Collaborative Research: Contribution of allochthonous dissolved organic nitrogen to biological nitrogen demand in the subtropical North Pacific

广阔的海洋，几乎覆盖了其表面的60%，是“沙漠”，由于营养缺乏，生命在那里挣扎着生存。这些地区位于副热带海洋的中心环流中，距离太远，无法通过垂直混合从陆地或较深水域获得大量的养分输入。营养物质供应不足会导致光合作用水平降低，食物网也不那么复杂。然而，观察结果表明，生命的生长速度与获得更高营养供应的地区相似。我们的项目旨在确定帮助维持这些生态系统的营养物质的缺失来源。这项工作将专注于溶解有机分子中的氮结合，这些有机分子产生于营养丰富的地区，包括加利福尼亚沿海地区，然后横向运输到亚热带北太平洋的“海洋沙漠”。一个为期一个月的研究考察将从加州圣地亚哥出发，在美国夏威夷州檀香山结束，研究溶解有机氮（DON）的产生、积累、化学成分和利用，并将其与北太平洋发生的光合作用联系起来。关于DON在满足亚热带北太平洋营养需求方面的作用的新知识将用于构建计算机模型，以衡量这种营养来源对亚热带海洋生态系统的全球重要性。为了解释亚热带海洋生态系统中观测到的净群落产量（NCP），需要在地下硝酸盐（NO3-）和N2固定通量之外的额外氮(N)来源。数值模式表明，侧向供给的异源DON可能支持副热带少营养环流中10-60%的NCP，但存在较大的不确定性。拟议的工作将实地验证有关海洋DON的生物生产和消耗的假设，以及外来DON是否是维持NCP的重要有机营养源。一艘北太平洋游轮将利用对DON浓度、叶绿素A浓度和N同位素[DON的15N/14N比率和NO3-的15N/14N比率]的观测，在分别使用O2/Ar和三氧同位素测量估计的净生物生产力和总生物生产力的背景下，确定净DON生产和消耗区域。新的有机地球化学工具将用于鉴定异域DON的分子组成。研究人员将利用船上孵育的方法研究深度对DON消耗的影响，将不同深度间隔的NO3-表面梯度收集的DON暴露给微生物群落。最后，将在全球海洋生物地球化学模型中编码一种新的半不稳定DON示踪剂，使其表现得像野外观测中表征的异域DON。DON和同位素数据集将用于约束半不稳定DON循环的模型参数化。最先进的半不稳定DON循环模型将用于定量评估由异位DON输送到北太平洋副热带环流所维持的NCP部分，并将这一评估扩展到全球海洋。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Vast stretches of the ocean, covering almost 60% of its surface, are ‘deserts' where life struggles to survive due to nutrient scarcity. Located in the central gyres of the subtropical ocean, these regions are too remote to receive significant nutrient inputs from land or from deeper waters by vertical mixing. The low supply of nutrients should result in reduced levels of photosynthesis and less complex food webs. However, observations instead indicate that life grows at rates similar to regions which receive higher nutrient supplies. Our project seeks to determine what the missing sources of nutrients are that help sustain these ecosystems. This work will focus on nitrogen bound within dissolved organic molecules that are produced in nutrient-rich regions, including coastal California, and then transported laterally to the ‘ocean desert’ of the subtropical North Pacific Ocean. A one-month research expedition will leave from San Diego, CA and end in Honolulu, HI USA to study the production, accumulation, chemical composition, and utilization of dissolved organic nitrogen (DON) and link it to photosynthesis occurring across the North Pacific. The new knowledge gained about the role of DON in satisfying the nutrient requirements of the subtropical North Pacific will be used to construct computer models that gauge the global importance of this nutrient source to subtropical ocean ecosystems. Additional nitrogen (N) sources beyond subsurface nitrate (NO3-) and N2 fixation fluxes are required to explain observed net community production (NCP) within subtropical ocean ecosystems. Numerical models indicate that laterally supplied allochthonous DON may support 10-60% of NCP in oligotrophic subtropical gyres, but with large uncertainties. The proposed work will field test hypotheses concerning the biological production and consumption of marine DON, and whether allochthonous DON is a significant organic nutrient source sustaining NCP. A North Pacific cruise will use observations of DON concentration, chlorophyll a concentration, and N isotopes [DON's 15N/14N ratio, and NO3-'s 15N/14N ratio] to identify regions of net DON production and consumption in the context of net and gross biological productivity estimated using O2/Ar and triple Oxygen isotopic measurements, respectively. Novel organic geochemistry tools will be used to identify the molecular composition of the allochthonous DON. The influence of depth on DON consumption will be investigated using ship-board incubations that expose surface DON collected across gradients in surface NO3- to microbial communities from several depth intervals. Finally, a new semi-labile DON tracer will be encoded in a global ocean biogeochemistry model, fashioned to behave like the allochthonous DON characterized from the field observations. The DON and isotopic data sets will serve to constrain model parameterizations of semi-labile DON cycling. The state-of-the-art model of semi-labile DON cycling will be used to quantitatively assess the portion of NCP sustained by allochthonous DON delivery to the North Pacific subtropical gyre and extend this assessment across the global ocean.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.