Architecture of a nanocrystalline biomineralised shell

Abstract

Marginopora sp. is an eukaryotic large benthic foraminifera that biomineralises a high magnesium calcitic test (shell) in which dinoflagellate symbionts are stored. The discoidal test has a complex architecture; a very large cellular organism expends considerable energy to assemble and maintain a complex shell. What does the organism realise from this architecture? What are the elemental features of the architecture? Researchers have studied the test’s microstructures and chemical composition; its functional value has been suggested but an endeavour to pinpoint any particular value can be extremely complicated in a biological structure, i.e. an intractable problem is presented. Form is related to and often determines the adaptive functional value of biological structures; an enhanced understanding of form precedes and informs an understanding of function. This work increases an understanding of the form and functional value of the complex architecture of the test. Particularly, it adds to existing knowledge by: extending to a nanoscale a detailed characterization of the microstructures of the test, revealing calcium carbonate nanostructures and their associations to form crystals; conceptualising and applying a hierarchical arrangement of the microstructures across nano-macro dimensions of scale; visualising in three dimensions (3D) the microstructures of the test through ontogeny to reveal anisotropy and symmetry in the morphology of microstructures and in the test; and discovering and interpreting structural patterns that were previously not visible. Based upon these findings, the architecture and microstructures of the test; properties of biomineralised structures such as bone and echinoderm skeletons; structural arrangement of adequate models; and predominantly the mechanical adaptive value of the test are correlated. This thesis advances ideas and suggestions for research, design and practical applications of the structural principles abstracted from these findings and interpretations towards a biomimetic design of structures and processes. A combination of advanced microscopy techniques at higher resolutions and magnifications than previously used in the research of the test were used to evaluate pristine specimens of the test of Marginopora sp. Data and images obtained by the use of high resolution and cryo-scanning electron microscopy, transmission electron microscopy, polarising microscopy and, X-ray micro computed tomography (X-ray MCT) of several specimens representing different ontogenic stages were analysed. Especially, the applicability of X-ray MCT as a technique was tested for the quantitative and qualitative analysis of 3D features of the test. Visualisation and statistical techniques used in network analysis were applied to the data obtained by X-ray MCT. A materials sciences approach was adopted towards interpreting the nano-macro scale structural features of the test. In lieu of a protracted and difficult experimental approach a method used in the field of artificial intelligence was adopted to find models that could adequately point to the adaptive value of the architecture of the test. Results obtained using this approach indicated that the adaptive values are self-assembly in a phyllotactic pattern that allows an effective mechanical flexural response using a minimum of materials; close-packing of a particular pore volume; and that enables large surface areas and transmission of light through a 3D biconcave disc.