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。光合作用的海洋浮游植物在这些过程中发挥着重要作用，有助于全球碳、氮和硫的循环。浮游植物不是简单的单细胞植物。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史，这些仍然鲜为人知。燃烧化石燃料导致的大气二氧化碳增加对气候有重大影响。海洋吸收大量二氧化碳的结果是表层海水的酸化--据预测，到本世纪末，酸碱度将从8.2降至7.7。由于海洋酸碱度几百万年来一直保持稳定，这可能对许多海洋生物产生深远影响，这些海洋生物在其最近的进化史上从未经历过这种酸碱度或变化速度。海洋酸化还会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降低本身对浮游植物生理学的直接影响在很大程度上被忽视了。海洋浮游植物和所有生物体一样，必须严格调节其细胞的pH值，以维持细胞过程的有利条件。我们一直在研究球藻基团的pH调节机制，球藻基团是一类重要的浮游植物，通过产生碳酸钙鳞片(球藻)在全球碳循环中发挥重要作用，在细胞死亡后沉入深海。我们已经发现，球虫体利用其外细胞膜上的蛋白质孔(通道)来调节细胞内的pH。当细胞中的酸度增加时，这些通道允许H+从细胞中流出，从而保持细胞内的pH恒定。这一点对球藻特别重要，因为球藻在细胞中的产生导致H+的不断产生，需要去除H+，否则细胞内的酸性将增加到危险的水平。这种新的机制对外界pH值的变化极其敏感，在不久的将来海洋pH值水平可能不再有效发挥作用。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、产量最高的一类。值得注意的是，我们发现在实验室中适应低pH生长的球藻细胞似乎不再使用H+通道。虽然这表明球藻生物体可能能够应对较低的pH，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事情。这个项目将详细研究球藻和硅藻中pH动态平衡的机制。我们的模拟研究预测，在大型和小型浮游植物物种中，细胞pH调节的机制可能不同，因为由于细胞大小对细胞表面扩散的物理影响，这些物种在细胞表面的pH波动将非常不同。我们认为，浮游植物物种所采用的不同的pH动态平衡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不同浮游植物物种目前和未来经历的酸碱度范围，并使我们能够评估它们对自然浮游植物种群多样性的影响，这些影响将在相关方案中进行研究。

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.