Elucidating the biomineralization pathway of calcite in coccolithophores

生物控制的碳酸钙（CaCO3）的形成可能是生物矿化现象普遍存在的最突出的例子。生物成因的碳酸钙主要以多晶型方解石和文石的形式沉积。碳酸钙的生物晶体通常采用显着的形态，这是未知的，从他们的非生物形成的同行。生物方解石晶体的成核和形态发生很长一段时间被认为遵循与非生物方解石相同的途径，但最近发现，在钙化后生动物中，结晶CaCO3通过无定形前体相形成。无定形碳酸钙（ACC）途径在钙化生物中的分布有多广，以及该途径在何处以及何时进化是一个悬而未决的问题。在这里，我们提出了一个项目，以阐明结晶途径的形成不寻常的形状的方解石晶体所产生的单细胞藻类称为颗石藻。这些晶体是被称为颗石的较大结构的一部分，它们装饰着每个颗石细胞的表面。为了研究颗石藻中的方解石结晶途径，我们已经建立了获得无碳酸钙细胞和切换颗石形成“开”和“关”的协议。这使我们能够研究时间分辨的颗石碳酸钙相的形成过程。利用X射线吸收光谱和共焦拉曼显微镜技术，这两种技术都能够识别ACC和任何其他富钙中间体，我们将确定在不同的时间点在一个颗石成熟过程中的CaCO3的相。目前，只有稀疏的资料，可利用的有机杂质的颗石方解石。现有的数据集目前太小，不能肯定地得出结论，颗石方解石一般不含晶内有机大分子，这是少数调查样品的建议。为了澄清这一点，我们计划在这里的一个系统的检查颗石方解石从不同物种的晶内生物分子，通过使用基于同步辐射的X射线衍射光谱。这里建议的实验结果将清楚地推进我们的理解的方解石矿化途径的颗石藻和显示的方式来破译和重新编程的方解石形态发生机制的颗石藻的一项重要工作。

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.