Dynamic change of calcium-rich compartments during coccolithophore biomineralization

Coccolithophores are abundant marine phytoplankton that produce biomineralized calcite scales, called coccoliths, which sequester substantial amounts of carbon and play a significant role in biogeochemical cycles. However, mechanisms underlying the storage and transport of ions essential for calcification remain unresolved. We used ptychographic X-ray computed tomography under cryogenic conditions to visualize intracellular calcium-rich structures involved in the storage of calcium ions in the coccolithophore species Chrysotila carterae. During calcification, we observed a range of structures, from small electron-dense bodies within larger compartments, to denser and distributed globular compartments, before returning to small bodies once scale formation is complete. Nanobeam-scanning X-ray fluorescence measurements further revealed these electron-dense bodies are rich in P and Ca (molar ratio of [~]4:1). We infer from the dynamic nature of structures that these bodies are part of required cellular calcium ion transport pathways, a fundamental process critical for understanding the response of coccolithophores to climate change.