Biomolecular Controls on Calcium Carbonate Formation by Amorphous and Classical Pathways: Insights from Measurements of Nucleation Rates and Isotope Tracers
Giuffre, Anthony J
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Calcified skeletons are produced within complex assemblages of proteins and polysaccharides whose roles in mineralization are not well understood. Researchers have long postulated that living organisms utilize the macromolecules of organic matrices to actively guide the formation of crystal structures. The timing and placement of the subsequent minerals that form are most easily controlled during nucleation; however, a physical and chemical picture of how organic functional group chemistry influences the initial stages of nucleation is not yet established. These processes are further complicated by the realization that carbonate biominerals can form by an amorphous to crystalline transformation process, which has prompted the question of how chemical signatures are recorded during mineralization. Investigations of mineralization processes such as the kinetics of nucleation and the transformation of amorphous calcium carbonate (ACC) to crystalline products are critical to building a better understanding of biomineral formation. Only from that fundamental basis can one begin to decipher changes in climate and seawater chemistry over geologic time and by recent anthropogenic effects. This dissertation presents the findings from experimental studies of the thermodynamics and kinetics of multiple mineral formation processes, including nucleation and transformation from an amorphous phase. The kinetics of calcite nucleation onto a suite of high-purity polysaccharide (PS) substrates were quantified under controlled conditions. Nucleation rates were measured as a function of 1) supersaturation extending above and below ACC solubility and 2) ionic strength extending to seawater salinity. These conditions decipher the chemical interactions between the PS substrate, calcite crystal, and solution. These investigations show the energy barrier to calcite formation is regulated by competing interfacial energies between the substrate, crystal, and liquid. The energy barriers to nucleation are PS-specific by a systematic relationship to PS charge density and substrate structure that is rooted in minimization of the competing substrate-crystal and substrate-liquid interfacial energies. The data also suggest ionic strength regulates nucleation barriers through substrate-liquid and crystal-liquid interfacial energetics. In a second experimental study, stable isotope labeling was used to directly probe the transformation pathway. Four processes were considered: dissolution-reprecipitation, solid-state, or combinations of these end member processes. Isotope measurements of calcite crystals that transform from ACC have signatures that are best explained by dissolution-reprecipitation. The extent of isotopic mixing correlates with the amount of ACC transferred and the time to transformation, suggesting the calcite crystals are recording the changing local solution environment during the transformation. These investigations into different mineralization mechanisms build a framework for how functional group chemistries of organic molecules regulate mineralization and the resulting isotopic and elemental signatures in the calcite. This may provide useful insights to interpreting chemical signatures of carbonate biominerals in fossil record and understanding ocean chemistry changes throughout geologic time.
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