Ultrastructure and Crystallography of Nanoscale Calcite Building

Mar 12, 2014 - Synopsis. The ultrastructure and crystallography of Rhabdosphaera clavigera heterococcoliths are characterized by electron microscopy t...
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Ultrastructure and Crystallography of Nanoscale Calcite Building Blocks in Rhabdosphaera clavigera Coccolith Spines Renée van de Locht,*,† Thomas J. A. Slater,‡ Andreas Verch,† Jeremy R. Young,§ Sarah J. Haigh,‡ and Roland Kröger*,† †

Department of Physics, University of York, Heslington, York, YO10 5DD, U.K. Schools of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. § Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, U.K. ‡

S Supporting Information *

ABSTRACT: Coccolithophores create an intricate exoskeleton from nanoscale calcite platelets. Shape, size, and crystal orientation are controlled to a remarkable degree. In this study, the structure of Rhabdosphaera clavigera is described in detail for the first time through a combination of electron microscopy techniques, including three-dimensional electron tomography. The coccolithophore exhibits several micrometer long 5-fold symmetric spines with diameters of approximately 0.5 μm. The nanorystals constituting the spine are arranged radially along the longitudinal axis, protruding from the almost flat disks that form the coccosphere. The stem of the spine is shown to consist of {104} calcite rhombohedra single crystalline platelets, arranged in five separate spiral “staircases”. The spine tip shows 15 structural elements: five large “panels” protruding outward along the lateral plane and five leaf-shaped smaller units which form the topmost steps of the staircases. The outer tip consists of five long thin platelets protruding along the length of the spine axis. This feature extends downward into the spine-core. This core-feature may serve as a base for crystal nucleation and assembly analogous to the proto-coccolith ring in the V/R growth model (Young J. R. et al. Nature, 1992, 356, 516−518). However, we find significant dissimilarities of the crystal elements constituting the spine in comparison to that model.

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INTRODUCTION Coccoliths are micrometer-scale disks built from single crystal calcite units and are produced by unicellular marine algae, called coccolithophores. They belong to the phylum Haptophyta. Coccolithophores play an important role in the ocean carbon cycle through carbon fixation during CaCO3 mineralization.2,3 They occur in great numbers and form a significant portion of the total marine primary production in open ocean environments. Furthermore, they are of great geological importance as fossil markers to determine sediment age in both rock formations and ocean floor drill cores. The complex structure of the mineral produced by these organisms shows little resemblance to inorganic mineral calcite, and it is yet impossible to synthetically reproduce such a complex nanocrystal assembly under controlled conditions. Coccoliths are exceptional in being able to fully control both cand a-axis orientation. Such stark control in creating functional structures motivates a detailed study of the microstructure with the aim of unveiling fundamental aspects of nanocrystal assembly in biological systems. The most common type of coccoliths is heterococcoliths, disk-shaped structures formed by radially arranged, interlocking calcite crystals. These disks are usually between 2 and 10 μm in size, depending on the species. The separate disks, or © 2014 American Chemical Society

coccoliths, interlock to form a hollow sphere, called the coccosphere. Hence, the individual coccoliths often have a concave proximal (inner) surface (Figures 1 and 2). Previous studies, using polarized light, scanning electron microscopy, and cytological transmission electron microscopy have shown that the coccoliths are formed within intracellular golgi-derived vesicles with growth commencing with nucleation of a “proto-coccolith ring” of minute crystallites with alternating crystallographic orientation, probably controlled by an organic template. Subsequently, these crystallites grow and interlock to form the elaborate final coccolith morphologies after which the coccolith vesicle migrates to the edge of the cell and merges with the cell membrane resulting in the expulsion (exocytosis) of the coccolith.4,5 Supersaturation enabling rapid mineralization has been found, in some species, to be regulated by acidic polysaccharides that bind Ca2+ which are transported into the vesicle.2,5 After the coccolith is fully formed, the remaining polysaccharide complexes disintegrate, and the organic residues bind to the crystals forming a coating.5 However, as of yet no such data exist for Rhabdosphaera clavigera. Received: December 11, 2013 Revised: February 11, 2014 Published: March 12, 2014 1710

