Nanobeam-scanning X-ray fluorescence microscopy reveals the elemental composition of dense intracellular bodies in biomineralizing coccolithophores.

Coccolithophore microalgae intracellularly produce nanostructured calcitic platelets, known as coccoliths, through a biologically-controlled mineralization process. Mature coccoliths are secreted to the cell surface and assembled into a shell that envelops the cell. The large-scale global production of coccoliths, followed by their sedimentation to the ocean floor, significantly contributes to carbon cycling. Despite progress in understanding the biomineralization pathway of coccoliths, we are still limited in our ability to predict how future climate conditions will impact coccolith formation and thus ocean carbon fluxes. Investigating coccolith biomineralization at the single-cell level is therefore critical to advance our understanding but remains challenging since current imaging techniques lack the combined spatial and temporal resolution coupled with element-specific detection to follow processes in situ. In light of this gap, nanobeam-scanning X-ray fluorescence microscopy (nano-XRF) in the hard X-ray regime is employed here to investigate the intracellular elemental distribution of the coccolithophore Gephyrocapsa huxleyi (formerly Emiliania huxleyi) achieving a resolution of 100 nm and elemental detection from phosphorus (P) to zinc (Zn). Calcium- and phosphorus-rich intracellular bodies, previously proposed to be involved in coccolith biomineralization, were observed in cells initially prepared ex situ by drying. Interestingly, nano-XRF imaging reveals metal species (e.g., Mn, Fe, Zn) within these bodies that were not detected in earlier studies, suggesting multiple biological roles for these structures. Moving towards native-state imaging, G. huxleyi was then imaged in the hydrated state using a dedicated liquid cell device. Measurements were performed on G. huxleyi cells both with and without coccolith shell in sea water medium and compared to those of dried cells, demonstrating comparable image quality. The future potential and limitations of liquid cell nano-XRF imaging for coccolithophores and other microorganisms are further discussed.