The morphology of coccoliths as revealed by coherent X-ray diffraction imaging

International audience ; INTRODUCTION: Coccoliths are micrometric biogenic calcium carbonate plates forming the external skeleton of coccolithophores, the main marine calcifying unicellular phytoplankton [1]. Understanding how environmental factors influence the degree of calcification of coccoliths is of significant interest [2]. However due to their minute size and three-dimensional structure the study of coccoliths is not trivial and new characterization methods are required. EXPERIMENTAL: To reveal the morphology of coccoliths we successfully applied synchrotron three-dimensional coherent X-ray diffraction microscopy to image whole coccolithophores of the Gephyrocapsa, Emiliania and Reticulofenestra families with a resolution close to 30 nm (Fig. 1). The quality of the images allows isolating individual coccoliths from a coccosphere using segmentation procedure. Hence the number of segments, volume and mass of each coccolith could be determined. RESULTS AND DISCUSSION: After isolating each coccolith from the coccospheres, we show that the number of segments and the mass of each coccolith depend on their central area perimeter. Assuming that this area is reminiscent of the organic base plate scale (obps) on which the biomineralization was initiated, our findings suggest the dominant role of the obps in the CaCO3 nucleation site number and the growth of coccoliths [3], strengthening the interplay between photosynthesis and biomineralization. CONCLUSIONS: By providing high resolution 3D images, CXDI emerges as a powerful technique for investigation of external skeleton of coccolithophores up to 7 microns in size. REFERENCES: 1.P. Westbroek, J. R. Young,K. Linschooten Authors, Coccolith Production (Biomineralization) in the Marine Alga Emiliania huxleyi, The Journal of Protozoology, 1989; 36:368–373 2.J. R. Young, P. Ziveri, Calculation of Coccolith volume and its use in calibration of carbonate flux estimate, Deep Sea Research Part II: Topical Studies in Oceanography, 2000, 47:1679-1700 3.T. Beuvier, I. ...