Investigating how environmental change affects benthic biogeochemistry

微型植物或浮游植物利用太阳能将大气中的二氧化碳（CO2）和水联合收割机结合，产生颗粒有机物（POM）。其中一部分沉入海底，细菌和动物将其用于能源生产，维持生物量和生长。这些生物体在其组织中保持碳（C），氮（N）和多不饱和脂肪酸（PUFA）的严格平衡。因此，它们的生长可能受到任何这些底物的限制，这取决于可用POM的数量和生化组成或“质量”。单一底物的限制需要所有其他底物过量，因此必须释放到环境中。这可以通过调节C和N同化的效率或通过将它们释放为CO2或溶解的无机N（DIN）来实现。因此，到达海底的POM的数量和质量，以及其居民的饮食需求，影响海洋沉积物中C和N的命运。任何逃脱摄入的POM，或者作为粪便颗粒释放的POM，都可能在沉积物中存在数千年。POM在海洋沉积物中的地质储存是一种可以降低不断增加的大气CO2浓度的手段。然而，它也从生物圈中移除N。这降低了浮游植物进一步生长的可能性，而浮游植物的生长受到DIN供应的限制。如果DIN仅通过海洋沉积物中POM的再循环提供，那么只有当到达海底的POM的C：N比大于释放的CO2：DIN的比率时，才会发生大气CO2进入海洋的持续净生物下降。从本质上讲，POM的数量和质量如何影响这些比率之间的抵消，或者它在未来如何变化，我们一无所知。人为的营养富集和气候变化已经改变了到达海底的POM的数量和质量，但我们不了解，也没有能力预测，这如何影响海洋沉积物在C储存和DIN释放中的作用。反过来，这极大地限制了我们在全球模型中准确表示C和N循环的能力，这些模型旨在对未来气候变化做出有意义的预测。我将种植不同比例的C，N和PUFA的浮游植物物种，因此它们的质量不同。浮游植物中的C和N将被13 C和15 N取代，这些元素的稳定同位素（SI）以相同的方式表现，但质量略有不同。它们很稀少，因此在自然环境中很容易追踪。我的研究将首次在海底引入越来越多的不同的双SI标记藻类，并跟踪13C和15N在13CO2，DI15N，细菌和动物生物量和沉积物中的命运，从而详细了解POM的数量和质量影响C的埋葬和DIN的释放的方式。实验将在沿海和深海（深度超过1公里）生境进行，因为这些生境被认为是全球海底碳周转的最重要区域。我的研究还将提供有关细菌和动物在浅水和深水栖息地的元素循环中发挥的相对作用的信息，这是一个目前仍在激烈辩论的话题。该项目的最终目标是产生一个机制的理解，其中POM的数量和质量影响的C和N在海洋沉积物中的命运的方式。这将被用来产生一个数学模型，该模型能够预测储存的C的数量，以及在已知POM的数量和质量的情况下，海底释放的DIN。这将是朝着能够准确反映海洋沉积物在全球气候模型中的作用迈出的重要一步。

Microscopic plants, or phytoplankton, use the sun's energy to combine atmospheric carbon dioxide (CO2) and water to produce particulate organic matter (POM). A proportion of this sinks to the seabed, where bacteria and animals use it for energy production, maintenance of biomass and growth. These organisms maintain a strict balance of carbon (C), nitrogen (N) and polyunsaturated fatty acids (PUFAs) in their tissues. Their growth can thus be limited by any of these substrates, depending on the quantity and biochemical composition, or 'quality', of the POM available. Limitation by a single substrate necessitates that all others are in excess and must therefore be released to the environment. This can be achieved by adjusting the efficiencies with which C and N are assimilated or by liberating them as CO2 or dissolved inorganic N (DIN). It follows that the quantity and quality of POM arriving at the seabed, and the dietary demands of its inhabitants, influence the fate of C and N in marine sediments. Any POM that escapes ingestion, or that which is released as faecal pellets, may persist in the sediments for thousands of years. Geological storage of POM in marine sediments represents a means by which the ever-increasing atmospheric concentration of CO2 can be reduced. However, it also removes N from the biosphere. This reduces the potential for further phytoplankton growth, which is limited by the supply of DIN. If DIN was only supplied via the recycling of POM in marine sediments, it follows that a sustained net biological drawdown of atmospheric CO2 into the oceans can only occur when the C:N ratio of the POM arriving at the seabed is greater than the ratio of CO2:DIN released. Essentially nothing is known about how POM quantity and quality affect the offset between these ratios, or how it is liable to change in the future. Manmade nutrient enrichment and climate change are already changing the quantity and quality of POM arriving at the seabed, but we do not understand, or have the capacity to predict, how this influences the roles that marine sediments play in C storage and DIN release. In turn, this greatly restricts our ability to accurately represent the cycles of C and N in global models that are designed to make meaningful forecasts about future climate change. I will grow species of phytoplankton with different ratios of C, N and PUFAs, which therefore differ in terms of their quality. The C and N in the phytoplankton will be replaced with 13C and 15N, stable isotopes (SIs) of these elements that behave in an identical manner, but differ subtly in their mass. They are scarce and hence easy to follow in the natural environment. My research will, for the first time, introduce increasing quantities of the different, dual SI-labelled algae onto the seabed and follow the fate of 13C and 15N into 13CO2, DI15N, bacterial and animal biomass and the sediments, thereby providing a detailed insight into the ways in which the quantity and quality of POM influence the burial of C and the release of DIN. The experiments will be conducted in coastal and deep-sea (> 1 km deep) habitats as these are considered to be the most important areas for global seabed C turnover. My research will also provide information on the relative roles that bacteria and animals play in elemental cycling in shallow and deep-water habitats, a topic that currently remains hotly debated. The ultimate goal of this project is to generate a mechanistic understanding of the ways in which POM quantity and quality affect the fate of C and N in marine sediments. This will be used to produce a mathematical model that is capable of predicting the quantities of C stored, and DIN released by the seabed, given a known quantity and quality of POM. This will represent a significant step towards being able to accurately represent the role of marine sediments in global climate models.