Collaborative Research: ETBC: Amazon iNfluence on the Atlantic: CarbOn export from Nitrogen fixation by DiAtom Symbioses (ANACONDAS)

研究人员推测，低氮磷比的大型热带河流羽流为硅藻-重固氮生物组合(DDA)提供了理想的生态位。他们提出，这些生物在表层海洋中固定氮气的能力是亚马逊河羽流中大量碳出口的原因。他们之前在亚马逊河羽流中的观察帮助揭示了水华由内共生的固氮蓝藻Richelia及其硅藻宿主(例如半毛藻)组成，是新生产和碳出口的重要来源。以前的工作主要集中在DDA对来自粉尘和河流输入的外部强迫的敏感性，因此这些生物的生态及其新生产的命运在很大程度上没有被研究。现在已经知道，DDA是亚马逊河羽流中二氧化碳排放量很大的原因，200米处的漂浮沉积物捕集器测得羽流下的质量通量是羽流外的4倍。这使得研究人员假设，这种更大的出口要么是由于DDA本身的聚集和下沉，要么是由于浮游动物对DDA的捕食。在这项研究中，研究人员将进行一系列现场、卫星和模拟研究，旨在了解生态和追踪DDA和生活在羽流中的其他浮游植物固定的C和N的命运。通过研究近海表层水中碳和硅酸盐(Si)的输出，他们将量化亚马逊河对控制C固存的生物过程的影响，以及这些区域过程对C、N和Si预算的影响。这项研究将超越以往的研究，因为它们将量化1)浮游植物物种演替背景下DDA的分布、营养需求和活动，2)二氧化碳减少对浮游植物混合的敏感性，3)导致下沉通量的放牧和聚集过程，4)这种通量的组成，以及5)这种物质到达海底的比例。这一努力确实代表了碳固定和泵效率的一种衡量标准。生态模拟将被用于将实地研究和卫星的观测结果置于具有热带气候年际和更长时间变异性的更大的大西洋盆地的背景下。智力优势：PI已经确定了一条潜在的意义重大但知之甚少的、生态系统控制的、对气候敏感的碳固存路径，这似乎违反了对低效开放海洋生物泵的预期。由于由外来氮源(如固定氮气)驱动的初级生产可以驱动C从大气到海洋的净生物中介转移，因此亚马逊河羽流中DDA对C的固存是一个具有区域意义的过程。由于在其他热带水系中也发现了DDA，它们可能代表了一种具有全球意义但以前被忽视的生物泵机制。更广泛的影响：亚马逊河比其他任何河流都更能激发公众的想象力。这项研究旨在利用如此高调的地球科学来促进我们所有公民的科学素养。该项目将通过培训邮轮支持研究生和博士后教育、本科生，以及通过COSEE-WEST、中大西洋COSEE和COSEEOS方案对K-12教师和本科生进行海洋科学教育。这项研究的结果将通过同行评议的出版物、公共数据库和水蟒网站向其他科学家公布，并通过旧金山州立大学皇家TC-湾区发现博物馆计划向公众公布。

The investigators hypothesize that large tropical river plumes with low N: P ratios provide an ideal niche for diatom-diazotroph assemblages (DDAs). They suggest that the ability of these organisms to fix N2 within the surface ocean is responsible for significant C export in the Amazon River plume. Their previous observations in the Amazon River plume helped reveal that blooms comprised of the endosymbiotic N2-fixing cyanobacterium Richelia and its diatom hosts (e.g. Hemiaulus) were a significant source of new production and carbon export. The previous work focused largely on the sensitivity of DDAs to external forcing from dust and riverine inputs, so the ecology of these organisms and the fate of their new production were largely unstudied. It is now known that DDAs are responsible for a significant amount of CO2 drawdown in the Amazon River plume, and floating sediment traps at 200 m measured 4x higher mass fluxes beneath the plume than outside the plume. This led the researchers to hypothesize that this greater export is due either to aggregation and sinking of DDAs themselves or to grazing of DDAs by zooplankton.In this study the researchers will undertake a suite of field, satellite and modeling studies aimed at understanding the ecology and tracing the fate of C and N fixed by DDAs and other phytoplankton living in the plume. By examining C and silicate (Si) export from offshore surface waters, through the upper oceanic food web, the mesopelagic, and down to the deep sea floor, they will quantify the impact of the Amazon River on biological processes that control C sequestration and the implications of these regional processes on C, N and Si budgets. The study will go beyond previous research because they will quantify 1) the distribution, nutrient demands, and activity of DDAs in the context of phytoplankton species succession, 2) the sensitivity of the CO2 drawdown to the mix of phytoplankton, 3) the grazing and aggregation processes contributing to the sinking flux, 4) the composition of this flux, and 5) the proportion of this material that reaches the seafloor. This effort truly represents a measure of C sequestration and pump efficiency. Ecological modeling will be used to place observational results from field studies and satellites into the context of the larger Atlantic basin with tropical climate variability on interannual and longer time scales.Intellectual Merit: The PIs have identified a potentially significant but poorly understood, ecosystem-controlled, climate-sensitive C sequestration pathway that seems to violate the expectation of an inefficient open-ocean biological pump. Since primary production fueled by allochthonous sources of N such as N2 fixation can drive a net, biologically mediated transfer of C from the atmosphere to the ocean, C sequestration by DDAs in the Amazon River plume is a regionally significant process. Because DDAs have been found in other tropical river systems, they may represent a globally significant, yet previously overlooked biological pump mechanism.Broader Impacts: The Amazon River has captured the public's imagination more than any other river. This study aims to take advantage of such high profile earth science to promote science literacy among all our citizens. This project will support graduate and postdoctoral education, undergraduates through training cruises, and ocean science education of K-12 teachers and undergraduates through the COSEE-West, the Mid Atlantic COSEE and the COSEEOS programs. The results of this research will be made available to other scientists through peer reviewed publications, public databases, and an ANACONDAS website, as well as to the general public through the SFSU RTC-Bay Area Discovery Museum Program.