Empirical determination of the interaction landscape for temperature, CO2 and nitrate for a model diatom

浮游植物是一种小而强大的单细胞光合生物，它们构成了大多数水生食物网的基础，影响营养循环，并负责地球上大约一半的碳固定。在浮游植物中，硅藻约占海洋初级产量的40%，占全球产量的20%，据信约占全球碳输出的40%，其中一些被封存在深海中。在中高纬度地区，如英国周围，它们提高了海洋食物网的营养效率，在初级生产、食草动物和大型海洋动物之间提供了直接联系。硅藻生活的海洋环境正在迅速变化，因为气候变暖，二氧化碳增加，光合作用发生的海洋上部营养水平（尤其是氮）下降。我们对浮游植物如何孤立地对这些环境变化作出反应有了相当好的了解。至少在高纬度地区，温度和二氧化碳的增加促进了浮游植物的生长，而氮供应的减少则减少了浮游植物的生长。然而，令人惊讶的是，关于浮游植物如何对所有这些同时变化的事物做出反应的实验数据很少。最近的研究表明，把环境变化放在一起考虑是很重要的，因为生物体同时对几种变化的反应不能从单独对每种变化的反应中预测出来。例如，低氮不仅会减少硅藻的总体种群规模，而且还会使它们对高温更敏感，因此，营养限制可能会加剧海洋变暖的影响。此外，我们有其他浮游植物的数据表明，二氧化碳浓度升高可以部分减轻其他一些环境变化的负面影响，但我们不知道这是否适用于硅藻。尽管硅藻在海洋生态系统中很重要，但很少有实验能让我们以一种普遍的方式了解硅藻是如何对温度、硝酸盐和二氧化碳同时变化做出反应的。这一点，再加上对未来海洋状况本身预测的不确定性，意味着目前我们甚至无法预测未来几十年全球初级生产预计会增加还是减少。这种不确定性使得准确预测生态系统服务（包括海洋碳输出的潜力）变得更加困难。我们的研究使用实验室实验来了解温度、硝酸盐和二氧化碳水平的同时变化如何影响硅藻的生长及其背后的生理学。我们的实验旨在测试和选择描述基本的、保守的生长过程如何在多个同时发生的环境变化下受到影响的一般方程。然后，我们将利用我们的实证研究结果，在一个广泛使用的全球海洋生态系统模型中，探索环境变化之间的相互作用如何影响硅藻空间分布和生长的未来预测。整个项目将提高我们预测海洋浮游植物生长变化的能力，从而改善对气候和生态系统服务的预测，以及我们对地球上最大的生态系统如何运作的理解。

Phytoplankton are the small but mighty single-celled photosynthetic organisms that form the base of most aquatic food webs, affect nutrient cycles, and are responsible for approximately half of the carbon fixation on Earth. Among the phytoplankton, diatoms are responsible for about 40% of marine primary production, which is up to 20% of global production, and are believed to be responsible for approximately 40% of global carbon export, some of which is sequestered in the deep ocean. At mid and high latitudes, such as around the UK, they increase the trophic efficiency of marine foodwebs, providing a direct link between primary production, grazers, and larger marine animals. The ocean environment where diatoms live is changing rapidly through warming, increases in CO2, and decreases in the nutrient levels (particularly nitrogen) in the upper part of the ocean where photosynthesis occurs. We have a reasonably good understanding of how phytoplankton react to these environmental changes in isolation. At high latitudes at least, increases in temperature and CO2 increase phytoplankton growth, whereas decreases in nitrogen availability decreases growth. Surprisingly, though, there is little experimental data on how phytoplankton react to all of these things changing simultaneously. Recent studies show that it is important to consider environmental changes together, as the organismal responses to several changes at once is not predictable from the organismal responses to each of the changes alone. For example, not only does lower nitrogen decrease the overall population size that diatoms can achieve, but it also makes them more sensitive to high temperatures, so that the effects of ocean warming may be exacerbated by nutrient limitation. In addition, we have data from other phytoplankton that suggest that elevated CO2 can partially mitigate negative effects of some other environmental changes, but we don't know if this applies to diatoms. Despite the importance of diatoms in marine ecosystems, there are very few experiments that enable us to understand, in a generalisable way, how diatoms respond to temperature, nitrate, and CO2 all changing at once. This, when combined with uncertainty in predictions about future ocean conditions themselves, means that currently, we are not even able to project whether global primary production is expected to increase or decrease over the coming decades. This uncertainty makes is harder to accurately project ecosystem services including the potential for carbon export in oceans. Our research uses laboratory experiments to learn how simultaneous changes in temperature, nitrate, and CO2 levels affect diatom growth and the physiology that underlies it. Our experiments are designed to test and choose between general equations describing how fundamental, conserved growth processes are affected under multiple simultaneous environmental changes. We will then use our empirical findings to explore how interactions between environmental changes affect future projections of diatom spatial distributions and growth in a widely-used global marine ecosystem model. The project as a whole will improve our ability to project changes to phytoplankton growth in the oceans, which can then improve projections of climate and ecosystem services, as well as our understanding of how the largest ecosystem on earth functions.