Solid State NMR Investigation of Structure and Composition of Biogenic Calcite in situ from Coccolithophores
Written by Ira Ben-Shir
Emiliania huxleyi is a unicellular alga, distinguished by its exquisitely sculptured calcium carbonate cell coverings known as coccoliths. These organisms control the finest details of the polymorph formed, its composition, its size and macroscopic assembly. In spite of vast biochemical and structural studies, the underlying factors that govern structure and composition of CaCO3 shells, the bioorganic-inorganic interfaces formed and their role, are not fully understood.
In this study, solid state NMR techniques, combined with stable isotope enrichment (13C and 15N), are employed. This enables for the first time direct, in situ elucidation of the structure and composition of the biomineralized calcite and its interface with bio-organic matter.
The solid state NMR measurements clearly distinguish interfacial calcite from bulk crystalline calcite. Within the bulk crystalline calcite, carbonates that reside adjacent to lattice defect sites are identified (below 2%). Embedded in these defects and proximate to these carbonates, REDOR NMR identifies small interstitial phosphorous and nitrogen moieties. Such proximate moieties were not detected in the interfacial calcite. Coccoliths from rich vs. poor growth media, 25-fold lower phosphorus concentration, exhibit very similar intra-crystalline phosphorous incorporation, implying that the incorporation level and defect formation with 'foreign' moieties is a highly regulated process. Unraveling for the first time in situ molecular level evidence about the structure and composition of bio-mineralized calcite, this study opens new venues to address diverse research areas ranging from environmental and systems biology to materials science and biomimetics.
