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Establishing a Physical and Chemical Framework for Amorphous Calcium Carbonate (ACC) Biomineralization

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Establishing a Physical and Chemical Framework for Amorphous Calcium Carbonate (ACC) Biomineralization Establishing a physical and chemical framework for Amorphous Calcium Carbonate (ACC) biomineralization Sebastian T. Mergelsberg Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Geosciences Patricia M. Dove, chair F. Marc Michel J. Donald Rimstidt Michael F. Hochella, Jr. Biswarup Mukhopadhyay April 27, 2018 Blacksburg, VA Keywords: Biomineralization, ACC, crustaceans, calcium carbonate, magnesium © Sebastian T. Mergelsberg, 2018 Establishing a physical and chemical framework for Amorphous Calcium Carbonate (ACC) biomineralization Sebastian T. Mergelsberg ABSTRACT Recent advances in high-resolution analytical methods have brought about a paradigm shift in our understanding of how crystalline materials are formed. The scientific community now recognizes that many earth materials form by multiple pathways that involve metastable intermediates. Biogenic calcium carbonate minerals are now recognized to develop by aggregating molecules or clusters to form amorphous phases that later transform to one or more crystalline polymorphs. Amorphous calcium carbonate (ACC) is now recognized as a precursor to CaCO3 biominerals in a wide variety of natural environments. Recent studies suggest an ACC pathway may imprint a different set of dependencies from those established for classical growth processes. Previous ACC studies provided important insights, but a quantitative understanding of controls on ACC composition when formed at near-physiological conditions is not established. The Mg content of ACC and calcite is of particular interest as a minor element that is frequently found in final crystalline products in calcified skeletons. This three-part dissertation investigated biological and well-characterized synthetic ACC using high-energy x-ray methods, Raman spectroscopy, and mechanical tests. The findings establish chemical and physical properties of ACC in the exoskeleton of crustaceans and show Mg and P levels are tuned in the mineral component to optimize exoskeleton function that could be sensitive to ecological or environmental conditions. Calcite and chitin crystallinity exhibit a similar body-part-specific pattern that correlates directly with the mechanical strength of the exoskeleton. Insights from this study suggest precise biological control of ACC chemistry in the to regulate exoskeleton properties. Laboratory measurements using quantitative methods and compositions that approximate the physiological conditions of crustaceans, demonstrate at least two types of ACC are formed by controlling Mg concentration and alkalinity. We also find temporal changes in the short-range ordering of ACC after precipitation that is dependent upon carbonate content. The findings from this study provide a quantitative basis for deciphering relationships between ACC structures, solution chemistry, and the final transformation products under biologically relevant conditions. Establishing a physical and chemical framework for Amorphous Calcium Carbonate (ACC) biomineralization Sebastian T. Mergelsberg PUBLIC ABSTRACT With the development of new imaging methods for nano-scale materials, scientists across diverse disciplines have recognized that many earth materials can form complex shapes by the formation and aggregation of nanocrystals or structureless (amorphous) particles. Biological minerals, such as shells and skeletons, are well-documented to form CaCO3 via both of these attachment pathways, particularly amorphous calcium carbonate (ACC). However, little is known about the ACC properties and the factors that determine the final composition of skeletal minerals. This three-part dissertation focuses on ACC and calcite in the exoskeletons of crustaceans to understand how animals form composite exoskeletons of calcium carbonate minerals. This knowledge is important because CaCO3 minerals are the primary component of the shells and skeletons of many economically important marine species. These minerals are also prevalent in the geological record as roadmaps for the evolutionary record. Amorphous and crystalline forms of CaCO3 are also used as inert 'filler' materials for pharmaceutical products. By designing a series of experiments to characterize ACC in exoskeletons from lobsters and crabs, one part of the dissertation shows relationships between chemical composition and physical behavior of the materials. Building on this biomineral information, a separate experimental study synthesizes ACC under near-physiological conditions to show how amorphous CaCO3 forms under controlled conditions. The findings have far-reaching consequences for understanding the complex chemistry that underlies the formation of calcium carbonate as a component of shells and skeletons, and what physical properties are optimized by the composition of these materials. TABLE OF CONTENTS ABSTRACT .................................................................................................................................... ii PUBLIC ABSTRACT ................................................................................................................... iii TABLE OF CONTENTS ............................................................................................................... iv LIST OF FIGURES ...................................................................................................................... vii LIST OF TABLES ......................................................................................................................... ix PREFACE ....................................................................................................................................... x CHAPTER 1. INTRODUCTION ................................................................................................ 1 1.1 MOTIVATION FOR STUDY ........................................................................................... 1 1.2 THE CRUSTACEAN EXOSKELETON – PATTERNS IN COMPOSITION ................ 2 1.3 THE CRUSTACEAN EXOSKELETON – PHYSICAL PROPERTIES .......................... 3 1.4 LITTLE IS KNOWN ABOUT ACC PROPERTIES ........................................................ 4 1.5 THE TWO TYPES OF ACC ............................................................................................. 5 1.6 ORGANIZATION OF THIS DISSERTATION ............................................................... 5 1.7 REFERENCES .................................................................................................................. 7 CHAPTER 2. TOWARD AN UNDERSTANDING OF COMPOSITION SYSTEMATICS IN THE EXOSKELETON OF THE AMERICAN LOBSTER, HOMARUS AMERICANUS ......................................................................................................................................................... 9 ABSTRACT ............................................................................................................................ 9 2.1 INTRODUCTION ........................................................................................................... 10 2.2 MATERIALS AND METHODS .................................................................................... 14 2.2.1. Animal and Sample Preparation. ............................................................................. 14 2.2.2. Dissolution of the organic and mineral fractions. ................................................... 15 2.2.3. Composition Analysis. ............................................................................................. 15 2.2.4. Pair Distribution Function (PDF) analysis. ............................................................ 16 2.3 RESULTS ........................................................................................................................ 17 2.3.1. Mineral component is primarily ACC...................................................................... 17 2.3.2. Mineral fraction contains variable Mg, P, and Ca levels with constant ratios. ...... 18 2.3.3. Average composition of the mineral fraction........................................................... 20 2.3.4. Low levels of Mg P, and Ca in the organic fraction. ............................................... 21 2.3.5. Bulk exoskeleton composition dominated by the mineral fraction. ......................... 24 2.4 DISCUSSION .................................................................................................................. 24 2.4.1. Chemistry of mineral component is highly regulated. ............................................. 24 2.4.2. No evidence for a separate phosphate phase. .......................................................... 26 iv 2.4.3. Enigma of a P/Mg signature. ................................................................................... 27 2.4.4. Broader pattern for multiple crustacean species? ................................................... 29 2.4.5. Physical basis for taphonomic bias in skeletal preservation. .................................. 30 2.5 CONCLUSIONS ............................................................................................................. 31 2.6 REFERENCES ................................................................................................................ 33 CHAPTER 3. RELATIONSHIPS OF α-CHITIN AND CARBONATE MINERAL COMPONENTS REVEAL BIOMINERALIZATION STRATEGIES OF CRUSTACEAN EXOSKELETONS .....................................................................................................................
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