Many organisms orchestrate the controlled precipitation of minerals. This physiological process takes place at ambient conditions, using soluble ions as building blocks. A widespread strategy for ...such crystallization processes is using a multistep route, where the initial phase is metastable and gradually transforms into the mature mineral phase. Even though the maturation of these intermediate phases has been intensively studied, it remains unclear how the initial, far from equilibrium phase can form within the cellular context. A model system for controlled biomineralization is the production of coccoliths by marine microalgae. Coccoliths are calcium carbonate crystalline arrays that form within the intracellular environment, at very low calcium concentrations. Here, we used coccolith-derived and synthetic polymers to study, in vitro, the chemical interactions between calcium ions and organic macromolecules that precede coccolith formation. We used in situ analyses, including state-of-the-art cryo-electron tomography and liquid-cell atomic force microscopy, to study the interactions in bulk solution and on organic surfaces simultaneously. The results unveil a chemical process in which a functional surface induces the precipitation of a polymer–Ca dense phase, or a coacervate, at chemical conditions where precipitation in solution is kinetically inhibited. This strategy demonstrates how organisms can form dense Ca-rich phases from the submillimolar concentration of calcium within organelles. This Ca-rich phase can then transform into a mineral precursor in a subsequent step, without posing challenges to cellular homeostasis.
Multistep mineralization processes are pivotal in the fabrication of functional materials and are often characterized by far from equilibrium conditions and high supersaturation. Interestingly, such ...‘nonclassical’ mineralization pathways are widespread in biological systems, even though concentrating molecules well beyond their saturation level is incompatible with cellular homeostasis. Here, we show how polymer phase separation can facilitate bioinspired silica formation by passively concentrating the inorganic building blocks within the polymer dense phase. The high affinity of the dense phase to mobile silica precursors generates a diffusive flux against the concentration gradient, similar to dynamic equilibrium, and the resulting high supersaturation leads to precipitation of insoluble silica. Manipulating the chemistry of the dense phase allows to control the delicate interplay between polymer chemistry and silica precipitation. These results connect two phase transition phenomena, mineralization and coacervation, and offer a framework to achieve better control of mineral formation.
Biomineralization processes exert varying levels of control over crystallization, ranging from poorly ordered polycrystalline arrays to intricately shaped single crystals. Coccoliths, calcified ...scales formed by unicellular algae, are a model for a highly controlled crystallization process. The coccolith crystals nucleate next to an organic oval structure that was termed the base plate, leading to the assumption that it is responsible for the oriented nucleation of the crystals via stereochemical interactions. In recent years, several works focusing on a well-characterized model species demonstrated a fundamental role for indirect interactions that facilitate coccolith crystallization. Here, we developed the tools to extract the base plates from five different species, giving the opportunity to systematically explore the relations between base plate and coccolith properties. We used multiple imaging techniques to evaluate the structural and chemical features of the base plates under native hydrated conditions. The results show a wide range of properties, overlaid on a common rudimentary scaffold that lacks any detectable structural or chemical motifs that can explain direct nucleation control. This work emphasizes that it is the combination between the base plate and the chemical environment inside the cell that cooperatively facilitate the exquisite control over the crystallization process.
Biological organic scaffolds can serve as functional surfaces that guide the formation of inorganic materials. However, in many cases the specific interactions that facilitate such tight regulation are complex and not fully understood. In this work, we elucidate the architecture of such amodel biological template, an organic scale that directs the assembly of exquisite crystalline arrays of marine microalgae. By using cryo electron microscopy, we reveal the native state organization of these scales from several species. The observed similarities and differences allow us to propose that the chemical microenvironment, rather than stereochemical matching, is the pivotal regulator of the process.
Display omitted
Diatoms are an abundant group of microalgae, known for their ability to form an intricate cell wall made of silica. Silicon levels in seawater are in the micromolar range, making it a challenge for ...diatoms to supply the rapid intracellular silicification process with the needed flux of soluble silicon. Here, we use three-dimensional cryo-electron microscopy and spectroscopy to quantitatively analyze, at submicrometer spatial resolution and sensitivity in the millimolar range, intracellular silicon in diatom cells. Our results show that the internal silicon concentration inside the cell is ~150 mM in average, three orders of magnitude higher than the external environment. The cellular silicon content is not compartmentalized, but rather unevenly distributed throughout the cell. Unexpectedly, under silicon starvation, the internal silicon pool is not depleted, reminiscent of a constitutive metabolite. Our spatially resolved approach to analyze intracellular silicon opens avenues to investigate this homeostatic trait of diatoms.
•Light absorption is attenuated in dehydrated C. pumilum leaves.•Chloroplasts shrink, fragment and exhibit senescence-like features during dehydration.•Senescence is avoided by unknown mechanisms, ...enabling revival of the plants upon rehydration.
