"Bio-molecule" based calcification in Nannoconus, an extinct group of planktonic algae approximately 150 million years old

Calcareous nannofossils (1-30 μm) are biomineralized calcitic remains of marine planktonic algae and are abundant in the marine sedimentary archives. Among nannofossils, the Nannoconus group was the main planktonic carbonate bio-producer in Early Cretaceous seas (~152 - ~120 Ma) with their heavy carbonate exoskeleton (~200 - 1100 picogram), responsible for massive carbonate accumulations. These massively produced bio-calcites must have certainly significantly modified the marine chemistry upon transfer to the ocean sedimentary record.

Nannoconus are conical in shape with interlocking arrangement of calcitic plates (length ~ 0.5 – 1 µm, thickness ~ 100 nm) spanned by a central canal. Different morpho-groups with different stratigraphic ranges have been recognized. However, the 3D structure of Nannoconus is not yet fully understood and needs to be investigated to analyze the inter-plates geometrical relation and reconstruct the growth pattern. Moreover, some studies show that bio-molecules, such as amino-acids, assert control on the shape of constituent elements, such as plates¹, the overlapping pattern, and the inter-elemental geometrical² relation in bio-calcified structures formed by individual components, with case studies in calcareous plankton. Similarly, an important question can be asked: Did the micro-structural arrangement of plates of Nannoconus (the bio-calcified structure) also result from a “particular biomolecule” assisted calcification? So far, there are not enough studies explaining the exoskeletal construction from the calcitic plates to answer the mentioned question. To understand the microstructure at the nanometer level, finer than the thickness of individual plate we have applied a set of Ptychography X-ray computed tomography (PXCT) with synchrotron radiation³ at SWING Beamline of SOLEIL (French synchrotron), Paris, France, on several well-preserved Nannoconus.

The experiment resulted in a series of tomographic image slices (with ~ 40 nm 3D resolution) for the whole exoskeleton of each specimen of Nannoconus. The images are combined in ORS-Dragonfly software to observe the 3-dimensional internal and external view of individual specimens and to separate a plate from each specimen of Nannoconus.

The 3D observation and the segmentation of the plates have shown different features. These plates rotate in an anticlockwise overlapping pattern with a proper angular increment between two successive plates. Such rotations lead to form layers arranged in a helical manner to create the entire exoskeleton. Each species of Nannoconus is characterised by a particular shape of the calcitic plate. Hence, the physical parameters such as anticlockwise overlapping, plate-shape, and geometrically regulated “plate-to-layers” micro-structures are very similar to structural features as seen in some bio-molecule influenced calcification^1,2 This suggests that bio-molecule(s) have substantial influence in regulating the micro-structure in Nannoconus. The regulation on the micro-structure to maintain a particular geometric relation among the plates possibly controlled the numbers of plates, creating a massive exoskeleton of Nannoconus.

¹Orme, C. A., et al., Nature 411.6839 (2001): 775-779, ²Jiang, W. et al., Nat. Commun. 8, 15066 (2017), ³Dierolf, M. et al., Nature 467, 436-439 (2010).