Coccolithophores put a CAP on calcification and carbon

Unlike the majority of biomineralization, mediated via proteins, the coccolithophores employ acidic polysaccharides (CAPs) as the template for the multitude of intricate calcium carbonate liths which are precipitated inside their cells and expelled to form an external spherical armour. Massive accumulations of these calcium carbonate coccoliths (we estimate a flux of ~ 6x10²⁵ liths/year) form sediments on the deep seafloor, a dominant sink of carbon from the atmosphere over geological timescales, and a buffer for seawater chemistry. This acidic, coccolith-associated polysaccharide (CAP) plays a dual role, likely determined by the calcifying vesicle chemistry, promoting and limiting precipitation.

We have developed and demonstrated novel techniques for the extraction and characterisation of these biomineral-mediating molecules, encapsulated and preserved within liths from sediments as old as the Mesozoic. Modern cultures of extant coccolithophores show that the composition of CAP is distinctive for different species, and even strains of e.g. Emiliania huxleyi, suggestive that interactions with galacturonic acid residues are key to dictating the architecture of the liths. Furthermore, it appears that the galacturonic acid content reflects an adaptive response of the CAP to the chemistry within the coccolith vesicle where calcification is induced; in particular the size of the internal carbon pool and inferred saturation state. Extraction of CAPs from the Coccolithale fraction of sediments suggests that the galacturonic acid content of these relatively large and inefficient carbon concentrators evolves in response to declining CO₂ availability and thus may provide a novel means for reconstructing paleo pCO₂.