Collaborative Research: Quantifying nitrous oxide sources across an oxygen gradient in the northern Benguela upwelling system

一氧化二氮是一种强有力的温室气体，也会破坏高层大气中的臭氧。大气中一氧化二氮的来源涉及海洋中微生物(细菌和古生菌)的活动，特别是在氧气稀缺的地方。尽管大西洋和太平洋都存在溶解氧含量较低的海洋环境，但有关海洋中微生物产生一氧化二氮的大部分已知信息都是基于东热带太平洋的数据。更少的研究集中在热带大西洋东部，如北部本格拉上升流系统(NBU)，在那里观察到了高一氧化二氮通量。缺乏测量，再加上对导致一氧化二氮产生的微生物途径的有限了解，给对未来海洋一氧化二氮排放的预测带来了不确定性。因此，该项目的总体目标是量化一氧化二氮的循环速率，并确定在世界上生产力最高的上升流系统-NBU中跨越氧气梯度的微生物路径。该项目将支持培训两名研究生和一名博士后研究员。私营部门负责人和研究生将参加非洲纳米比亚的非洲地区海洋学研究生网络(RGNO)发现营，因此，拟议的研究完全融入了更广泛的影响活动。此外，国际和平研究所将在第二年和第三年接待来自非洲纳米比亚大学佛罗里达州立大学的一名研究生，传授有关微生物生态学、生物信息学和统计分析所用方法的知识。该项目有两个具体目标，第一个目标是利用15N示踪剂以及自然丰度同位素和同位素分析确定一氧化二氮产生率的空间分布和机制。第二个目标是确定导致一氧化二氮产生的溶解氧浓度与微生物群落结构、微生物功能和代谢活动之间的关系，解决导致第一个目标中观测到的同位素数据的活跃微生物途径。不同途径对一氧化二氮积累的相对贡献将通过15N示踪实验、自然丰度同位素和同位素分析以及16S rRNA基因、变基因组和变转录组测序相结合来测试。拟议的工作将使用硝酸盐和亚硝酸盐同位素测量作为输入，模拟一氧化二氮的总体和特定地点的同位素比率的分布，并将测试孵化实验中确定的速率和环境控制，以评估与较长期同位素测量的一致性。分子数据还将为同位素数据的解释提供信息，进一步解析一氧化二氮源机制和活动。这一裁决反映了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。

Nitrous oxide is a potent greenhouse gas that also contributes to ozone destruction in the upper atmosphere. Sources of nitrous oxide to the atmosphere involve the activity of microbes (bacteria and archaea) in the ocean, particularly where oxygen is scarce. Although marine environments low in dissolved oxygen occur in both the Atlantic and Pacific oceans, much of what is known about microbial nitrous oxide production in the ocean is based on data from the Eastern Tropical Pacific. Substantially less research has focused on the Eastern Tropical Atlantic, such as the northern Benguela Upwelling System (nBUS), where high nitrous oxide fluxes have been observed. The lack of measurements, combined with a limited understanding of the microbial pathways that lead to nitrous oxide production, creates uncertainty in predictions about future ocean emissions of nitrous oxide. Therefore, the overall project goal is to quantify nitrous oxide cycling rates and determine microbial pathways across an oxygen gradient in the world’s most productive upwelling system – the nBUS. The project will support training of two graduate students and a postdoctoral researcher. The PIs and graduate students will participate in the African Regional Graduate Network in Oceanography (RGNO) Discovery Camp in Namibia, Africa, thus the proposed research is fully nested in broader impacts activities. Further, the PI will host a graduate student from the University of Namibia, Africa at Florida State University to transfer knowledge regarding methods used in microbial ecology, bioinformatics and statistical analyses in years two and three. This project has two specific objectives, with the first being to determine the spatial distribution of nitrous oxide production rates and mechanisms using 15N tracers and natural abundance isotope and isotopomer analyses. The second objective is to determine the relationship between dissolved oxygen concentrations and microbial community structure, microbial function and metabolic activities that lead to nitrous oxide production, resolving the active microbial pathways that lead to the observed isotopic data from the first objective. The relative contributions of different pathways to nitrous oxide accumulation will be tested through a combination of 15N tracer experiments, natural abundance isotope and isotopomer analyses, and 16S rRNA gene, metagenome and metatranscriptome sequencing. The proposed work will model the distributions of nitrous oxide bulk and site-specific isotope ratios using nitrate and nitrite isotope measurements as inputs and will test the rates and environmental controls determined in incubation experiments to assess consistency with the longer-term isotopic measurements. The molecular data will also inform the interpretation of the isotopic data, further resolving nitrous oxide source mechanisms and activity.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.