Investigating how environmental change affects benthic biogeochemistry

微观植物，或浮游植物，利用太阳的能量将大气中的二氧化碳（CO2）和水结合，产生微粒有机物质（POM）。其中一部分沉入海底，细菌和动物利用它来生产能源，维持生物量和生长。这些生物在其组织中保持碳(C)、氮(N)和多不饱和脂肪酸（PUFAs）的严格平衡。因此，它们的生长可能受到这些底物中的任何一种的限制，这取决于可用POM的数量和生化组成或“质量”。单一基质的限制意味着所有其他基质都是过量的，因此必须释放到环境中。这可以通过调整碳和氮被同化的效率或通过释放它们作为二氧化碳或溶解的无机氮（DIN）来实现。由此可见，到达海底的POM的数量和质量以及其居民的饮食需求影响着海洋沉积物中C和N的命运。任何未被摄入的聚甲醛，或以粪便颗粒的形式释放出来的聚甲醛，都可能在沉积物中存留数千年。海洋沉积物中聚甲醛的地质储存是降低大气中不断增加的二氧化碳浓度的一种手段。然而，它也从生物圈中去除氮。这减少了浮游植物进一步生长的潜力，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的再循环来提供，那么只有当到达海底的POM的C:N比大于释放的CO2:DIN比时，大气中CO2进入海洋的持续净生物减少才会发生。基本上，对于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。人为的营养物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释放方面所起的作用。反过来，这极大地限制了我们在全球模式中准确表示碳和氮循环的能力，这些模式旨在对未来气候变化做出有意义的预测。我将种植不同种类的浮游植物，它们的C、N和PUFAs的比例不同，因此它们的质量也不同。浮游植物中的碳和氮将被13C和15N取代，13C和15N是这些元素的稳定同位素（si），它们的行为方式相同，但质量略有不同。它们很稀少，因此在自然环境中很容易被跟踪。我的研究将首次在海床上引入越来越多的不同的双si标记藻类，并跟踪13C和15N进入13CO2、DI15N、细菌和动物生物量以及沉积物的命运，从而详细了解POM的数量和质量如何影响C的埋藏和DIN的释放。这些实验将在沿海和深海（50 - 10公里深）栖息地进行，因为这些被认为是全球海底C周转最重要的区域。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所扮演的相对角色的信息，这是一个目前仍有激烈争论的话题。该项目的最终目标是对POM的数量和质量影响海洋沉积物中C和N的命运的方式产生机制理解。这将用于产生一个数学模型，该模型能够在已知POM的数量和质量的情况下预测海底储存的C和释放的DIN的数量。这将是朝着能够准确地表示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.