Structure and composition of Unio pictorum shell: arguments for the diversity of the nacroprismatic arrangement in molluscs

Investigation of Bioinspired Nacreous Structure on Strength and Toughness

The toughening mechanism of the nacre was widely investigated in recent decades, which presents a great prospect for designing high performance composite materials and engineering structures with bioinspired structures. To further elucidate which structural parameters and which kinds of morphology of the nacre-inspired structure are the best for improving tensile strength without sacrificing too much toughness is extremely significant for composite materials and engineering structures. The “brick-and-mortar” structure is a classical nacre-inspired bionic structure. Three characteristic structural parameters, including the aspect ratio ρ of the brick length and width, the thickness ratio β between the thickness of brick and mortar, and the spacing ratio τ between the width of brick and mortar, were used as variables to study their effect on tensile strength and toughness. It was found that ρ was the most prominent factor in determining the strength and toughness, and τ could improve the strength and toughness almost simultaneously. Racked and wedged morphology of the structural unit were established based on the structural parameters of the regular staggered unit, and were used to compare tensile behavior. It was found that the model with the wedged unit possessed the highest strength and toughness, and could absorb more strain energy during fracture crack growing. The crack propagation path further illustrated that the crack resisting ability of the wedged unit was the best. Our simulation results presented the connection between three characteristic structural parameters with the strength and toughness, and proved that the wedged staggered unit was the best in improving the strength and toughness.

Polymorphism of CaCO3 and the variability of elemental composition of the calcareous skeletons secreted by invertebrates along the salinity gradient of the Baltic Sea

Biomineralization is of great importance in ecosystem functioning and for the use of carbonate skeleton as environmental proxies. Skeletal formation is controlled to different degrees by environmental parameters and biological mechanisms. While salinity is one of the most important factors affecting ecological processes and ocean physiochemistry, the goal of this investigation was to identify how salinity influences the mineral type and the concentrations of chemical elements in the whole skeleton of invertebrates from the Baltic Sea. In this model system, the surface salinity decreases from marine values (27.2) to almost fresh water (6.1). The selected organisms, mussels (Mytilus spp.), bryozoans (Einhornia crustulenta, Cribrilina cryptooecium, Cryptosula pallasiana, Electra pilosa, Escharella immersa), barnacles (Amphibalanus improvisus, Semibalanus balanoides), and polychaetes (Spirorbis tridentatus), precipitated skeleton composed of calcite and aragonite, most likely as a result of various interacting environmental and biological factors. The concentrations of all elements in bulk skeleton were highly variable between species from the same location, underlining the role of the biological mechanisms in skeletal formation. The concentration of Ca, Mg, Sr, and Na increased in the bulk skeleton of stenohaline organisms with increasing salinity, while in the bulk skeleton of euryhaline species, only the concentration of Na increased with increasing salinity. The concentrations of Mn, Ba, Cu, Pb, Y, V, Cd, and U in the skeleton of euryhaline species generally decreased at higher salinities, most likely reflecting the lower bioavailability of elements at higher salinity. However, the concentrations of elements in the skeleton of stenohaline organisms were highly variable with no clear salinity impact. This study suggests that, although the composition of skeleton of calcifying organisms along the salinity gradient of the Baltic Sea is to a large extent affected by biological mechanisms, it also reflects the responses to environmental conditions.

Heterogeneous distribution of shell matrix proteins in the pearl oyster prismatic layer

Shell formation in molluscan bivalves is regulated by organic matrices composed of biological macromolecules, but how these macromolecules assemble in vitro remains elusive. Prismatic layer in the pearl oyster Pinctada fucata consists of polygonal prisms enveloped by thick organic matrices. In this study, we found that the organic matrices were heterogeneously distributed, with highly acidic fractions (EDTA-soluble and EDTA-insoluble) embedded inside the prism columns, while basic EDTA-insoluble faction as inter-column framework enveloping the prisms. The intra-column matrix was enriched in aspartic acid whereas the inter-column matrix was enriched in glycine, tyrosine and phenylalanine. Moreover, the intra-column matrix contained sulfo group further contributing to its acidic property. Proteomics data showed that the intra-column proteins mainly consisted of acidic proteins, while some typical matrix proteins were absent. The absent matrix proteins such as shematrin family and KRMP family were highly basic and contained aromatic amino acids, suggesting that electric charge and hydrophobic effect might play a role in the matrix heterogeneity. Interestingly, chitin metabolism related proteins were abundant in the inter-column matrix, which may be involved in reconstructing the prism organic matrix. Overall, our study suggests that each single prism grew in an enclosed organic envelope and the organic matrix undergoes rearrangement, thus leading to the peculiar growth of the prismatic layer.

