Tracer studies of faunal organic matter transformation in marine sediments, and the wider role of seafloor faunal communities in C cycling

海洋沉积物中有机物（OM）的埋藏是全球碳循环的一个主要因素，因此也是气候的一个主要因素。生活在海底的生物在决定最终埋在沉积物中的OM的数量和组成方面起着主要作用。这些生物依靠到达海底的有机质作为食物，并通过进食和挖洞活动混合和灌溉沉积物，从而影响其命运。然而，这些不同的过程仍然知之甚少，底栖生物在沉积有机质循环中的作用仍然是目前对海洋碳循环认识的主要空白。在生物学和地球化学的交汇处，需要一种新的方法，将示踪剂实验与最先进的分析工具结合起来。拟议的工作将调查海底动物的消化过程和沉积物的有机地球化学之间的联系。动物群在OM循环和埋藏中的整体作用将在一系列对比环境中进行调查，从浅河口到深大陆边缘地点。这项工作将通过对重要的个体物种以及整个完整的底栖动物群落进行喂养实验来完成。研究的第一部分将包括对来自荷兰斯海尔特河口Molenplaat的两种大型的、生态上重要的、对比鲜明的物种进行控制喂养实验（与NIOO的科学家合作）。将用标记有稳定同位素13C的藻类喂养标本长达4周。使用新技术，将分析样品中含有13c标记的生物化学物质。氨基酸、碳水化合物和脂质将首次被追踪，从食物进入肠道内容物、动物组织、粪便物质和周围沉积物。对这一系列生物化学物质的研究将对动物消化过程所带来的有机质变化产生批判性的理解，其细节和完整性是前所未有的。相对较长的实验时间将使动物消化、沉积物地球化学和有机质保存之间的直接联系成为可能。研究的第二部分将涉及类似的实验，但在自然条件下进行完整的底栖生物群落。主要目的是构建定量的C预算，以对比沿海地区与不同社区的差异。这些C预算将通过跟踪13C标签来构建。整个社区的实验将首先在不同的河口环境中进行；埃提湖（苏格兰西部）和伊森河口（苏格兰东部）。这两个地点也有强烈对比的底栖生物群落和物理条件，但包括初步微观世界实验的目标动物，因此提供了有价值的地面真相。湖湖实验已经开始了，样品也已经到手了。有足够的样品可用于13C标签的定量追踪，以及对一些标记的氨基酸、碳水化合物和脂类样品的分析。Ythan河口实验将在现场进行，使用阿伯丁大学提供的mesocosm技术。最后，作为日本主导的国际合作的一部分，在阿拉伯海大陆边缘进行的一套现场实验将作为上述实验的补充。其中将包括在深度约150米至1850米、沉积物OM含量、底水含氧量和底栖生物群落显著不同的地点进行底栖生物群落C循环的13C示踪研究。最先进的海底实验将通过潜水器和遥控操作装置的可用性而成为可能。我将为实验的运行和取样做出贡献，但最重要的是将13C追踪纳入全套主要生化物质中。

Burial of organic matter (OM) in marine sediments is a major factor in the global C cycle, and thus climate. Organisms living on the sea floor play major roles in determining the amount and composition of OM ultimately buried in sediments. The organisms depend on the OM reaching the sea floor as food, and also influence its fate by mixing and irrigating the sediments through feeding and burrowing activities. However, these diverse processes remain poorly understood, and the roles of benthic organisms in sedimentary OM cycling remain a major gap in current understanding of the marine C cycle. A new approach is required, at the interface between biology and geochemistry, that will combine tracer experiments with state-of-the-art analytical tools. The proposed work will investigate the links between the digestive processes of seafloor fauna and the organic geochemistry of the sediments. The overall role of fauna in the cycling and burial of OM will be investigated in a range of contrasting environments, from shallow estuarine to deeper continental margin sites. The work will be accomplished through feeding experiments performed on important individual species, as well as on whole, intact benthic faunal communities. The first part of the study will involve controlled feeding experiments on two large, ecologically important and contrasting species from Molenplaat in the Scheldt estuary in The Netherlands (in collaboration with scientists at NIOO). Specimens will be fed with algae labelled with the stable isotope 13C for up to 4 weeks. Using novel techniques, samples will be analysed for the 13C-labelled biochemicals they contain. Amino acids, carbohydrates and lipids, will for the first time be traced, from the food into gut contents, faunal tissues, faecal materials and surrounding sediments. The study of this range of biochemicals will yield a critical understanding of OM alteration brought about by faunal digestive process, the detail and completeness of which are unprecedented. The relatively long duration of experiments will allow the first direct links to be made between faunal digestion, sediment geochemistry and OM preservation. The second part of the study will involve similar experiments, but with whole, intact benthic communities under natural conditions. A primary aim will be to construct quantitative C budgets for contrasting coastal sites with different communities. These C budgets will be constructed by tracking the 13C label. The whole-community experiments will firstly be conducted in contrasting estuarine settings; Loch Etive (western Scotland), and the Ythan estuary (eastern Scotland). The two sites also have strongly contrasting benthic communities and physical conditions, but include fauna targeted in the preliminary microcosm experiments, thus providing a valuable ground-truthing. The Loch Etive experiment has already been carried out, and the samples are in hand. Sufficient samples are available to allow both the quantitative tracing of the 13C label, and analysis of some specimens for labelled amino acids, carbohydrates and lipids. The Ythan estuary experiment will be conducted in situ, using mesocosm technology available at the University of Aberdeen. Finally, the above experiments will be complimented by a suite of in situ experiments conducted on the continental margin of the Arabian Sea, as part of a Japan-led international collaboration. Among these will be 13C tracer studies of C cycling by benthic communities at sites ranging in depth from ca. 150m to 1850m, and with dramatically different sediment OM content, bottom-water oxygen levels and benthic communities. State-of-the-art seafloor experimentation will be made possible by the availability of a submersible and a Remotely Operated. I will contribute to the running of experiments, and also to sampling, but most significantly through the inclusion of 13C tracing into the full suite of major biochemicals.