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，并吸收了大约30-40%的人为产生的二氧化碳。人们早就知道，海洋生物群在很长时间尺度（100年至1000年）的二氧化碳封存中起着重要作用。最近的证据还表明，海洋生物群在较短的时间尺度（10-100年）内发挥重要作用。浮游植物光合作用和群落呼吸作用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养分。然而，目前尚不清楚是什么控制了海洋中的氮含量。与大多数限制氮的浮游植物不同，固氮蓝藻（重氮营养菌）以氮气的形式提供无限的氮。固氮生物在海洋养分和生物地球化学循环中发挥着重要作用，因为它们是氮的主要来源，在营养贫乏的海洋地区为高达50%的初级生产力提供氮。氮固定是调节海洋在10至10,000 y的时间尺度上固碳能力的关键过程。氮固定的限制导致其他主要生产者的氮可用性降低，从而降低了海洋固碳的潜力。虽然形成毛菌群被认为是最重要的海洋重氮营养体，但最近在海洋中发现了一系列新的重氮营养体。这给我们带来了一个问题“是什么限制了海洋中氮的固定量？”，以及“是什么决定了重氮营养体在海洋中的种类分布？”铁似乎是限制N2固定的关键环境因素，这是基于最近在大西洋中观察到的铁和N2固定之间的直接联系，铁决定了海洋表面P循环。该项目的目标是定量研究大西洋中铁供应和N2固定之间的联系，为此，了解各种铁来源的重要性和优势至关重要。铁的来源被认为是西北非洲上升流区的大气粉尘沉积和低氧陆架沉积物。这些来源的强度预计在未来会随着尘埃沉积的变化和海洋中氧气最低带的扩大而改变。因此，在未来气候情景下对氮固定进行模式估计的关键是确定和量化来源。本研究将探讨（亚）热带大西洋重氮营养菌群落结构与铁、磷的供给和生物地球化学循环对N2的固定作用。我们将结合船上观察和放射性示踪剂吸收实验，以及涉及营养素和铁的建模活动来开展这项研究。我们将通过使用化学源示踪剂，从大气和海洋环流中量化重氮营养型群落的这些元素的来源。我们将把营养物质和铁的供应与重氮营养体的活性和种类分布联系起来。分子技术将用于确定研究区域的不同重氮营养物。我们将在（亚）热带大西洋进行一次专门的巡航，涉及非洲大陆架的东西样带，以表征氧最小带的微量元素梯度，从而表征大陆架横向平流的可能性。该巡航还将穿越研究区域的尘埃/氧化还原羽流，并描绘沿尘埃/氧化还原羽流边缘的水平微量元素梯度。我们将对常见重氮营养菌Trichodesmium进行取样，并使用放射性示踪剂研究其对铁和磷的吸收。我们将使用循环模型为该项目提供大规模的环境，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关于营养循环和模拟的海洋学研究，并着眼于未来海洋作为碳汇的重要性。

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.