Microstructure Analysis and Chemical and Mechanical Characterization of the Shells of Three Freshwater Snails

The shells of freshwater snails are discarded as waste, which qualify as biological materials with prospective multiple uses. To substantiate this proposition, an attempt was made to elucidate the physical and chemical properties of the shells of three freshwater snails, namely, Bellamya bengalensis, Pila globosa, and Brotia costula. The shells were prepared for electron microscopy and assessment of the calcium carbonate content, apart from the Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and nanoindentation studies. The results indicated that the calcium carbonate content (y) of the shells ranged between 87 and 96% of the total weight (x) and complied with a power regression equation: y = 0.801x 1.016; R 2 = 0.994; r = +0.998; P < 0.001. Observations through SEM depicted different snail species-specific arrangement patterns of calcium carbonate crystals in the diverse layers of shells. The XRD, FTIR, and EDS observations revealed the dominance of the aragonite form of the calcium carbonate crystal in the microstructures of each snail shell with the occurrence of different shell surface functional groups. The Brunauer-Emmett-Teller analysis elucidated the surface textures of shell dust taken from each snail species; in addition, the nanohardness properties indicate the shells as a tough biocomposite exoskeleton. Species-specific variations in the shell morphology, microstructure, and calcium carbonate content were prominent for the three freshwater snails considered for the study. Nonetheless, the physical and chemical properties substantiate that the shells of B. bengalensis, P. globosa, and B. costula qualify as biological materials for sustainable use in various fields including bioremediation, biocatalyst, biomedical applications, and a source of lime. Since the shells of the freshwater snails are discarded as aquaculture waste, subsequent use as a biological material will support the "waste made useful" paradigm in sustainability, both from ecological and economic perspectives.

An insight into the structure, composition and hardness of a biological material: the shell of freshwater mussels

The shell of the freshwater mussel (Mollusca: Bivalvia) is a composite biological material linked with multifunctional roles in sustaining ecosystem services. Apart from providing mechanical strength and support, the shell is an important site for adherence and growth of multiple types of algae and periphyton. Variations in the shell architecture are observed in the mussels both within a species and among different species. Considering the prospective utility of the shell of the freshwater mussels as a biological material, an assessment of the shell characteristics was accomplished using Corbicula bensoni and Lamellidens marginalis as model species. The calcium carbonate (CaCO3) content of the shells, physical features and mechanical strength were assessed along with the morphometric analysis. The CaCO3 content of the shell (upto 95% to 96% of the shell weight) of both the mussels was positively correlated with the shell length, suggesting increased deposition of CaCO3 in shells with the growth of the species. The cross sectioned views of FE-SEM images of the shells exhibited distinct layered structure with external periostracum and inner nacreous layer varying distinctly. In the growing region, the growth line was prominent in the mussel shells revealed through the FESEM images. In addition XRD, FTIR and EDS studies on the mussel shells confirmed the existence of both aragonite and calcite forms of the calcium carbonate crystals with the incidence of various functional groups. The mechanical strength of the mussel shells was explored through nanoindentation experiments, revealed significant strength at the nanoparticle level of the shells. It was apparent from the results that the shell of the freshwater mussel L. marginalis and C. bensoni qualify as a biological material with prospective multiple applications for human well-being and sustaining environmental quality.

Identification of a long non-coding RNA (LncMSEN2) from pearl oyster and its potential roles in exoskeleton formation and LPS stimulation

Long non-coding RNAs (lncRNAs) play regulatory roles in various biological processes, including exoskeleton formation and immune response. The exoskeleton-based mantle-shell defense system is an important defense mechanism in shellfish. In this study, we found a novel lncRNA, herein formally named, LncMSEN2, from the pearl oyster Pinctada fucuta martensii, and its sequence was validated via polymerase chain reaction (PCR). LncMSEN2 was highly expressed in mantle tissues, especially in the central region (P < 0.05), and was also expressed in the pearl sac as detected by quantitative real-time PCR. In situ hybridization experiments revealed that LncMSEN2 had a strong positive signal in the inner and outer epidermal cells of the mantle pallial and central regions. RNA interference experiments showed that interference of LncMSEN2 expression with dsRNA in mantle tissues led to an abnormal crystal structure of the nacre. In addition, LncMSEN2 expression significantly increased 6 h after lipopolysaccharide stimulation in mantle tissues (P < 0.05). These results indicated that LncMSEN2 may be a novel regulator of the mantle-shell defense system of pearl oyster.

Biomineralization as a Paradigm of Directional Solidification: A Physical Model for Molluscan Shell Ultrastructural Morphogenesis

Molluscan shells are a model system to understand the fundamental principles of mineral formation by living organisms. The diversity of unconventional mineral morphologies and 3D mineral-organic architectures that comprise these tissues, in combination with their exceptional mechanical efficiency, offers a unique platform to study the formation-structure-function relationship in a biomineralized system. However, so far, morphogenesis of these ultrastructures is poorly understood. Here, a comprehensive physical model, based on the concept of directional solidification, is developed to describe molluscan shell biomineralization. The capacity of the model to define the forces and thermodynamic constraints that guide the morphogenesis of the entire shell construct-the prismatic and nacreous ultrastructures and their transitions-and govern the evolution of the constituent mineralized assemblies on the ultrastructural and nanostructural levels is demonstrated using the shell of the bivalve Unio pictorum. Thereby, explicit tools for novel bioinspired and biomimetic bottom-up materials design are provided.