Micro- to nano-scale architecture and aspects of skeletal growth in marine calcifiers
thesisposted on 28.03.2022, 11:50 by Laura Meret Otter
Bivalve shells are nano-composite materials consisting of crystalline calcium carbonate phases (i.e.aragonite and calcite) that are intimately arranged with organic constituents into different hierarchical architectures. This explains their enhanced mechanical properties that outperform their monolithic geological counterparts. Shells grow incrementally throughout their lifetime and record environmental conditions in their trace element and isotopic signatures. Our understanding of how shells grow has seen a recent paradigm change away from classical ion-by-ion crystallization models to non-classical crystallization pathways via colloid attachment and transformation of nanogranular amorphous calcium carbonate precursor phases. These findings complicate any paleo-environmental reconstructions and lead to fundamental questions, such as: which impact do the new crystallization pathways have on trace element partitioning? How are these pathways affected by physiological effects on sub-micron growth processes? These questions are addressed in this thesis by using living shells in controlled aquaculture conditions. Four different bivalve species with different architectural ultrastructures were studied here: Anadaratrapezia (crossed-lamellar and complex crossed-lamellar architectures), Katelysia rhytiphora (compound composite prismatic and crossed-acicular), Mytilus galloprovincialis (simple prismatic and nacre) and Pinctada imbricata fucata (simple prismatic and nacre). These bivalves were periodically subjected to pulse-chase labelling experiments with strontium-enriched seawater to create "snapshots" of submicron shell growth processes correlatively visualized with micro- to nano-analytical instrumentation such as EPMA, FEG-SEM, Nano-SIMS, Micro-Raman, EBSD, HR-(S)TEM, and Atom Probe Tomography. As non-nacreous architectures are virtually not studied, pulse-chase labelling experiments combined with architectural investigations provide a powerful tool for detailed characterization at the micron scale: the Sr-label in the shell is shown to cut across all architectural units independent of hierarchy and local orientation, indicating growth rates across a uniform growth front during shell mineralization. While this growth process may appear intuitive, this thesis presents here the first direct evidence compared to previous inferences from natural environments without rigorous time-resolution. This growth process across all architectural hierarchies in the shell is fundamentally different to that of nacroprismatic shells, which are thought to grow in a two-step process of forming organic compartments prior to calcification. Material properties of non-nacreous shells are investigated via stereographic visualization of Young's moduli and reveal a girdle-like arrangement of elastically stiffer orientations that result in quasi-isotropic planes perpendicular to the growth direction across the entire shell. For nacroprismatic shells at the nano-scale, the thesis presents new insights into the toughening processes investigated via innovative TEM-based nanoindentation experiments. At low compression, it is found that organic interfaces confine strain propagation within each tablet and nanogranules as well as organic inclusions rotate and deform, while at high compression tablets interlock and fully recover to their original state. Nacre is by far the best-studied biomineral, but its insulating properties and high organic content present significant challenges when taking geochemical investigations to the nano-scale. Therefore, a best practice methodological protocol for Atom Probe Tomography is developed for nacre that minimises the generation of artefacts in the mass spectrum and allows for correlative analysis of the insulating material. This protocol paves the way for innovative studies focussing on the growth dynamics within individual nacre tablets. The correlated, multianalytical approach is further used to study the ultrastructure and composition of bimineralic cheilostome bryozoan skeletons that consist of primary, calcitic lateral walls that are covered with fine-grained fibrous aragonite on their distal side while the colony's frontal and basal walls are fully aragonitic. This arrangement of mineral phases was previously unknown in cheilostome bryozoa. Further, an organic membrane is situated between both mineral phases that may serve as a template for biomineralisation. The last part of the thesis investigates another facet of nacre - lustrous pearls by mapping out the knowledge gaps existing in the area of provenance identification and presenting the world's only source of untreated Akoya cultured pearls. These untreated pearls are an invaluable source to investigate the natural colour-palette that are linked to pigments within the nacre.