Assessing how cell size constrains carbon uptake in diatoms using direct measurements of cell surface carbonate chemistry

硅藻是一组单细胞藻类，代表了我们星球上一些最重要的光合生物。硅藻在营养丰富的沿海地区尤其丰富，在那里它们构成了食物网的基础，支持着渔业和海产品产业。据估计，硅藻贡献了全球光合作用的20%。因此，令人惊讶的是，关于硅藻吸收碳的形式以及这些机制如何受到细胞大小的影响存在重大的不确定性。硅藻从非常小到非常大（直径5-200微米），甚至可以形成菌落，以细胞链的形式连接在一起。这种巨大的大小多样性对每个物种从其环境中获取营养的能力有重大影响，对较大物种的供应可能受到其扩散边界层的限制。因此，了解细胞大小如何限制营养获取是我们理解硅藻生态学和不同物种分布的核心，尽管缺乏对细胞周围扩散边界层的直接测量。尽管海水中含有大量溶解的无机碳，但其中只有一小部分以二氧化碳的形式存在。因此，通过扩散向细胞提供的二氧化碳不足以支持在硅藻中观察到的高光合作用速率。这个问题在大型物种中更为严重，因为在细胞表面周围有明显的扩散边界层。因此，硅藻和其他海洋浮游植物必须利用碳酸氢盐池（HCO3-），要么通过积极地将其运送到膜上，要么通过一种酶（细胞外碳酸酐酶）催化其转化为二氧化碳，然后二氧化碳可以扩散到膜上。然而，在技术上很难测量这些不同机制所吸收的碳的比例，不同的硅藻种类表现出相当大的差异。此外，胞外碳酸酐酶的作用一直存在争议。由于这种不确定性，我们对二氧化碳供应的变化如何影响硅藻群落的组成没有一个机制上的理解。在未来的几个世纪里，海水中二氧化碳的浓度预计会发生巨大的变化，这种不确定性阻碍了我们预测不同物种对二氧化碳可用性变化的反应。如果我们要了解硅藻细胞如何从海水中获取碳，就必须提高对硅藻细胞周围微环境的认识。我们已经开发出微小的离子选择性微电极，可以放置在单个硅藻细胞的表面。通过测量pH值和碳酸盐（CO32-），我们可以计算出碳在膜上的通量，并估计到细胞表面的二氧化碳供应可能受到限制的程度。本项目将利用这些新技术来解决与硅藻获取碳有关的一些主要问题。我们希望研究二氧化碳供应在多大程度上限制了不同大小的细胞，并研究如何调整碳吸收机制以应对碳供应（例如二氧化碳升高）或碳需求（例如在强光下需要更高的碳固定率）的变化。我们还将研究二氧化碳的供应是否会影响某些物种形成链的能力，以便了解我们可能期望这些物种成功生存的环境。这些使用微电极的研究将辅以分子遗传学方法来研究酶的作用，细胞外碳酸酐酶似乎在一些硅藻的二氧化碳供应中起着关键作用。最后，我们还将研究硅藻的自然种群，以了解在典型硅藻华的整个过程中，它们如何受到二氧化碳可用性变化的影响。

The diatoms are a group of unicellular algae that represent some of the most important photosynthetic organisms on our planet. Diatoms are particularly abundant in nutrient rich coastal regions where they form the base of the food web, supporting fishing and seafood industries. It is estimated that diatoms contribute up to 20 % of global photosynthesis. It is therefore surprising that there are major uncertainties relating to the form of carbon taken up by diatoms and how these mechanisms are influenced by the size of the cell.Diatoms range from very small to very large (5-200 micrometre diameter) and can even form colonies, in the form of chains of cells linked together. This huge diversity in size has a major influence on the ability of each species to acquire nutrients from its environment, with the supply to larger species potentially limited by their diffusive boundary layer. Understanding how cell size constrains nutrient acquisition is therefore central to our understanding of diatom ecology and the distribution of different species, although direct measurements of the diffusive boundary layer around cells are lacking.Although seawater contains a plentiful supply of dissolved inorganic carbon, only a small proportion of this is present as carbon dioxide (CO2). The supply of CO2 to the cell by diffusion is therefore not sufficient to support the high rates of photosynthesis observed in diatoms. This problem is much greater in large species, due to the significant diffusive boundary layer around the cell surface. Diatoms, and other marine phytoplankton, therefore have to utilise the pool of bicarbonate (HCO3-), either by actively transporting it across the membrane or by using an enzyme (extracellular carbonic anhydrase) to catalyse its conversion to CO2, which can then diffuse across the membrane. However, it is technically difficult to measure the proportion of carbon taken up by these different mechanisms and different diatom species show considerable variability. Moreover, the role of the enzyme extracellular carbonic anhydrase has been much disputed. Because of this uncertainty, we do not have a mechanistic understanding of how changes in CO2 supply can influence the composition of diatom communities. With the concentration of CO2 in seawater predicted to change dramatically in the coming centuries, this uncertainty hampers our ability to predict how different species may respond to the changing availability of CO2. Improved knowledge of the microenvironment around diatom cells is necessary if we are to understand how they acquire carbon from seawater. We have developed tiny ion-selective microelectrodes that can be placed at the surface of a single diatom cell. By measuring pH and carbonate (CO32-), we can calculate fluxes of carbon across the membrane and estimate to what extent the supply of CO2 to the cell surface may be limiting.This project will use these new techniques to address some of the major questions relating to carbon acquisition by diatoms. We wish to examine the extent to which CO2 supply is limiting to cells of different sizes and examine how mechanisms of carbon uptake are adjusted to cope with changes in carbon supply (e.g. elevated CO2) or carbon demand (e.g. a greater rate of carbon fixation is needed at high light). We will also examine whether the supply of CO2 influences the ability of certain species to form chains, in order to understand the environments where we might expect these species to be successful. These studies using microelectrodes will be complemented by a molecular genetic approach to study to the role of the enzyme, extracellular carbonic anhydrase, which appears to play a critical role in the supply of CO2 to some diatoms. Finally, we will also examine natural populations of diatoms to see how they are influenced by changes in the availability of carbon dioxide throughout the progression of a typical diatom bloom.