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Figure 1. SEM images of Rhabdosphaera clavigera. (A) Complete coccosphere. (B) Detached coccolith with spine. (C) Side view of the tip of the spine.

mineralization mode but not much is known about the crystallographic orientation of its crystal units and ultrastructure. In this study, we investigate the crystallographic orientation of the calcite crystal elements of R. clavigera spines by transmission and scanning electron microscopy (TEM and SEM) in conjunction with selected area electron diffraction (SAED) analysis. We combine this with a nanoscale threedimensional (3D) tomographic reconstruction using high angle annular dark field (HAADF) imaging by scanning transmission electron microscopy (STEM). The focus of this study lies in the detailed understanding of the nanocrystals arrangement and the characterization of the crystal units. This may have implications for better understanding the formation mechanisms involved.

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EXPERIMENTAL DETAILS

Specimens. We studied R. clavigera heterococcoliths extracted from a plankton filter sample from the Alboran Sea, W. Mediterranean (MATER 15-12B 5m LM (stub 276)). The specimens were observed directly from the filter membrane in the SEM, with the deposition of a thin 3 nm carbon coating to prevent surface charging. For the TEM and HAADF-STEM tomography specimens were selected using an optical microscope (50× objective; large working distance) in combination with a micromanipulator device. Coccospheres were lifted by an approximately 10 μm glass needle to be deposited onto a TEM lacy carbon grid (200 mesh, Cu). In addition, a single spine was extracted and deposited onto a similar TEM grid for electron tomography investigations. Plane-Wave Transmission Electron Microscopy. TEM images and selected area electron diffraction patterns were obtained utilizing a JEM 2011 (LaB6 filament) and JEM 2200 FS (spherical aberration corrected, FEG) TEM at 200 kV, at the York JEOL Nanocentre. The selected area apertures used for diffraction have an area size of approximately 30 nm. Diffraction patterns were analyzed using JEMS software.8 Electron energy loss spectroscopy (EELS) using an in-column Omega filter setup in the JEM 2200 FS was performed for elemental analysis of the crystalline and amorphous phases. Secondary electron images were collected using an FEI Sirion S-FEG FESEM. Z-Contrast STEM Tomography. Scanning transmission electron tomography was performed on an FEI Tecnai F30 operating at 300 kV using a Fischione 2020 single tilt tomography holder and a highangular annular dark field detector (HAADF). The tilt series was acquired using Xplore3D acquisition software at angular increments of 2° from −60° to +60° and 1° between ± (60° to 70°). Alignment and reconstruction were performed using the IMOD software package.9 The tilt series was aligned using cross correlation and a patch tracking

Figure 2. SEM micrographs of coccolith disk. (A) Top view of a coccolith without spine, with interlocking crystals in the upper rim (yellow) and imbricated radial cycles of central elements (red). (B) Detail of coccolith underside with indicated upper rim elements (yellow), lower rim elements (green), and radial cycles of tile elements at the center (red). (C) Detached coccolith with spine, viewed from below.

Young et al. suggested a model of coccolith crystal ultrastructure which can be applied to most heterococcoliths: the V/R model.1,6 It is based on the observation that the coccolith rims are characteristically formed of two cycles of interlocking crystal units, one with subvertical c-axes, termed Vunits, and the other with subradial c-axes, R-units. These originate as alternating nuclei in the proto-coccolith ring. However, the central area of the coccoliths may show rather different structures. R. clavigera is a dramatic example of this, exhibiting a central area of spiral cycles of small units with a 5fold symmetric spine several micrometers long (diameter of approximately 0.5 μm). Crystals are arranged radially along the longitudinal axis, protruding from the center of the concave disks forming the sphere structure (see Figure 1). The rim of the disk in R. clavigera does appear to comply with the V/R structure model. It has been suggested that the central area and the spine are both separately nucleated7 or via a different 1711

