Collaborative Research: Evaluating the contribution of small eukaryotes to nitrate-based new production in the North Pacific Subtropical Gyre

中纬度地区的亚热带海洋是“海洋沙漠”，表层水中的营养物质相对较少。氮是一种特别稀缺的重要营养素。海洋中的氮以硝酸盐或铵的形式存在。由于亚热带环流中的营养物质有限，那里的海藻或浮游植物生长缓慢。在地表以下较深的水域，有更多的营养物质。然而，人们尚不清楚夏季生活在表层水域的浮游植物如何获取更深水域的营养。该奖项将研究北太平洋副热带环流 (NPSG) ALOHA 站的浮游植物如何获取地下氮。浮游植物将根据大小和色素成分进行分类。研究人员将测量分类种群的稳定氮同位素比率，并确定浮游植物从哪里获取所需的营养物质，要么是来自地下的硝酸盐，要么是来自地表水的铵。研究人员将利用宏转录组分析来研究不同浮游植物种群吸收所需氮的生理途径。宏转录组分析涉及分析浮游生物转录的 RNA 序列，将该序列与记录的基因序列进行比较，并映射到记录的生理通路。研究人员将评估哪些分类群正在使用硝酸盐和回收的氮源。他们将检验真核浮游植物主要使用硝酸盐而原核浮游植物主要使用回收的氮源的假设。通过这些测量，研究人员将帮助社会预测气候对亚热带环流生产力的影响。该项目将支持两名研究生和一名博士后研究员进行实验室和船基研究。康涅狄格州的首席研究员 (PI) Granger 将通过 AGU 的桥梁合作伙伴计划吸引高中生并资助代表性不足群体的研究生。北卡罗来纳州的 PI Marchetti 将资助一名来自当地少数民族县的高中生实习。夏威夷的 PI White 将通过海洋监测土著合作伙伴计划与夏威夷高中生互动，指导和教授基本的研究技能。该项目的研究人员将调查 NPSG 的 ALOHA 站生产力的驱动因素。他们将利用高灵敏度的自然丰度氮同位素分析方法（“过硫酸盐反硝化剂”方法）来评估通过流式细胞术分选的浮游生物的分类学不同成分对硝酸盐的依赖程度与地表水中减少的氮源的程度。我们还将通过宏转录组分析查询浮游生物的生理氮途径，以推断哪些进化枝依赖硝酸盐。我们将量化硝酸盐推动的初级生产的比例，以表征季节性趋势，并探索硝酸盐向亮光带及其混合层供应的潜在机制。这里提出的工作将解决从海洋内部混合到 ALOHA 站表面生产的硝酸盐的相对贡献，以更好地定义 NPSG 中生物泵的季节性动态。我们将确定对硝酸盐生产做出主要贡献的浮游生物群，并解决在氮严重受限的环境中赋予适应性的生态生理策略。将考虑事件水文学来评估硝酸盐对总氮产量贡献的季节性趋势，以推断硝酸盐供应的潜在机制。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。

The subtropical oceans in the middle latitudes are “ocean deserts,” where there are relatively little nutrients in the surface waters. Nitrogen is an important nutrient that is particularly scarce. Nitrogen in the ocean occurs as nitrate or ammonium. As a result of limited nutrients in the subtropical gyres, marine algae or phytoplankton there grow slowly. In deeper waters below the surface, there are more nutrients. However, it is not well understood how phytoplankton living in surface waters in summer can access the nutrients in the deeper waters. This award will investigate how phytoplankton access subsurface nitrogen at Station ALOHA in the North Pacific Subtropical Gyre (NPSG). Phytoplankton will be sorted by size and pigment composition. The researchers will make measurements of the stable nitrogen isotope ratios of sorted populations and determine where phytoplankton are getting the needed nutrients, either nitrate from the subsurface or ammonium from the surface waters. The researchers will use metatranscriptomic analysis to investigate the physiological pathways that different phytoplankton populations use to take up the needed nitrogen. The metatranscriptomic analysis involves analyzing the RNA sequences transcribed by plankton, comparing the sequence to documented gene sequences and mapped to documented physiological pathways. The researchers will assess which taxonomic groups are using nitrate versus recycled sources of nitrogen. They will test the hypothesis that eukaryotic phytoplankton primarily use nitrate whereas prokaryotic phytoplankton mostly use recycled sources of nitrogen. With these measurements, the researchers will help the society anticipate climate impacts on the productivity of subtropical gyres. This project will support two graduate students and a post-doctoral researcher in laboratory and ship-based research. Principal Investigator (PI) Granger in Connecticut will engage high school students and sponsor graduate students from underrepresented groups via AGU’s Bridge Partner Program. PI Marchetti in North Carolina will sponsor the internship of a high school student from a local minority county. PI White in Hawaii will engage with Hawaiian high school students though the Indigenous Partnership for Ocean Monitoring program to mentor and teach essential research skills. The researchers of this project will investigate the drivers of productivity at station ALOHA in NPSG. They will exploit a high-sensitivity method for natural abundance N isotope analysis (the ‘persulphate-denitrifier’ method) to evaluate the extent to which taxonomically distinct components of the plankton sorted by flow cytometry rely on nitrate vs. reduced N sources in surface waters. We will also query physiological nitrogen pathways of the plankton with metatranscriptomic analysis to infer which clades are reliant on nitrate. We will quantify the fraction of primary production fueled by nitrate to characterize seasonal trends and explore potential mechanisms of nitrate supply to the euphotic zone and its mixed layer. The work proposed here will resolve the relative contribution of nitrate mixed from the ocean interior to surface production at station ALOHA, to better define seasonal dynamics of the biological pump in the NPSG. We will identify plankton groups that contribute dominantly to nitrate-based production and resolve eco-physiological strategies that confer fitness in a severely N-limited environment. Seasonal trends in the contributions of nitrate to total nitrogen production will be evaluated considering incident hydrography to infer potential mechanisms of nitrate supply.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.