Deepening on coccolithogenesis by mapping a highly-calcifying species of coccolithophores, Helicosphaera carteri

Coccolithophores are micrometer-sized, unicellular microalgae that play a key role in the global carbon cycle and climate change mitigation. The peculiarity of these organisms is to cover their cell with an exoskeleton made of intricate calcium carbonate scales, called coccoliths. The production and accumulation into the deep-sea sediments of this biogenic calcite represent an effective carbon sequestration mechanism crucial in the global carbon removal. The physiological processes involved in the coccolith formation, known as "coccolithogenesis", remain only partially known, but are fundamental for understanding the biological strategies involved in the crystal design.
Studying the calcium (Ca) accumulation centers within the coccolithophore cell is a mandatory step to move forward our knowledge in this field. These centers are formed by intracellular granules of Ca ions, together with polyphosphate, that accumulate and contribute to the coccolith formation. Only two species, Chrysotila carterae and Emiliania huxleyi, with slightly calcified coccoliths compared to the target species Helicosphaera carteri, have been already studied using the cryoSXT returning important results in defining the Ca accumulation centers, and thus, in the coccolithogenesis. The advanced imaging techniques available at cryoSXT on calcium L-edge will allow studying the nanometric structure and distribution of Ca within the inner cell of H. carteri, deepening on a heavily calcifying coccolithophore. To do so, the cells will be decalcified with hydrochloric acid (HCl), and then allowed to recalcify for specific periods (1 and 2 hours). This approach will provide a detailed temporal mapping of Ca accumulation centers along the formation of one coccolith, and when possible, also Ca chemical state.
A deeper understanding of the mechanisms controlling coccolithogenesis is crucial not only for advancing knowledge on the physiology and evolutionary adaptation of this group of microalgae, but also for opening new perspectives in crystal design (biomimetc field).