Collaborative Research: Transparent exopolymer and phytoplankton vertical migration as sources for preformed nitrate anomalies in the subtropical N. Pacific Ocean

海洋通常是分层的，光和氧气在温暖的表面，营养物质在较冷的深处。生物和物理过程决定了这种分布。海藻生长在光照充足的上层，但需要营养来生长。然而，在亚热带，海洋中最大的生物群落，氧和硝酸盐（光合作用所需的关键营养物质）之间的关系与预期不同。有两个过程可以解释这一现象。营养物质可以通过迁移的巨型单细胞藻类（浮游植物）向上运输。另一种解释是，一种被称为透明外聚物（TEP）的有机物质的产生吸收了碳，而不像光合作用那样使用营养物质或向深处输出碳。虽然这两个过程都可能发生，但浮游植物迁移与TEP的相对贡献将告诉我们观察到的上层海洋中的氧气模式是否来自光合作用。这个问题涉及到营养物质在哪里以及如何到达光照充足的地表水以进行光合作用的一般问题。这些假设在夏威夷海洋时间序列上进行了测试，使用现场摄像系统对巨型浮游植物进行成像和量化，并直接采集水样来测量TEP的垂直分布。这些数据被输入到数值模型中，以计算硝酸盐与氧的关系，并添加有关碳循环的信息。除了培养本科生和一名博士后外，这次巡游还提供了一个机会，培养一批交流人员，他们将开发材料和媒体，包括视频，向公众翻译这一高度复杂的科学概念。社交媒体活动#SaveOur70提供了一个接触和参与公众的宝贵场所。量化亚热带环流中养分的运输、利用及其与碳减少的关系是我们理解碳循环的基础。长期以来，来自夏威夷和百慕大附近具有良好特征的时间序列地点的地球化学分布，一直用于确定在缺乏已知营养来源的情况下，地下硝酸盐场、夏季溶解无机碳（DIC）减少和净群落产量之间的未知联系。最近提出了两个重要的过程来解释亚热带上层海洋中溶解碳、氧和硝酸盐分布的异常：1)垂直迁移的巨型浮游植物的营养物质运输；2)低氮有机质在混合层和上层营养线之间以透明的外聚合物颗粒（TEP）或凝胶状有机物（GLOM）的形式循环。虽然在基本水平上有联系（浮游植物是TEP的生产者），但这两个过程的结果是不同的。浮游植物的垂直迁移是将营养线中获得的硝酸盐主动运输到海面。这意味着随后的还原，光合作用的碳固定和最终的出口。TEP/GLOM循环导致DIC明显下降，但没有从表层净输出，也不需要在混合层中添加额外的营养源。本项目收集数据，量化这两个过程对夏威夷时间序列站观测到的硝酸盐对氧分布异常的贡献。这是通过列举垂直迁移的鞭毛虫菌群（VMF）来实现的，实施垂直迁移的一维模型，并将这些结果与由TEP和碳水化合物测量得出的上层水柱n -贫碳循环模式贡献的一维模型相结合。联合VMF和TEP/GLOM 1-D模型用于模拟HOT的溶解氧、碳和硝酸盐预算，允许将这两个假设过程归因于观察到的预形成硝酸盐分布、其形成速率和夏季无机碳减少。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。

The ocean is usually layered, with light and oxygen in the warmer surface and nutrients at the cooler depths. Biological and physical processes determine this distribution. Marine algae grow in the well-lit upper layers but need nutrients to grow. However, in the subtropics, the ocean's largest biome, the relationship between oxygen and nitrate (a key nutrient required for photosynthesis) is different from expected. Two processes could explain this. Nutrients could be transported upward by migrating giant single-celled algae (phytoplankton). Another explanation is that the production of an organic material called transparent exopolymer (TEP) takes up carbon without using nutrients or exporting carbon to depth, as would occur in photosynthesis. While both processes could be occurring, the relative contribution of migrating phytoplankton versus TEP would tell us whether the observed oxygen pattern in the upper ocean results from photosynthesis. This problem relates to the general question of where and how nutrients reach the well-lit surface waters to enable photosynthesis. These hypotheses are tested at the Hawaii Ocean Time-Series using in-situ camera systems to image and quantify the giant phytoplankton and direct water samples to measure the vertical distribution of TEP. The data are entered into numerical models to calculate the nitrate to oxygen relationships and add information about the carbon cycle. In addition to training of undergraduate students and a postdoctoral fellow, the cruises provide an opportunity to prepare a cadre of communication fellows who will develop materials and media, including videos, to translate this highly complex scientific concepts for the general public. The social media campaign #SaveOur70 provides a valuable venue to reach and engage with the public. Quantifying nutrient transport, utilization, and its relationship to carbon drawdown in the subtropical gyres is fundamental to our understanding of the carbon cycle. Geochemical distributions from the well-characterized time-series sites near Hawaii and Bermuda have long-served to identify previously unknown links between subsurface nitrate fields, summertime dissolved inorganic carbon (DIC) drawdown, and net community production in the absence of known nutrient sources. Two recently suggested processes rise to prominence to explain anomalies in subtropical distributions of dissolved carbon, oxygen, and nitrate in the upper ocean: 1) nutrient transport by giant phytoplankton that vertically migrate, and 2) cycling of low N organic matter between the mixed layer and the upper nutricline as transparent exopolymer particles (TEP) or gel-like organic material (GLOM). While linked at a fundamental level (phytoplankton are TEP producers), the outcome of the two processes are distinct. Vertical migration of phytoplankton is an active transport of nitrate, acquired in the nutricline, to the surface. There is an implication of subsequent reduction, photosynthetic carbon fixation and eventual export. TEP/GLOM cycling results in apparent DIC drawdown but there is no net export out of the surface layer and no requirement for additional nutrient sources in the mixed layer. This project collects the data to quantify the contribution of these two processes to the observed anomalies in nitrate to oxygen distribution at the time-series station at Hawaii (HOT). This is accomplished by enumerating the vertically migrating, aflagellate flora (VMF), implementing a 1-D model on vertical migration, and coupling these results with a 1-D model of the contribution of N-poor carbon cycling patterns in the upper water column derived from TEP and carbohydrate measurements. The combined VMF and TEP/GLOM 1-D models are used to model the dissolved oxygen, carbon, and nitrate budgets at HOT allowing for attribution of both hypothesized processes to the observed preformed nitrate distribution, its formation rate, and summertime inorganic carbon drawdown.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.