. Furthermore, in the forming end of the tooth, we observe that CaCO(3) is present as amorphous calcium carbonate (ACC). We demonstrate that co-orientation of the nanoparticles in the polycrystalline matrix occurs via solid-state secondary nucleation, propagating out from the previously formed fibers and plates, into the amorphous precursor nanoparticles. Because amorphous precursors were observed in diverse biominerals, solid-state secondary nucleation is likely to be a general mechanism for the co-orientation of biomineral components in organisms from different phyla.