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Figure 3. SAED patterns accompanying the TEM bright field micrograph, white circles indicate the position of the selected area aperture. (A) Diffraction pattern of left panel (I) indicating the {104} family of planes in reciprocal space (long yellow line), which is perpendicular to the plane it represents in real space (short yellow line). (B) The same is indicated for the right panel (II). (C) Projection of the {104} planes in reciprocal and real space onto the image shows that the panel crystals are delimited on both top and side edge by the {104} plane (or {1014̅ } plane in 4-fold notation: (hkl) = (hk, (−h−k), l)). method before reconstruction using weighted back-projection. Visualization was performed using ImageJ and Avizo software.

coccoliths. Surprisingly, the spines are always consistent in quantity (i.e., five equally long staircases and the tip has 15 units), size, and shape of crystal elements, whereas the central parts of the coccolith plate frequently contain irregularities. Spine Morphology. The spine structures protruding from R. clavigera coccoliths in our samples are 5.0−5.5 μm in length and have a diameter of approximately 500 nm. A detailed TEM analysis in conjunction with electron diffraction and SEM studies show that the spine consists of single crystal calcite elements (Figure 3) that are arranged in a 5-fold chiral symmetric structure around its longitudinal axis. The calcite crystal platelets are organized in five discrete “staircase” structures. The long crystal platelets that comprise the staircase steps are approximately 80 × 200 × 1000 nm and have a 12− 14° angle between steps. In total, five staircases revolve around the spine’s longitudinal axis and show twisting toward the tip, each staircase effecting a near rotation of approximately 280° (Figure 1). The chirality of the spine structure has been observed to be constant over more than 50 R. clavigera coccospheres within the SEM image collection of the Natural History Museum London. These observations were made from diverse samples on which chirality could be determined reliably, including specimens from diverse localities in the North and South Atlantic, Pacific Ocean, Caribbean, and the Mediterranean Sea. Similarly, the pointed outer tip itself always consists of five elements. Bright-field transmission electron microscope images show the presence of a homogeneous 25 nm thick and apparently

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RESULTS AND INTERPRETATION Coccolith Base Plate: Rim and Central Area. The coccolith rim consists of a cycle of interlinking crystal units (roughly 200 nm × 300 nm × 50 nm); the upper rim elements appear to have an exterior downward kink to interlock with the lower rim elements toward the center as shown in Figure 2. A yellow patch in the images indicates the crystal unit. The rim is attached to the central elements via the lower rim elements which can be observed in the bottom view (Figure 2B,C; indicated in green). These rodlike elements are inclined by approximately 20° angle relative to the upper rim elements, tilted toward the center, and are approximately 100 nm × 300 nm × 50 nm in size. For every upper rim element, there is a lower rim element. This rim structure is conserved across the Rhabdosphaeraceae.6,10,11 The upper rim elements are V-units and the lower rim elements are thought to be R-units, although this cannot be proven by classic low magnification techniques. The center is formed by radial cycles of imbricate tiles, with the left tile consistently overlapping its right-hand neighbor, forming an anticlockwise spiral toward the center (Figure 2A; indicated in red). The tiles look almost square in top view and are about 200 nm × 200 nm × 50 nm, although they show irregularities in both shape and size as well as in overlap step. The innermost central tiles appear to push each other up, forming a crest that serves as the spine-base in spine-bearing 1712

