H+ fluxes in phytoplankton - a mechanistic and modelling study of their physiological roles and impact upon community responses to ocean acidification

海洋去除了我们释放到大气中的大约一半的二氧化碳（CO2），并产生了我们呼吸的大约一半的氧气。进行光合作用的海洋浮游植物在这些过程中发挥着重要作用，促进了全球碳、氮和硫的循环。浮游植物不仅仅是单细胞植物。它们代表了一个极其多样化的藻类集合，具有许多新的特征和复杂的进化历史，但仍然知之甚少。由于燃烧化石燃料而造成的大气中二氧化碳的增加具有重大的气候影响。海洋吸收大量二氧化碳的一个结果是海洋表面沃茨酸化--预计到世纪末，pH值将从8.2下降到7.7。由于海洋pH值保持稳定数百万年，这可能对许多海洋生物产生深远的影响，这些生物在最近的进化史中以前没有经历过这种pH值水平或变化率。海洋酸化还将改变碳酸盐和营养离子的水平，所有这些都可能对海洋浮游植物的生理产生重大影响。虽然其中一些影响正在被深入研究，但pH值下降本身对浮游植物生理的直接影响在很大程度上被忽视了。海洋浮游植物，像所有生物一样，必须严格调节其细胞的pH值，以保持细胞过程的有利条件。我们一直在研究颗石藻的pH调节机制，颗石藻是一组重要的浮游植物，通过产生碳酸钙鳞片（颗石藻）在全球碳循环中发挥重要作用，碳酸钙鳞片在细胞死亡后沉入深海。我们已经发现，颗石藻利用其外细胞膜上的蛋白质孔（通道）来调节细胞内的pH值。这些通道允许H+在细胞中的酸性增加时离开细胞，从而保持细胞内的pH恒定。这对于颗石藻来说尤其重要，因为细胞中颗石藻的产生会导致H+的不断产生，而H+需要被去除，否则细胞内的酸度会增加到危险的水平。这种新机制对外部pH值的变化非常敏感，在不久的将来海洋pH值水平下可能不再有效。我们还在硅藻中发现了这种形式的H+通道，硅藻是浮游植物中数量最多、生产力最高的群体。 值得注意的是，我们已经发现，球石藻细胞在实验室中适应生长在较低的pH值不再出现使用H+通道。虽然这表明颗石藻可能能够科普较低的pH值，但我们不知道这种机械开关的更广泛或长期的生理意义。这显然是我们迫切需要了解的。这个项目将详细研究颗石藻和硅藻的pH稳态机制。我们的建模研究预测，细胞的pH值调节机制可能会有所不同，在大型和小型浮游植物物种，因为这些将经历非常不同的pH值波动在细胞表面由于细胞大小的物理效应在细胞表面扩散。我们建议，浮游植物物种采用的pH值稳态的不同机制可能在这些生物对海洋酸化的反应中发挥重要作用。 为了衡量浮游植物生理学的这些新方面将如何在更广泛的范围内影响海洋生态系统，我们将使用建模方法来研究浮游植物细胞中的细胞H+通量如何响应环境的变化。这些数学模型将使我们能够预测目前和未来不同浮游植物物种所经历的pH值范围，并使我们能够评估它们对将在相关方案中研究的天然浮游植物种群多样性的影响。

The oceans remove about half of the carbon dioxide (CO2) that we release into the atmosphere and produce about half of the oxygen that we breathe. The photosynthetic marine phytoplankton play a major role in these processes, contributing to global carbon, nitrogen and sulphur cycling. Phytoplankton are not simply single-celled plants. They represent an extremely diverse collection of algae with many novel traits and complex evolutionary histories which are still poorly understood. The increase in atmospheric carbon dioxide due to the burning of fossil fuels has major climatic implications. A result of the oceans absorbing much of this CO2 is the acidification of surface ocean waters - a drop from pH 8.2 to pH 7.7 is predicted by the end of the century. As ocean pH has remained stable for many millions of years this may have profound effects on many marine organisms that have not previously experienced this level of pH or rate of change during their recent evolutionary history. Ocean acidification will also change the levels of carbonate and nutrient ions, all of which may have significant impacts on the physiology of marine phytoplankton. While some of these impacts are being intensively studied, the direct effect of decreased pH itself on phytoplankton physiology has been largely overlooked. Marine phytoplankton, like all organisms, must tightly regulate their cellular pH by in order to maintain favourable conditions for cellular processes. We have been studying mechanisms of pH regulation in coccolithophores, an important group of phytoplankton that play a major role in the global carbon cycle through their production of calcium carbonate scales (coccoliths) which sink to the deep ocean following cell death. We have discovered that coccolithophores use protein pores (channels) in their outer cell membrane to regulate pH inside the cell. These channels allow H+ to exit from the cell whenever acidity in the cell increases, thus acting to keep pH inside the cell constant. This is particularly important for coccolithophores as the production of coccoliths in the cell results in a constant production of H+ which need to be removed or the acidity inside of the cell would increase to dangerous levels. This novel mechanism is extremely sensitive to changes in external pH and may no longer function effectively at near future ocean pH levels. We have also found this form of H+ channel in diatoms, the most numerous and productive group of phytoplankton. Remarkably, we have found that coccolithophore cells acclimated in the laboratory to growth at lower pH no longer appear to use a H+ channel. While this suggests coccolithophores may be able to cope with lower pH, we do not know the wider or long-term physiological implications of this mechanistic switch. This is clearly something we urgently need to understand. This project will examine in detail the mechanisms of pH homeostasis in coccolithophores and diatoms. Our modelling studies predict that mechanisms of cellular pH regulation are likely to differ in large and small phytoplankton species as these will experience greatly different fluctuations in pH at the cell surface due to physical effects of cell size on diffusion at the cell surface. We propose that different mechanisms of pH homeostasis employed by phytoplankton species may play a major role in the response of these organisms to ocean acidification. In order to gauge how these novel aspects of phytoplankton physiology will impact upon marine ecosystems on a broader scale, we will use modelling approaches to examine how cellular H+ fluxes in phytoplankton cells respond to changes in their environment. These mathematical models will enable us to predict the ranges of pH experienced by different phytoplankton species both currently and in the future and will allow us to evaluate their impact on the diversity of natural phytoplankton populations that will be studied in related programmes.