|M.Sc Student||Gertman Ronen|
|Subject||Solid State NMR Investigation of Structure and Composition|
of Biogenic Calcite in situ from Coccolithophores
|Department||Department of Chemistry||Supervisor||Professor Asher Schmidt|
|Full Thesis text|
Biomineralization, particularly the formation of calcium carbonate structures by organisms under ambient conditions, is of vast fundamental and applied interest. Organisms finely control all aspects of the formation of the biomaterials: composition, polymorph, morphology, and macroscopic properties. While in situ molecular level characterization of the resulting biominerals is a formidable task, solid state MAS NMR is one of the most powerful analytical techniques for this purpose. It is employed in this study to elucidate the structure and composition of biogenic calcite formed by the three coccolithophores - Emiliania huxleyi, Pleurochrysis carterae and Gephyrocapsa oceanica, unicellular algae distinguished by their exquisitely sculptured calcite cell coverings known as coccoliths. The three species (CCMP strains 371, 645, and 2051, respectively) were grown and harvested from 15N and 13C enriched growth media, with biosynthetic labeling to enhance the sensitivity of the NMR measurements. Crystalline and interfacial calcite environments were selectively probed using direct and indirect (CP) 13C excitation, respectively. Different crystalline environments, in particular structural defect sites at concentrations of up to 1.4 % (quantitative measurements were accomplished only for E. huxleyi) with P and N moieties incorporated, were identified using 13C REDOR NMR. REDOR derived geometrical constraints show that the P and N atoms at the defect sites are 3.2 and 2.3 (±0.2) Å apart from a crystalline carbon carbonate. The phosphorous and nitrogen moieties within the biogenic calcite are identified as small, non-protonated moieties, attributed to inorganic ions such as PO43- and NO3-. The carbonates adjacent to these defects are chemically indistinguishable from bulk crystalline carbonates, yet their immediate environments experience reduced rigidity as reflected by substantial T1(13CO32-) shortening. Interfacial carbonates, on the other hand, reside in structurally/chemically perturbed environments as reflected by heterogeneous line broadening. This study is the first to unravel directly evidence on the state of the crystalline carbonates adjacent to structural defects. It is also the first to report on the calcitic intracrystalline and interfacial structure and composition of coccoliths in situ and on the prevailing similarities among three species.