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amorphous layer surrounding each calcite platelet as shown in Figure 4B. We analyzed this layer using electron energy loss

spectroscopy (EELS, see Supplementary Figure S1, Supporting Information) and found that it does not contain calcium but does exhibit a strong carbon signal. This suggests that the layer does not consist of amorphous calcium carbonate (ACC). ACC has often been observed as a precursor phase in biomineral systems12−15 but has not been observed in coccoliths. The presence of oxygen is also suggested, which has low-loss peaks at approximately 5 and 10 eV. The spectrum of the crystal has a peak at 10 eV, and the amorphous layer has a poorly defined peak at 5 eV, which may result from different atomic arrangements in crystalline and amorphous material. We speculate that the amorphous layer is organic, presumably related to polysaccharides previously identified in other coccoliths species.4,16−18 Such organic coatings were first observed using TEM in 1955 by Halldal and Markali in decalcified holococcoliths.19 SEM imaging of the bottom of the detached coccolith (Figure 2C) suggests that the spine has a hollow core where it meets the plate. TEM images of the spine tip show a lighter contrast in the center (Figure 3C), suggesting that a hollow or low density noncrystalline center persists along the length of the spine. Indeed, a 3D tomographic reconstruction using HAADF STEM imaging confirms that the core of the spine consists of a material that has much lower electron density compared to the surrounding crystalline platelets or might even be hollow (Figure 5). Tip of the Spine. SEM imaging (Figure 4A) combined with tomographic data (Figure 5) reveals that the pointed tip of the spine consists of five, roughly 500 nm long intertwining platelets, extended parallel to the long axis of the rod. A crosssection perpendicular to the long-axis direction of the spine, just below the base of the pointed tip crystals, shows that this region consists of 15 individual elements (Figures 5A and 6A). Furthest from the core, there are five broad side “panels” (roughly 100 nm × 200 nm × 400 nm in size), orientated with their longest edge parallel to the long axis of the spine and

Figure 4. (A) SEM image of the tip of the spine. The angles between facets were determined with a tolerance of ±5°; the crossed circle indicates that the c-axis is roughly oriented into the page. (B) TEM bright field image of the side view of a crystal panel showing the position of the acute angle at the outer edge of the crystal panel. Surrounding the panel is a homogeneous amorphous layer of ±25 nm as shown in detail in the inset. (C) Schematic drawing of {104} calcite rhombohedra with obtuse and acute angles of 102° and 78°, respectively. (D) Calcite unit cell with (104) planes indicated by the green colored areas on the left; the dashed line indicates the edge of the unit cell and does not relate to angles in the crystal facets. The right schematic shows the atomic arrangement viewed along the [104] direction (Ca: blue, C: gray, and O: red).

Figure 5. 3D STEM tomography reconstruction of the spine tip. (A) Cross section perpendicular to the long-axis of the spine at the base of the pointed outer-tip crystals, revealing a distinct arrangement of five nanocrystals (assigned by a white arrow, the contour of one of the nanocrystals is highlighted by a dashed line). (B) Segmented crystal units forming the lower part of the spine tip. This consists of 10 units; however due to tilting limitations of the tomography holder and shadowing from the lacy carbon substrate, only nine elements could be identified using this method. The large purple segment consists of two crystals as can be interpreted from TEM and SEM data and is indicated in the schematic with a dashed line. (C) Cross section along the longitudinal axis of the spine, with the darker core revealing a continuous void or presence of less electron dense material at the center of the spine. The arrows and brackets highlight crystal elements that extend into the core of the structure. 1713

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Figure 6. Schematic of ultrastructure of the spine. (A) Frontal view of tip, without top extension, showing panels, leaf-shaped crystals (i.e., top of crystal staircases), and centrally placed nanocrystals. (B) Side view of the top of a staircase with the panel crystal intertwining with the terminal staircase crystal. Crystal surfaces are indexed; the red curve indicates the obtuse angle (102°), and the green curve indicates the acute angle (78°) in {104} calcite facets.

Figure 7. Schematic two-dimensional representation of two coccolith disk cross sections. (A) A generalized model for species without central spine structures. The coccolith has an overall concave proximal (inner) shape but has a central depression, which, depending on the species, is void, partly filled with crystals or completely crystalline. (B) A generalized model for R. clavigera, the V/R rim units are indicated as well as the central elements that do not conform the V/R growth model. The coccolith has an overall concave proximal shape and has a central protrusion leading to possible spine formation.

