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

海洋的广阔延伸，覆盖了近60%的表面，是“沙漠”，由于营养缺乏，生命在那里挣扎着生存。这些区域位于亚热带海洋的中央环流，距离太远，无法通过垂直混合从陆地或更深的沃茨获得大量的营养物输入。营养供应不足应导致光合作用水平降低，食物网复杂性降低。然而，观察结果却表明，生命的增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。这项工作的重点是氮结合在溶解的有机分子，产生在营养丰富的地区，包括沿海加州，然后横向运输到亚热带北太平洋的“海洋沙漠”。为期一个月的研究考察将从加利福尼亚州圣地亚哥出发，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、化学组成和利用，并将其与北太平洋发生的光合作用联系起来。关于DON在满足北太平洋亚热带营养需求方面的作用的新知识将用于构建计算机模型，以衡量这种营养源对亚热带海洋生态系统的全球重要性。亚热带海洋生态系统中观测到的净群落生产力（NCP）需要更多的氮（N）源来解释亚热带海洋生态系统中亚表层硝酸盐（NO3-）和N2固定通量。数值模拟结果表明，在贫营养的副热带环流中，横向供应的外来DON可能支持10-60%的NCP，但具有很大的不确定性。拟议的工作将实地测试有关海洋DON的生物生产和消费的假设，以及异地DON是否是维持NCP的重要有机营养源。北太平洋巡航将使用DON浓度，叶绿素a浓度和N同位素[DON的15 N/14 N比和NO3-的15 N/14 N比]的观测，以确定净DON生产和消费的区域，分别使用O2/Ar和三重氧同位素测量估计的净和总生物生产力。新的有机地球化学工具将被用来确定外来DON的分子组成。深度对DON消耗的影响将使用船上培养，暴露表面DON收集跨梯度表面NO3-从几个深度间隔的微生物群落。最后，一个新的半不稳定的DON示踪剂将被编码在全球海洋地球化学模型，塑造的行为像外来的DON从现场观测的特点。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。半不稳定DON循环的最先进模型将用于定量评估NCP的一部分，该部分由异地DON输送到北太平洋亚热带环流，并将此评估扩展到全球海洋。该奖项反映了NSF的法定使命，并被认为值得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来支持。

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.