NSFGEO-NERC An unexpected requirement for silicon in coccolithophore calcification: ecological and evolutionary implications.

海洋覆盖了地球表面的四分之三以上，海洋中的微小藻类负责地球上一半的光合作用。这些被称为浮游植物的单细胞生物构成了海洋食物网的基础，它们的活动可能对我们星球的地质产生巨大影响。有一类浮游植物被称为颗石藻，它们能产生一层碳酸钙板（颗石藻），并能在海洋中形成大量水华。当颗石藻死亡时，它们的颗石体沉降到海底，导致白垩和石灰石等沉积岩的形成。在海洋的许多地方，营养物质（如氮和磷）的低可用性限制了浮游植物的生长。对营养盐的竞争在决定哪些浮游植物物种可以在不同的环境中生长方面起着重要的作用。现代海洋中最成功的浮游植物群体之一是硅藻，它们生长迅速，使许多其他浮游植物类型难以与它们竞争。然而，硅藻需要大量的溶解硅来制造它们的硅细胞壁。在某些海洋环境中，硅的存在限制了硅藻的生长，使其他浮游植物（不需要硅）在它们的位置上生长。通常认为钙化颗石藻不需要硅。然而，我们最近发现，一些重要的颗石藻物种实际上拥有硅转运蛋白，这是类似的硅藻使用。值得注意的是，我们发现这些颗石藻使用硅来制造碳酸钙颗石藻。因此，硅藻中二氧化硅的形成过程和颗石藻中方解石的产生过程，以前被认为是不同的过程，显示出完全出乎意料的联系。这些发现对浮游植物生物矿化的演化以及颗石藻和硅藻之间的竞争相互作用具有重要意义。并非所有的颗石藻都需要硅。我们发现，负责大规模颗石藻花，Emiliania huxleyi，不具备硅转运蛋白，并表现出不需要硅的钙化过程中的物种。不需要硅可能使水华形成物种能够在硅含量低的地区生长得更好（例如硅藻水华之后）。因此，有一个明确的需要了解的作用，硅在颗石藻biology.In本提案中，我们将解决这个问题，使用实验室实验和计算建模方法相结合。首先，我们将使用分子遗传学和实验室实验来确定哪些主要的颗石藻物种表现出对硅的需求。然后，我们将选择物种进行详细的生理分析，以确定硅如何有助于球石的形成，以及球石藻如何从周围的海水中吸收硅。这些研究将使我们能够研究硅需求的进化历史，并确定某些谱系何时似乎失去了这种特性。利用我们实验工作中获得的硅吸收和利用参数，我们将使用计算机模拟来模拟全球颗石藻分布，并确定对硅的需求在颗石藻生态学中起重要作用的环境。这项研究将为颗石藻的生理学、生态学和进化提供新的见解，包括颗石藻是如何产生的以及为什么产生的信息，目前人们对此知之甚少。这项研究还将使我们了解颗石形成的演变，这对于我们了解颗石如何受到地球气候过去变化的影响以及它们如何应对未来的变化至关重要。

The oceans cover more than three quarters of the surface of the Earth and tiny algae in our seas are responsible for half of all photosynthesis on our planet. These single celled organisms, known as phytoplankton, form the basis of marine food webs and their activities can have an enormous impact on the geology of our planet. One group of phytoplankton known as the coccolithophores produce a covering of calcium carbonate plates (coccoliths) and can form vast blooms in the oceans. When the coccolithophores die, their coccoliths settle to the ocean floor, leading to the formation of sedimentary rocks, such as chalks and limestones.In many parts of the ocean the low availability of nutrients (such as nitrogen and phosphorus) limits phytoplankton growth. Competition for nutrients plays an important role in determining which phytoplankton species can grow in different environments. One of the most successful phytoplankton groups in modern oceans is the diatoms, which are fast-growing, making it difficult for many other phytoplankton types to compete with them. However, diatoms need lots of dissolved silicon to make their silica cell walls. In some marine environments, the availability of silicon limits the growth of diatoms, allowing other phytoplankton (which do not need silicon) to grow in their place.It is commonly thought that the calcifying coccolithophores have no requirement for silicon. However, we have recently discovered that some important coccolithophore species actually possess silicon transporters that are similar to those used by diatoms. Remarkably, we found that these coccolithophores use silicon to make their calcium carbonate coccoliths. Therefore the processes of silica formation in diatoms and calcite production in coccolithophores, which were previously believed to be distinct processes, show a completely unexpected link. These findings have important implications for the evolution of biomineralisation in phytoplankton and for the competitive interactions between coccolithophores and diatoms.Not all coccolithophores show a requirement for silicon. We found that the species responsible for the massive coccolithophore blooms, Emiliania huxleyi, does not possess silicon transporters and exhibits no need for silicon in the calcification process. The absence of a requirement for silicon may have enabled bloom-forming species to grow better in areas where silicon is low (e.g. after a diatom bloom). There is therefore a clear need to understand the role of silicon in coccolithophore biology.In this proposal we will address this issue using a combination of laboratory experiments and computational modelling approaches. Firstly, we will use molecular genetic and laboratory experiments to determine which of the major coccolithophore species exhibit a requirement for silicon. We will then select species for detailed physiological analysis, to determine how silicon contributes to the formation of coccoliths and how coccolithophores take up silicon from the surrounding seawater. These studies will allow us to examine the evolutionary history of the requirement for silicon and determine when certain lineages appear to have lost this trait. Using parameters on Si uptake and usage derived from our experimental work, we will use computer simulations to model global coccolithophore distributions and identify environments where the requirement for Si appears to be playing an important role in coccolithophore ecology.The research will provide novel insight into physiology, ecology and evolution of coccolithophores, including information on how and why coccoliths are produced, which is currently poorly understood. The research will also inform us on the evolution of coccolith formation, which will be vitally important if we are to understand how coccolithophores have been influenced by past changes in the Earth's climate and how they may respond to changes in the future.