Physical and chemical forcing of diazotrophy in the (sub)-tropical Atlantic Ocean

海洋在全球碳循环中发挥着核心作用，并已承担了约1000万吨的碳排放。30-40%的温室气体产生的二氧化碳。人们早就知道，海洋生物群在很长一段时间（>1000年）的二氧化碳封存中发挥着重要作用。最近的证据还表明，海洋生物群在较短的时间尺度（10-100年）上发挥着重要作用。浮游植物光合作用和群落呼吸之间的平衡决定了海洋吸收二氧化碳的能力。氮（N）通常被认为是限制浮游植物光合作用的营养物质。然而，目前还不清楚是什么控制着海洋中的氮含量。不像大多数浮游植物，这是氮有限的，氮固定蓝藻（固氮生物）有一个无限的供应氮的形式N2气体。固氮菌是氮的主要来源，在营养盐贫乏的海洋地区，为初级生产力提供高达50%的氮，因此在海洋营养盐和海洋地球化学循环中发挥着重要作用。N2固定是一个关键过程，它在10至10，000年的时间尺度上调节海洋封存CO2的能力。N2固定的限制导致其他初级生产者的氮可用性降低，减少了海洋固碳的潜力。虽然形成束毛藻属的殖民地被认为是最重要的海洋固氮生物，但最近在海洋中发现了一系列新的固氮生物。这给我们带来了这样的问题：“是什么限制了海洋中N2的固定量？”“，以及”是什么决定了海洋中固氮生物的种类分布？铁似乎是限制N2固定的关键环境因素，这是基于最近观察到的大西洋Fe和N2固定之间的直接联系，Fe决定了表层海洋P循环。该项目的目标是定量研究大西洋铁供应和N2固定之间的联系，为此，必须了解各种铁源的重要性和优势。铁的来源被认为是在非洲西北部上升流区的大气尘埃沉积物和低氧陆架沉积物。随着尘埃沉积的变化和海洋中最低含氧区的扩大，这些来源的强度预计今后将发生变化。因此，确定和量化的来源是关键，在未来的气候情景下进行N2固定的模型估计。该建议将涉及供应和生态地球化学循环的铁和磷固氮和群落结构的（亚）热带大西洋。我们将进行这项研究，结合船上的观察和放射性示踪剂的吸收实验，并涉及营养物质和铁的建模活动。我们将从大气和海洋环流中量化这些元素对固氮生物群落的来源，并使用化学源示踪剂。我们将把营养物和铁的供应与固氮生物的活性和物种分布联系起来。分子技术将被用来确定不同的固氮生物在研究区域。我们将在（亚）热带大西洋进行一次专门的巡航考察，其中涉及从非洲大陆架向东西方向的断面，以确定最低含氧区的微量元素梯度，从而确定大陆架横向平流的可能性。这次航行还将穿越研究区域的尘埃/氧化还原羽流，并确定沿尘埃/氧化还原羽流边缘沿着水平微量元素梯度的特征。我们将采样常见的固氮生物束毛藻，并使用放射性示踪剂研究其对铁和磷的吸收。我们将使用一个循环模型为该计划提供一个大规模的背景，根据观察研究调整营养物质和铁的来源和循环。这项研究最终将有助于海洋学研究的营养循环和建模，着眼于未来的重要性，海洋作为碳汇。

The oceans play a central role in the global carbon cycle, and have taken up ca. 30-40% of the anthropogenically produced CO2. It has long been known that ocean biota play a major role in sequestering CO2 on very long time scales (>1000 y). Recent evidence also suggests that the ocean biota play an important role on shorter time scales (10-100 y). The balance between phytoplankton photosynthesis and community respiration determines the ability of the oceans to take up CO2. Nitrogen (N) is generally considered to be the nutrient that limits phytoplankton photosynthesis. However, it is unclear what controls the amount of N in the ocean. Unlike most phytoplankton, which are N-limited, N2 fixing cyanobacteria (diazotrophs) have an unlimited supply of N in the form of N2 gas. N2-fixers play a significant role in ocean nutrient and biogeochemical cycles as they are a major source of N, providing N for up to 50% of primary productivity in nutrient poor oceanic regions. N2 fixation is a key process that modulates the ability of the oceans to sequester CO2 on time scales of 10 to 10,000 y. Limitation of N2 fixation results in lowered N availability for other primary producers reducing the potential of oceans to sequester carbon. Whilst the colony forming Trichodesmium is considered the most important oceanic diazotroph, recently a range of new diazotrophs have been discovered in the ocean. This brings us to the questions of 'what constrains the amount of N2 fixation in the ocean?', and 'what determines the species distribution of diazotrophs in the ocean?' Iron appears to be the key environmental factors constraining N2 fixation based on a recently observed direct link between Fe and N2 fixation in the Atlantic, with Fe determining surface ocean P cycling. The goal of this project is to investigate quantitatively the link between iron supply and N2 fixation in the Atlantic, and for this it is essential to understand the importance and strengths of various iron sources. The iron sources are considered to be atmospheric dust deposition and low oxygen shelf sediments in the NW African upwelling region. The strengths of these sources are expected to change in future with changes in dust deposition and expansion of the oxygen minimum zones in the oceans. Identification and quantification of the sources is hence key to undertake model estimates of N2 fixation under future climate scenarios. This proposal will relate the supply and biogeochemical cycling of Fe and P to N2 fixation and the community structure of diazotrophs in the (sub)-tropical Atlantic Ocean. We will undertake this research using a combination of ship-board observations and radiotracer uptake experiments, and modeling activities involving nutrients and Fe. We will quantify sources of these elements for the diazotroph community from the atmosphere and ocean circulation, and by use of chemical source tracers. We will link the supply of nutrients and Fe to the activity and species distribution of diazotrophs. Molecular techniques will be used to determine the different diazotrophs in the study region. We will undertake the work on a dedicated cruise in the (sub)-tropical Atlantic which involves east-west transect from the African shelf to characterise the trace element gradient in the oxygen minimum zone and thus the potential for lateral advection from the shelf. The cruise will also traverse the dust/redox plumes in the study region and characterize the horizontal trace element gradients along the edges of the dust/redox plumes. We will sample the common diazotroph Trichodesmium and study its uptake of Fe and P using radiotracers. We will use a circulation model to provide a large scale context for the programme, with sources and cycling of nutrients and Fe adapted according to the observational studies. This research will ultimately assist with oceanographic studies on nutrient cycling and modeling with a view on the future importance of the oceans as C sink.