Deciphering coccolith formation: advanced microscopy insights from the biomineralization of Gephyrocapsa huxleyi

Coccolithophores are unicellular marine phytoplankton that produce complex and intricately shaped mineralized scales called coccoliths. Coccoliths are produced in an intracellular vesicle where crystal nucleation occurs, from which several individual calcite units develop with anisotropic crystallographic facets, prompting studies into the cellular mechanisms that control crystal growth within the cell. Here, we characterize those morphological developments in three dimensions that occur during the formation of coccoliths by the species Gephyrocapsa huxleyi using cryo-ptychographic X-ray computed tomography. This technique is ideally suited to study coccolith mineral development, as intracellular structures can be imaged intact in their native state without needing to disrupt cells. Combined with additional imaging of developing coccoliths using cryo-transmission electron microscopy and scanning electron microscopy, we report a quasi-continuous sequence of coccolith growth with the respective calculated mass for each state. Within this growth sequence, we identified that a significant proportion of the coccoliths were at the proto-coccolith ring stage, consistent with a rate-limiting step in their formation that we correlated to the shift in growth of the crystals from isotropic to anisotropic, potentially owing to constraints imposed by adjacent crystal units. This study provides unique insights into the morphological evolution of coccoliths during their development, and strengthens our understanding of this important biomineralization process.