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crystal units of the tip. It is also suggested that this core-like structure is repeated lower down in the spine as can be observed in the cross-sectional slices through the tomogram. Possibly, these central elongated nanoplatelets act as a base onto which the other crystal units have assembled or nucleated, guiding the overall crystal orientation in a similar way as the proto-coccolith ring. Whether this nanoplatelet core structure is in itself guided by an organic template, as has been found for the proto-coccolith ring, remains unclear. It has, however, been observed by light microscopy, that the coccoliths with attached spines are, like all heterococcoliths, fully formed within the cell membrane (see Supplementary Figure S2, Supporting Information). A remarkable feature in R. clavigera, and coccoliths in general, is the tight size and shape control of each cycle/group of elements. The rim units have a fixed size, as do the central tiles of the base plate. All the spine staircase elements are nearly identical to each other, as are the tip side panels and outer tip platelets. In rims formed of V/R units, their shape and size are largely a function of the original spacing of the nuclei. Thus, we can surmise for R. clavigera that the upper and lower rim elements were originally evenly spaced around the protococcolith ring, and subsequent regular growth rates of these crystals, until the interlock, resulted in the consistent size of these elements. However, with the central area of the base plate and the spine structure, it is less obvious how nucleation spacing and orientation are regulated. The core platelets in R. clavigera are a first indication toward a controlled templating mechanism for central protrusion features, but it is far from conclusive. The element size and shape in central protruding features appear, if anything, even more strongly regulated than in rim units, as can also be seen in related species such as Algirosphaera robusta and Discosphaera tubifera. Another influential factor in size and shape control is the crystallization in confinement. The mechanical stresses applied by the expanding vesicle during crystal growth, or in other words “the shaping by cytoskeletal pull”, as well as organic molecules acting as crystal growth inhibitors have been suggested to play an important role in shape and size control in coccolith crystal formation.7 Properties of the Basic Building Blocks: Calcite (104) Facets. Synthetic crystals grow if no additives are used during the precipitation, perfect rhombohedra with stable {104} faces. However, in coccoliths, like in most other biomineralizing systems, the mineralization is largely controlled by the biochemistry of the organism leading to an intricate nanocrystal assembly thought to be associated with coccolith associated polysaccharides (CAPs).17 Although the exact process is not understood, it is commonly accepted that the crystal planes on which biomolecules are preferentially adsorbed are expressed via inhibition of these facets. Hendriksen et al.17 found that CAPs from Emiliania huxleyi interact differently with crystal surface sites defined by acute rather than obtuse angles and that acute sites are blocked during dissolution and growth. In this way, the CAPs can lead stable {104} facets to extend on the obtuse edges, thus stimulating the development of faces with lower angles to the c-axis. However, in the R. clavigera spine the stable “easy” {104} rhombohedra structure seems to be preferred, even though a homogeneous organic coating around each crystal element is present. Furthermore, no crystal steps were observed on the elements constituting the spine. It appears that, the complex laterally extended structure aside, the calcite crystals are significantly different from those nucleated

inclined such that they protrude from the core. Closer to the spine core there are five smaller “leaf”-shaped crystals which form the final step of the staircase toward the pointed tip (Figure 6A). The side panels and leaf crystals occur in turn at a 90° angle to each other. The panel crystals cut through, or are intergrown with, the penultimate crystal in each staircase (Figure 6B). Closest to the core there are five smaller units, which interconnect the “panel” and “leaf” crystals (Figures 5A and 6A). The tomographic data reveals that these small units are in fact the base of the outer tip crystal platelets which are extended downward into the core of the spine. Furthermore, the tomographic cross section along the long-axis of the spine suggests that a similar platelet core structure is present lower down the spine (Figure 5C). This appears to be a repeating feature, albeit no tomography data exist for this section of the spine. SAED patterns show that the side panels are delimited by the calcite {104} planes on the top and side edges. In combination with angle measurements from several panels over different spines, it is concluded that they have a {104} calcite rhombohedra structure (Figures 3 and 4). Angle measurements from multiple leaf-shaped crystal elements of diverse coccosphere spines exhibit an average obtuse and acute angle which suggests that the leaves are also {104} rhombohedra (Figure 4). The atomic arrangement at this particular crystallographic orientation is shown schematically in Figure 4D. It was not possible to obtain conclusive SAED data from the leaf-shaped crystals (i.e., staircase platelets) as well as the outer tip crystals, since they sterically overlapped in the beam direction. Hence, an unambiguous analysis of the resulting superimposed diffraction patterns was not possible. A focused ion beam sample preparation approach could be considered for further analysis of these elements in the future.

