Elucidating the biomineralization pathway of calcite in coccolithophores

• Applicants: Dr. Damien Faivre; Professor Dr. André Scheffel
• Subject Area: Synthesis and Properties of Functional Materials ; Biomaterials ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2014 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 257409621

The biologically controlled formation of calcium carbonate (CaCO3) is likely the most prominent example for the widespread phenomenon of biomineralization. Biogenic CaCO3 is predominantly deposited in form of the crystalline polymorphs calcite and aragonite. Biogenic crystals of CaCO3 often adopt remarkable morphologies, which are unknown from their abiotically formed counterparts. The nucleation and morphogenesis of biogenic calcite crystals was for a long time believed to follow the same pathway as known for abiotic calcite but recently it was discovered that in calcifying metazoans crystalline CaCO3 forms via an amorphous precursor phase. How widely distributed the amorphous calcium carbonate (ACC) pathway among calcifying organisms is and where and when this pathway evolved is an open question. Here we propose a project to elucidate the crystallization pathway underlying in the formation of unusually-shaped calcite crystals produced by unicellular algae known as coccolithophores. The crystals are parts of larger structures termed coccoliths which decorate the surface of each coccolithophore cell. To study the calcite crystallization pathway in coccolithophores, we have established protocols to obtain CaCO3-free cells and to switch coccolith formation 'on' and 'off'. This enables us to study the formation process of the coccolith CaCO3 phase time-resolved. By using X-ray absorption spectroscopy and confocal Raman microcopy, which both are techniques able to identify ACC and any other calcium-rich intermediate, we will determine the phase of the CaCO3 at different time points during the maturation of a coccolith.A relevant factor in crystal morphogenesis in general is the incorporation of inorganic and organic molecules. At present, only sparse information are available about organic impurities in coccolith calcite. The available data set is currently too small to certainly conclude that coccolith calcite is in general free of intra-crystalline organic macromolecules, which is what the few investigated samples suggest. To clarify this point, we have planned here a systematic examination of coccolith calcite from different species for intra-crystalline biomolecules by using synchrotron-based X-ray diffraction spectroscopy. The outcome of the here suggested experiments will clearly advance our understanding of the calcite mineralization pathway in coccolithophores and display an important piece of work on the way to decipher and reprogram the calcite morphogenesis machinery in coccolithophores.

• DFG Programme: Research Grants
• Participating Persons: Dr. Luca Bertinetti, Ph.D.; Dr. Admir Masic, Ph.D.; Professorin Dr. Yael Politi, Ph.D.