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。生物成因碳酸钙主要以晶型方解石和文石的形式沉积。CaCO3的生物晶体通常具有显著的形貌，这是它们在非生物形成的晶体中所没有的。长期以来，生物型方解石晶体的成核和形态发生一直被认为遵循与非生物方解石相同的途径，但最近发现，在钙化的后生动物中，晶态CaCO3是通过无定形前驱物相形成的。无定形碳酸钙(ACC)途径在钙化生物中的分布如何，以及这条途径是在哪里以及何时进化的，这是一个悬而未决的问题。在这里，我们提出了一个项目，以阐明由单细胞藻类产生的异常形状的方解石晶体形成的结晶途径，称为球石发酵体。这些晶体是更大的结构的一部分，称为球石，装饰着每个球石细胞的表面。为了研究球石中的方解石结晶途径，我们建立了获得无CaCO3细胞和切换球石形成的方案。这使得我们能够研究球石CaCO3相的形成过程。通过使用X射线吸收光谱和共聚焦拉曼显微镜，这两种技术都可以识别ACC和任何其他富钙中间体，我们将确定球石成熟过程中不同时间点的CaCO3物相。晶体形态形成的一个相关因素通常是无机和有机分子的结合。目前，关于球石方解石中的有机杂质的信息很少。目前可用的数据集太小，不能肯定地得出结论，球石方解石一般不含晶内有机大分子，这是少数几个研究样本所表明的。为了阐明这一点，我们计划利用基于同步加速器的X射线衍射谱对来自不同物种的球石方解石进行晶内生物分子的系统研究。本文提出的实验结果将明显地促进我们对球石中方解石矿化途径的理解，并在破译和重新编程球石中的方解石形态发生机制方面展示了一项重要的工作。

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.