The microstructure, crystallography and mechanical performances of biomineralized oyster shell materials : ocean acidification perspective

Abstract

Marine calcifiers, such as oysters, build structurally complex shells to defend against predators using CaCO3 crystals through biomineralization. However, they are now facing a novel threat from ocean acidification (OA) in the process of which their biomineralization is damaged by the decreasing availability of carbonate ions in the unfavourable CO2-induced chemical environment. The inability to form their protective shell, with proper structure and mechanical features, is a major threat to marine biodiversity, shellfish aquaculture industries, coastal fisheries and thus global food security. It is not possible to predict the impact that OA will have on these species unless we understand the response of the shell structural mechanics to changing seawater chemistry. For these reasons, my thesis is particularly focused on a unique biological material, oyster shell, and the responses of their hierarchical microstructure to the near-future level of OA. I borrowed tools from marine biology, climate change science and aquaculture to obtain biological minerals from a variety of ecologically and economically important oyster species with unique structural and mechanical features. I then, mapped the architecture of CaCO3 crystals embedded in an organic matrix in those materials, and also modeled its mechanical features using tools borrowed from mechanical engineering (e.g. nanoindentation and finite element modelling). Beyond the immediate benefit to marine climate change science and aquaculture industry, the interdisciplinary knowledge gained here will provide novel insights for the materials science design of light and strong biomaterials. My research, combining laboratory experiments and biomaterial analysis, is structured around four tasks or chapters. In chapter 2, the integrated comparison of structural and mechanical features of shells of two related species, Hong Kong oyster (Crassostrea hongkongensis) and the Portuguese oyster (C. angulata), demonstrates that the superior microstructure and organization of CaCO3 crystals in foliated layer are contributed to a stronger shell (Hong Kong oyster), which draws a baseline for the rest OA-related Chapters. In chapter 3, I have found that Portuguese oyster shells are highly vulnerable to OA due to their porous microstructure which directly leads less stiff and softer foliated layers at a near-future level of OA at pH 7.8. In chapter 4, Hong Kong oysters keep their superiority in structural and mechanical resistance to OA even at the pH 7.6 by realignments of crystal units in foliated layers. Indeed, I have found, in the final chapter 5, that the Pacific oyster (C. gigas) shells show a high degree of plasticity, in terms both microstructure (i.e. shorter laminas but unchanged porosity) and crystal orientation, in response to OA at pH levels from 7.6 to 8.1. Thus, this hypothesis-driven (based on chapter 2), interdisciplinary (science and engineering) and comparative (three different species) study has illustrated the trade-offs between the microstructural and crystallographic characteristics and the mechanical features in oyster shells of different species in near-future coastal oceans with elevated anthropogenic CO2-driven OA, which not only unveils the intriguing capability of calcifiers in biomineralization but also provides critical knowledge for accurate forecasting the survival and production of edible oysters in the near-future ocean.