Gross nitrogen fixation in the oligotrophic North Pacific Ocean

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Just as plants convert carbon dioxide into biomass for growth, some microorganisms can convert nitrogen gas from the atmosphere into biomass. This process is called nitrogen fixation and it is important ecologically because it allows microorganisms to live in habitats where they would otherwise be unable to grow. Nitrogen fixation occurs both on land and in the oceans. One of the key questions facing oceanographers today is resolving how much nitrogen fixation is occurring in the surface ocean and how it varies temporally, on timescales of days to seasons, and spatially, across different biomes. This work will develop high throughput analysis of nitrogen fixation at sea so that measurements can be made in real-time onboard oceanographic research vessels. This will help improve targeted sampling of the water-column and facilitate the comparison of nitrogen fixation with rates of photosynthesis which are currently conducted on timescales of minutes to hours. The work will be done at the Center for Microbial Oceanography: Research and Education (C-MORE) where Native Hawaiians and Pacific Islanders majoring in an ocean or earth science-related field are given opportunities for scientific training and laboratory experience. To engage the high-school students in the work and inform them about opportunities in scientific careers, we will hold one to two outreach events per year targeting sixth to eighth grade girls at local public charter schools.This research will use a highly-sensitive reduced gas analyzer to quantify the production of hydrogen gas which is a metabolic by-product of the nitrogen fixation process. To improve the detection limit, seawater samples will be sparged with the inert gas argon to inhibit the reduction of nitrogen gas by the nitrogenase enzyme. Under this scenario, nitrogenase transfers all the electrons used to reduce nitrogen gas into the production of hydrogen. This technique of amplifying the production of hydrogen via nitrogen fixation is familiar to the biohydrogen research community and will be developed for sea-going oceanographic purposes. This proposal will test the method on laboratory cultures of diazotrophs to verify the ratio of hydrogen gas production to traditional methods of nitrogen fixation analysis such as stable isotope incorporation into biomass.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.