Award: OCE-1756524

Biological nitrogen fixation has important implications for ecosystem structure and function because it represents a source of bioavailable nitrogen. To constrain the rates of nitrogen fixation in a given ecosystem and thereby improve our understanding of its relevance to biological productivity and the importance of microorganisms that can fix nitrogen requires a substantial increase in the spatial-temporal resolution of measurements compared to what is currently available. To date, the majority of the instantaneous rate measurements of nitrogen fixation have depended on the acetylene reduction assay, a process whereby the nitrogenase enzyme converts acetylene into ethylene which becomes a proxy for nitrogen fixation. During this three year project, an alternative approach was evaluated whereby an argon gas mixture was used to manipulate nitrogen fixing microbes into maximizing the production of hydrogen from nitrogenase, and using the resulting measurements of hydrogen gas as the proxy for nitrogen fixation. The first two years were spent evaluating the method using aquatic photoautotrophs in the laboratory. The method proved to be highly successful and based on the promising results obtained in the laboratory, a prototype system was assembled for conducting measurements in the field. The prototype system was tested in September 2020 and captured the diel variability in nitrogen fixation observed in the surface waters of the North Pacific Ocean. The equipment is now being improved so that it can be routinely used at sea. The results from this project have been presented at scientific conferences and resulted in eight scientific publications. As part of the Broader Impacts associated with the project, the Principal Investigator contributed to the science curriculum for local schoolchildren in Honolulu. Last Modified: 05/04/2021 Submitted by: Samuel Wilson