Pathways of biomineralization and microstructure evolution in brachiopod shells

Simonet Roda, Maria del Mar (2021): Pathways of biomineralization and microstructure evolution in brachiopod shells. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

Biominerals of shells, bones and teeth are composits of minerals and organic tissue components precipitated by organisms. Accordingly, it is very important to understand (1) the relation between the soft and hard tissues in composite materials of living organisms, (2) the resulting micro- and nanostructure of the constituting biominerals (3) and the function of the minerals of the biomineralization epithelial cells in producing these materials. Brachiopod shells were selected to be the principal subject of this work as they are major geochemical archives for paleo-environmental reconstruction of sea water conditions. The shell of modern brachiopods is secreted by the outer mantle epithelium (OME) of the animal. Despite several decades of research, it is still unknown how the mineral is transported from OME cells to the site of mineralization. For brachiopod shells the biomineralization process was not yet described and often biomineralization of mollusc shells was used as a reference. In order to understand mineral transport and shell secretion, we investigated the ultrastructure of OME cells and their spatial relation to the growing shell for the terebratulide brachiopod Magellania venosa (Chapters 2.1 and 2.2). The animals were chemically fixed and high pressure frozen. We worked with high resolution panorama images formed of up to 350 TEM images. This ensured a general overview as well as a detailed description of the ultrastructure of the OME. We found and described the specific differences between (1) the OME ultrastructure at the commissure and that at central shell regions as well as (2) differences between areas in the central region where active secretion takes place and those areas where secretion is finished. The OME at the commissure consists of several cell layers, while at central shell regions it is single-layered. It is significantly thinner at the central shell region in comparison to the commissure. Especially at sites of actively forming calcite fibres, OME cells are only a few tens of nanometre thin. Where the mineral deposition takes place, the apical membrane of OME cells is in direct contact with the calcite of the forming fibre. At these sites the extracellular organic membrane at the proximal convex surface of the fibre is absent. When mineral secretion is finished the cells form an extracellular organic membrane which lines the proximal surface of fibres. The extracellular organic membrane is attached to the apical cell membrane via apical hemidesmosomes. Tonofilaments cross the cell, connect apical to basal hemidesmosomes, stabilize the contact between epithelium and fibres and keep the mantle attached to the shell. Furthermore, communication and cooperation of neighbouring OME cells could be proved in this work as individual fibres are secreted by several cooperating cells at the same time (Chapters 2.1 and 2.2). The extracellular space, the space between the epithelium and the growing fibres, is either absent or very narrow. Quantitative analysis demonstrated that there are no significant differences in the volume fraction of vesicles between secreting and non-secreting regions of the OME. The latter and the extreme reduction in cell thickness at sites of mineral secretion suggest that for Magellania venosa shells mineral transport to the sites of mineralization does not occur by transport with organelles such as vesicles but via ion transport mechanisms through the cell membrane. For the central shell region the previously discussed data was complemented with atomic force microscopy (AFM) and electron backscatter diffraction (EBSD) measurements. In the central region of the shell the fibrous layer is secreted. The fibrous layer of modern terebratulide brachiopod shells has an overall plywood-like organization with the basic mineral units, the calcite fibres, being assembled with a microstructure resembling an ‘anvil-type’ arrangement (Chapter 2.2). The observations on the TEM images and on etched sample surfaces under AFM lead us to develop a model for calcite fibre secretion and fibre shape formation for Magellania venosa is described as a dynamic process coordinated by outer mantle epithelium cells (OME). The secretion process consists of the following steps: (i) local detachment of epithelial cell membrane from the organic membrane of previously formed fibres, (ii) onset of secretion of calcite at these sites, (iii) organic membrane formation along the proximal, convex side of the forming fibre during achievement of the full width of the fibre, (iv) start of membrane secretion at the corners of fibres progressing towards the centre of the fibre, (v) attachment of the cells via apical hemidesmosomes to newly formed organic membrane, and (vi) suspension of calcite secretion at sites where the proximal, organic membrane of the calcite fibre is fully developed and the apical cell membrane is attached to the latter with apical hemidesmosomes. Thecideide brachiopods are an anomalous group of invertebrates. Their position within the phylogeny of the Brachiopoda and the identification of their origin is still not fully resolved. Studies of morphological features such as shell structure and body size aimed to shed more light on thecideide evolution. However, none of these did provide a definitive answer, possibly because of their complex and diverse evolutionary track. In this thesis (Chapter 2.3) we attempt to trace thecideide shell evolution from a microstructure and texture point of view. We describe for this group of brachiopods the appearance and disappearance of a variety of calcite biocrystals that form the shells and trace these from Late Triassic to Recent times. The results and conclusions are based on EBSD measurements that form the basis of a phylogenetic tree. With this thesis we present a new phylogenetic hypothesis for the evolution of Thecideida. This is the first study that links microstructure and texture results gained from EBSD measurements with phylogenetic analysis and implications derived from phylogenetic evolution. BSD measurements demonstrated the presence of a large variety of mineral units within thecideide shells throughout the geological record. With geologic time there is a progressive loss of the fibrous layer in favour of highly disordered acicular and granular microstructures. This loss can be seen as a paedomorphic pattern in the complex mosaic of evolutionary changes characterizing thecideide brachiopods. The Upper Jurassic species has transitional forms. The shells are composed of stacks of acicles on the external part of the shell. The fibrous layer is kept only in some regions next to the soft tissue of the animal. The regularity of biocrystal shape, mineral unit size, and the strength of calcite co-orientation decreases from the Late Triassic to Recent species. Even though, since the Upper Jurassic the thecideide shell microstructure shows the same type of mineral unit morphologies made of (i) nanometric to small granules, (ii) acicles, (iii) fibres, (iv) polygonal crystals, (v) large roundish crystals. I deduce from my studies that the change in microstructure and texture of thecideide brachiopods may be related to the ecological strategy to exploit distinct niches and life styles, in particular attachment to hard substrates. The clear and well defined microstructure of this brachiopod group is well distinguishable and can help to unravel the phylogenetic relationships between different taxa. Brachiopods are one of the very few marine organism groups which have a complete fossil record. First species appeared in early Cambrian. The end-Permian extinction erased the majority of Paleozoic brachiopod taxa and reset taxonomic, morphological, functional and ecological brachiopod diversity. A few groups survived end-Permian extinction, diversified and occupied new ecological niches. Representatives of these form today the extant orders of the Lingulida, Craniida, Rhynchonellida, and Terebratulida. The Thecideida appeared after the end-Permian crisis, in the Triassic. The geological record shows that brachiopods were and are able to adopt to many marine environments. Accordingly, a large diversity in body plans as well as morphological, structural and chemical features of their shell became developed. With this thesis I highlight structural features of the shells of selected terebratulide, rhynchonellide, thecideide and craniide taxa. Chapter 2.4 describes the difference in shell structure for brachiopods with different life-styles, highlights the distinctness between the structure of the primary shell layer of Terebratulida, Rhynchonellida and the shell structure of Thecideida. I detail the nanometer scale calcite organization of Rhynchonellide and Terebratulide fibers, describe some advantages of a hierarchical composite hard tissue, address possible determinants for primary, fibrous and columnar shell calcite of Terebratullida and discuss a possible usage of thecideide shell for paleoenvironment reconstruction.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 3. März 2021
• 1. Bericht­erstatter:in: Schmahl, Wolfgang