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DISCUSSION Summary of the Ultrastructure. The coccolith rim and disk are constituted of three structural elements as indicated in Figure 1: A upper rim of interlocking complex-shaped (exterior downward kink) crystals (analogous to V-units), connected via lower rim elements (interpreted as R-units) to central imbricated tiles forming an anticlockwise spiral around the disk center. At the center, the imbricated tiles push each other up to form a small crest. This acts as the base of the spinein those coccoliths that exhibit a spinealthough the elements are distinct from the spine elements. A schematic drawing of a cross section of the disk is shown in Figure 7B. The rim structure conforms to the general V/R growth model and nucleates intracellularly from a proto-coccolith ring as described in refs 1, 6, 7, 20, and 21. In contrast to the complex rim elements, the spine is formed from an arrangement of simple calcite {104} rhombohedral building blocks at constant angles to each other in a chiral 5fold symmetric structure; this is drawn schematically in Figure 6. Three building blocks can be distinguished as (1) elongated platelets (80 nm × 200 nm × 1000 nm) making up five staircases (the topmost platelets are the five “leaf”-shaped elements in the tip), (2) five broad panels (±100 nm × 200 nm × 400 nm) that protrude into the lateral plane of the tip of the spine like the petals of a flower, and (3) five elongated platelets (±500 nm in length) extending along the longitudinal axis making up the outer tip. The bottommost part of the outer tip crystal platelets appears to form a core structure for the other surrounding 1715

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could be related to a confinement mechanism or certain organic molecules controlling the ion transport for CaCO3 precipitation, as was suggested earlier for shape and size control of crystal elements. From our structural and crystallographic investigation of the complex ultrastructure of the spine, it is a feature we can highlight from our observations, but determining a controlling formation mechanism would require an extensive biochemical, cytological and cytoskeletal study. We can nevertheless conclude from our data on R. clavigera and the structure of other well-described species that a central protrusion in the coccolith disk appears to be a prerequisite for spine formation. Many coccolith species instead have a central depression, as for example in Emiliana huxleyi. Although Emiliana and others have a widely varying physical appearance, in general, they exhibit a central area that forms a depression in the disk; this area can be filled or partly filled with crystal elements or be void. In contrast, species that form protruding spines, hoods, or other features such as in the Rhabdosphaeraceae, generally have an “up-sloping” central area from the rim inward that culminates in a crest promoting an upward extending, central canal, around which a central feature may develop. These concepts are illustrated by a schematic in Figure 7. The formation of central protruding features seems therefore to be intimately linked to the ultrastructure of the rim formation itself. This reinforces the argument that both mineralization modesthat of the V/R rim and the disjunct central areaare related although the crystal units themselves are distinct. It is interesting to note that the rim structure is conserved across the Rhabdosphaeraceae; by contrast the central areas show widely varying structures: spines in Acanthoica and Rhabdosphaera, a hood in Algirosphaera, and a trumpet in Discosphaera. But all the central individual elements are simple in shape and exceptionally regular in size. Five-Fold Chiral Symmetries. The pentameral symmetry seems to be predominantly expressed in the central protrusions of coccoliths as in the staircase and tip of R. clavigera. The symmetry appears to be predetermined from the crystal nucleation stage, as crystals grow in a regular fashion and interlock at a later stage without changing their growth direction (side panels intergrown with penultimate staircase elements). Also in Cyrtosphaera the papillae at the tip of the spines have a 5-fold symmetry. Other calcite biomineralizing systems known to have a 5-fold symmetry are the echinoderms, for example, sea urchins, starfish, and brittle stars. However, this symmetry is only superimposed on a primary 2-fold symmetry.24 In nanoparticles, adopting a 5-fold symmetry often reduces surface energies, which may overcome internal strain caused by some lattice distortions, this is often achieved through multiple twinning, which results from crystal subgrains sharing a common crystallographic plane in a cyclic fashion.25 On the microscale, synthetic multiple polymeric nanobristles attached to a substrate have been shown to naturally assemble in a chiral helical pattern upon drying, by introducing more flexibility (bristle length) and adhesion to the materials.26 Assembly in a chiral fashion is energetically advantageous and by introducing a controlled preassembly tilt the neighboring nanostructures can only approach one another from one side and thus induce uniform left-hand or right-hand spiralling depending on the tilt.26 Another study showed that the amount of left-hand to righthand chiral helical inorganic crystals could be tuned by the