The vegetative tissues of resurrection plants are able to withstand severe protoplasmic dehydration and revive quickly upon rehydration. Resurrection species defined as ‘homoiochlorophyllous’ retain most or part of their chlorophyll and photosynthetic complement in the dry state, and rely on various mechanisms to protect themselves against photo-damage. In this study, we investigated the changes in chlorophyll distribution, light absorption gradients as well as the alterations in ultrastructure that take place during dehydration of the homoiochlorophyllous species Craterostigma pumilum. Chlorophyll fluorescence profiles show that light absorption is attenuated in dry leaves, likely minimizing generation of reactive oxygen species. These are accompanied by changes that take place in the supramolecular organization of the photosynthetic protein complexes, and ordered functional adjustments of the photosynthetic apparatus, further lessening the excitation and electron pressures. Albeit these, the ultrastructural studies reveal that chloroplasts in dehydrated leaf tissues exhibit features indicative of oxidative stress, which are also reminiscent of senescing chloroplasts. These include mass proliferation of plastoglobules, variable degrees of thylakoid dismantling, as well as chloroplast fragmentation and seemingly vacuolar degradation of such fragments. In addition, unique vesicular structures between the two chloroplast envelope membranes were visualized, some of which appeared to detach from chloroplasts, likely en route to degradation. Together, the data indicate that homoiochlorophyllous resurrection species handle photo-induced damage during dehydration on two levels. Minimization of photo-damage is achieved by attenuation of light absorption and other photo-protective mechanisms. When this is insufficient and significant damage does occur, elimination of damaged components takes place via processes resembling senescence. Nevertheless, these processes are reversible, enabling the plants to avoid the terminal steps of senescence and, hence, to recover.
Summary
The development of calcification by the coccolithophores had a profound impact on ocean carbon cycling, but the evolutionary steps leading to the formation of these complex biomineralized ...structures are not clear. Heterococcoliths consisting of intricately shaped calcite crystals are formed intracellularly by the diploid life cycle phase. Holococcoliths consisting of simple rhombic crystals can be produced by the haploid life cycle stage but are thought to be formed extracellularly, representing an independent evolutionary origin of calcification.
We use advanced microscopy techniques to determine the nature of coccolith formation and complex crystal formation in coccolithophore life cycle stages.
We find that holococcoliths are formed in intracellular compartments in a similar manner to heterococcoliths. However, we show that silicon is not required for holococcolith formation and that the requirement for silicon in certain coccolithophore species relates specifically to the process of crystal morphogenesis in heterococcoliths.
We therefore propose an evolutionary scheme in which the lower complexity holococcoliths represent an ancestral form of calcification in coccolithophores. The subsequent recruitment of a silicon‐dependent mechanism for crystal morphogenesis in the diploid life cycle stage led to the emergence of the intricately shaped heterococcoliths, enabling the formation of the elaborate coccospheres that underpin the ecological success of coccolithophores.
See also the Commentary on this article by Mock, 231: 1663–1666.
Summary
Unicellular organisms are known to exert tight control over their cell size. In the case of diatoms, abundant eukaryotic microalgae, two opposing notions are widely accepted. On the one hand, ...the rigid silica cell wall that forms inside the parental cell is thought to enforce geometrical reduction of the cell size. On the other hand, numerous exceptions cast doubt on the generality of this model.
Here, we monitored clonal cultures of the diatom Stephanopyxis turris for up to 2 yr, recording the sizes of thousands of cells, in order to follow the distribution of cell sizes in the population.
Our results show that S. turris cultures above a certain size threshold undergo a gradual size reduction, in accordance with the postulated geometrical driving force. However, once the cell size reaches a lower threshold, it fluctuates around a constant size using the inherent elasticity of cell wall elements.
These results reconcile the disparate observations on cell size regulation in diatoms by showing two distinct behaviors, reduction and homeostasis. The geometrical size reduction is the dominant driving force for large cells, but smaller cells have the flexibility to re‐adjust the size of their new cell walls.
Plant cystoliths are mineralized objects that are formed by specialized cells in the leaves of certain plants. The main mineral component of cystoliths by volume is amorphous calcium carbonate (ACC) ...and the minor component is silica. We show that the silica stalk is formed first and is essential for ACC formation. Furthermore, the cystolith is shown to be composed of four distinct mineral phases with different chemical properties: an almost pure silica phase grades into a Mg‐rich silica phase. This Mg‐rich silica is overlaid by a relatively stable ACC phase. A bulky and less stable ACC phase encapsulates the first ACC phase. This architecture poses interesting questions about the role of Mg in the silica phase and suggests a strategy for ACC stabilization that takes advantage of a precise regulation of the mineral‐growth microenvironment.
The fantastic four: Cystoliths are mineralized objects that are mainly composed of amorphous calcium carbonate (ACC), which is found in the leaves of several plants. They have a unique composition and architecture of four distinct amorphous phases. A Mg‐rich silica phase is essential for the formation of two distinct ACC phases. The inner ACC phase has inherently higher stability, presumably required by the sequential formation mechanism.
The formation of intricately shaped crystalline minerals by organisms is orchestrated by specialized biomacromolecules. The macromolecules associated with coccoliths, nanometer-sized calcite crystal ...arrays produced by marine microalgae, can form a distinct calcium-rich phase
via
macromolecular recognition. Here, we show that this calcium-rich phase can be mineralized into a thin film of single-crystalline calcite by the balanced addition of carbonate ions. Such a crystallization process provides a strategy to direct crystalline products
via
local interactions between soluble macromolecules and compatible templates.
Soluble macromolecules and insoluble organic templates of biological origin facilitate a two-step crystallization process that results in thin, single-crystalline films of calcite.
Highlights • We examine factors affecting the use of children's dental checkups. • We examine whether children's dental checkups have increased after Israeli dental reform. • Socio-demographic status ...and mothers’ health beliefs affect children's checkups. • After the reform, children checkups have increased among vulnerable populations. • The Israeli dental reform has helped reduce gaps in Israeli society.
PDF
