The structure of calcifying marine organisms : a materials science approach

• Main Author: Teniswood, Clara
• Format: Thesis
• Language: English
• Published: The Australian National University 2015
• Subjects: Southern Ocean; Antarc*; Antarctica; Carbonic acid; Limacina helicina; Ocean acidification
• Online Access: https://dx.doi.org/10.25911/5d6119ce57439; https://openresearch-repository.anu.edu.au/handle/1885/150287

Increasing levels of carbon dioxide in the atmosphere due to anthropogenic emissions have been shown to alter ocean chemistry. The absorption of carbon dioxide in the ocean increases concentrations of hydrogen ions, carbonic acid and bicarbonate ions, while concentrations of carbonate ions decrease. The pH of the ocean has already dropped from 8.2 to 8.1 since preindustrial times and is predicted to fall further to 7.9 by 2050. Many marine organisms build their external skeletons or shells from calcium carbonate. As both the ocean pH and the availability of dissolved carbonate ions decreases, it is likely that these organisms will find it increasingly difficult to form and maintain their shells. One such group of calcifiers are pteropods, also known as sea butterflies. Common in polar waters, these are small planktonic free-swimming molluscs, which produce shells of aragonite. Pteropods are an integral part of the marine food chain of the Southern Ocean as many organisms, from zooplankton to whales, rely on them as a food source. Therefore, changes in their abundance or distribution could have a substantial flow-on effect for the whole Southern Ocean ecosystem. Another group of important calcifiers are foraminifera, which are simpler unicellular organisms. The species studied in this work makes its shell from low Mg-calcite and resides in the warmer Papua New Guinea waters. Although there have been some studies on the effects of pH on foraminifera and pteropods, there have been no thorough studies on the structure or composition of these calcifiers, an understanding of which is crucial for predicting and measuring how they are affected by changing ocean chemistry. In this work, the structure and mechanical properties of the calcified shells of Limacina helicina antarctica pteropods and Amphistegina lessonii foraminifera are studied. The macro- and micro-structure of these species is investigated using electron microscopy techniques, the composition is analysed using Raman spectroscopy and the mechanical properties are determined using nanoindentation. With the aim of quantifying the effects of ocean acidification, the mechanical properties of pteropods collected in 1998 and 2007 are compared, as well as foraminifera collected at various distances from a carbon dioxide volcanic vent in PNG. These are unique 'natural' experiments in which the samples have been analysed directly after removal from their habitat and thus, do not have the uncertainties, such as adaptation, feeding or life cycle effects, that 'capture and keep' experiments introduce. The distribution of the organic matrix, which is believed to determine the growth of biogenic calcium carbonate, has been mapped, with a rich organic layer discovered in the pteropod shells. This finding provides valuable information on the structure-property relations of these shells and the possible explanations for the presence of this layer are discussed. The detrimental effects and implications of routine chemical sample preparation treatments, such as bleaching, on the mechanical integrity of calcified structures is investigated using electron microscopy, nanoindentation and a custom three point bend system. Finally, the suitability of materials science techniques for quantifying the effects of ocean acidification on calcifiers is evaluated.