The 3D submicron-scale skeletal reconstruction of Nannoconus (Cretaceous calcareous nannofossil)-Insights into biomineralization

Abstract

Nannoconus (g1/4 5-20 μm) was a major planktonic producer in the Early Cretaceous seas (g1/4 150-120 Ma). The heavy calcitic skeletons (micaliths; g1/4 200-1400 picogram) of this extinct nannoplankton genus have contributed to massive carbonate accumulations for over g1/4 30 million years. The micalith microstructure is characterized by an interlocking arrangement of calcitic lamellae spanned around a central canal. The biomineralization process involved in producing the sophisticated micalith is investigated for the first time. Ptychography X-ray computed tomography (PXCT) with synchrotron radiation is applied to an isolated micalith, to obtain a 3D set of tomographic images with g1/4 40 nm spatial resolution. This 3D set was processed to virtually segment the individual calcitic lamella and reconstruct the full micalith through constraining different lengths and angles. The lamellae are repetitively stacked in two distinct inclinations, one following the other, and producing segments combined to form the entire micalith. Individual lamellae were calcified in a "template"of organic layer containing amino acid(s)/biomolecule(s), responsible for creating the interlocking arrangement. Our study of Nannoconus provides a simple yet potent approach to the analysis of biomineralized microstructures characterized by the repetitive arrangement of calcitic units as commonly seen in the calcareous nannoplankton.