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

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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.