via the V/R model. It seems that the organism compensates for the complex lateral structure with the use of simple, stable {104} rhombohedral building blocks, following a 5-fold symmetric model. An explanation for the role of the organic coating on R. clavigera spine crystals may be related to the findings of Cooke et al.,22 as the coating does not always seem to enable a surface stepping mechanism or the promotion of faces with lower angles to the c-axis. Their work showed, based on molecular dynamics simulations and atomic force microscopy of ethanol/ water mixtures with calcite crystals, that a stable and highly ordered organic layer attaches to the {104} calcite face making it difficult for incoming ions to penetrate to the crystal surface. The organic coating on the spine may help to prevent recrystallization on the {104} surface and decrease direct calcite/seawater interactions. Ultrastructure of R. clavigera in Context. As already mentioned, the ultrastructure of the R. clavigera spine is distinct from that of most coccoliths and does not comply with the V/R model. Note that within the Rhabdosphaeraceae family many other species show a disjunct central structure often with elongated central protrusions (e.g., Algirosphaera, Acanthoica, Anacanthoica, and Discosphaera, among others). Algirosphaera robusta for example, has a central structure in the form of a prominent “hood”.23 Probert et al.23 described the hood as consisting of several cycles of elements, with “tiles” positioned at regular interfaces and angles spiraling to the top of the structure. The inner part of the hood was found to be void. Although having a very different physical appearance, there seem to be some similarities between the building blocks. Both the “tiles” mentioned by Probert et al.23 and the staircase platelets form a spiral structure along an elongated axis, albeit with a much less pronounced rotation in the case of A. robusta. These crystals are found at regular intervals and angles of each other, forming the bulk of the protruding central structure of the coccolith. A. robusta was also observed to have vertical rods at its center, and both structures appear to have an inner cavity. It is probable that both species have a similar biomineralization mode for these central protruding features. A summary of the crystal elements present in both species is given in Table S1 of the Supporting Information. As mentioned before, the mineralization mode has some strong dissimilarities to the V/R coccolith rim nucleation mechanism, as was also previously observed by Probert et al.23 Most importantly, the R. clavigera spine is formed of discrete simple and exceptionally regular crystal units that never grow in noncrystallographic directions, contrary to the complex-shaped rim elements. However, there are also correlative features between the spine and the rim structures suggesting an altered but related nucleation mechanism. First, the spine consists of regular spiraling elements. Second, a nanocrystal platelet corestructure is observed in the spine, possibly acting as a base for the outer crystalline units, guiding the overall structural and crystallographic orientation, in a similar fashion as the protococcolith ring in the V/R model. However, this is purely hypothetical at this stage. Interestingly, only about half the R. clavigera coccoliths develop a central spine structure, while others do not (Figures 1 and 2). This is a regular feature of the species and the nonspine bearing coccoliths form an outer layer covering gaps between spine-bearing coccoliths. Thus, they fulfill a particular role in the coccosphere architecture, and therefore spine development versus spine-inhibition is likely biologically controlled. This 1716

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addition of chiral molecules, such as D- and L-glutamic acids.27 The chiral molecules recognize the mirror surface of triclinic crystals and change the growth behavior of the helical morphology.27 Chiral structures thus seem, although their appearance is complex, relatively easy to be established by controlling the initial nucleation template.

