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

合作研究：透明外聚合物和浮游植物垂直迁移作为北太平洋副热带硝酸盐异常的来源

• 批准号: 1923687
• 负责人: Robert Letscher
• 金额: $ 49.13万
• 依托单位: University of New Hampshire
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-09-15 至 2023-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1923687&HistoricalAwards=false
• 关键词: Collaborative; Research; Transparent; exopolymer; phytoplankton

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.

海洋通常是分层的，较温暖的表面有光和氧气，而较冷的深处有营养物质。生物和物理过程决定了这种分布。海藻生长在光线充足的上层，但需要营养才能生长。然而，在海洋最大的生物群落亚热带地区，氧气和硝酸盐（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点。营养物质可以通过迁移巨型单细胞藻类（浮游植物）向上输送。另一种解释是，一种称为透明外聚合物（TEP）的有机材料的生产会吸收碳，而不使用营养物或将碳输出到深处，就像光合作用中发生的那样。虽然这两个过程都可能发生，但迁移浮游植物与 TEP 的相对贡献将告诉我们在上层海洋中观察到的氧气模式是否是光合作用的结果。 这个问题涉及营养物质在哪里以及如何到达光照充足的地表水域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中进行了测试，使用现场摄像系统对巨型浮游植物进行成像和量化，并直接提取水样来测量 TEP 的垂直分布。 将数据输入数值模型以计算硝酸盐与氧气的关系并添加有关碳循环的信息。 除了培训本科生和博士后研究员外，巡游还提供了培养传播研究员骨干的机会，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 提供了一个接触公众并与​​公众互动的宝贵场所。 量化养分运输、利用及其与亚热带环流碳减少的关系对于我们理解碳循环至关重要。 夏威夷和百慕大附近特征明确的时间序列地点的地球化学分布长期以来一直用于确定地下硝酸盐田、夏季溶解无机碳（DIC）下降以及在缺乏已知营养源的情况下净群落生产之间以前未知的联系。 最近提出的两个过程引起了人们的重视，可以解释上层海洋中溶解碳、氧和硝酸盐的亚热带分布异常：1）垂直迁移的巨型浮游植物的养分运输，2）混合层之间低氮有机物的循环上层 Nutricline 为透明外聚物颗粒 (TEP) 或凝胶状有机材料 (GLOM)。 虽然在基本层面上存在联系（浮游植物是 TEP 生产者），但这两个过程的结果是截然不同的。 浮游植物的垂直迁移是从营养线中获得的硝酸盐主动运输到地表的过程。这意味着随后的减排、光合作用碳固定和最终出口。 TEP/GLOM 循环导致明显的 DIC 下降，但表面层没有净输出，并且混合层中不需要额外的营养源。 该项目收集数据以量化这两个过程对夏威夷 (HOT) 时间序列站观察到的硝酸盐到氧气分布异常的贡献。 这是通过枚举垂直迁移的鞭毛菌群 (VMF)、实施垂直迁移的一维模型并将这些结果与上部水体中贫氮碳循环模式的贡献的一维模型耦合来实现的来自 TEP 和碳水化合物测量。组合的 VMF 和 TEP/GLOM 一维模型用于模拟 HOT 时的溶解氧、碳和硝酸盐预算，从而将两个假设过程归因于观察到的预先形成的硝酸盐分布、其形成速率和夏季无机碳减少。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。