AUTHOR INFORMATION

Corresponding Authors

*(R.vdL.) E-mail: [email protected]. *(R.K.) E-mail: [email protected]. Tel: + 44 (0)1904 324622. Fax: +44 (0) 1904 322214. Webpage: http://wwwusers.york.ac.uk/∼rk523/index.html.

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Notes

CONCLUSIONS The R. clavigera coccolith rim consists of complex-shaped interlinked crystals which conform to the V/R growth model suggested by Young et al.1 The central area of the coccolith base plate is made of imbricated tiles that spiral in an anticlockwise direction toward its center, forming the base of the spine. Not all R. clavigera coccoliths have spines; some central area spirals only result in a small crest. The spine is constituted by discrete simple {104} calcite rhombohedra, arranged in a winding staircase fashion along a longitudinal axis with a consistent 5-fold chiral symmetry. The tip of the spine has a few extra elements, namely, laterally protruding side panel crystals and longitudinally extended outer top platelets. Conversely, the latter may be a reoccurring feature, partway observed lower down in the core of the spine. The discrete simple elements, that never grow in noncrystallographic directions, of the spine are dissimilar from the complex-shaped intergrown coccolith rim units. Furthermore, we find that the spine crystals show a homogeneous 25 nm wide organic coating. We hypothesize that the core-platelets in the R. clavigera spine may act as a template for the outer crystalline units, guiding the overall structural assembly and crystallographic orientation, like the proto-coccolith ring in the rim structure. As each tip side panel is intergrown with the penultimate corresponding staircase element, it appears that the crystallographic growth direction is predetermined from the nuclei stage. We conclude, therefore, that the biomineralization mode for central protruding features in coccolith formation is separate and different from, but related to the V/R model, with exceptionally stark control of crystal orientation and size. Additionally, we derive from our data and other welldocumented coccolith species that a central protrusion is a prerequisite for spines and other elaborate central protrusions to develop, as coccolith species with a central depression do not develop such structures. Finally, although a crystal templating mechanism appears to guide the structural assembly in R. clavigera and tight size and shape control may be linked to growth in confinement, the data presented here do not allow for a derivation of a detailed formation mechanism.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank UK Engineering and Physical Sciences Research Council (EPSRC) (Grant No. EP/I001514/1) for funding and the York JEOL Nanocentre for the use of their facilities. The School of Materials of the University of Manchester is thanked for making the STEM tomography instrument available for this study. T.J.A.S. and S.J.H. thank the EPSRC, NowNANO doctoral training centre for funding support.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1. (A) Low loss EELS spectra of three point measurements on a panel crystal from the tip of the spine. (B) TEM bright field micrograph of the analyzed crystal. Figure S2. Light microscope images of R. clavigera showing a fully developed spine-bearing coccolith inside the coccosphere before exocytosis. Table S1. Summary of the crystal elements present in heterococcoliths of R. clavigera and A. robusta, in the base plate and the central disjunct area. This information is available free of charge via the Internet at http://pubs.acs.org/. 1717

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Article

(24) Cuif, J.-P.; Dauphin, Y.; Sorauf, J. E. In Biominerals and Fossils through Time; Cambridge University Press: Cambridge, 2011; Chapter 4, pp 244−245. (25) Hofmeister, H. In Encyclopedia of Nanoscience and Nanotechnology; American Scientific Publishers: Valencia, CA, 2004; Vol. 3, pp 431. (26) Aizenberg, J. MRS Bull. 2010, 323−330. (27) Oaki, Y.; Imai, H. J. Am. Chem. Soc. 2004, 126, 9271−9275.

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