RESEARCH: N₂ fixation

N₂ fixation is the primary pathway by which nitrogen is added to the ocean and is mediated by microbes with the ability to reduce N₂ gas to ammonium.

	I am on the Leadership Team of the Aquatic N2 fixation Research Coordination Network
	Picture by A. Knapp in the Gulf of Mexico, June 2015, of a surface slick of Trichodesmium spp.
	Since N₂ fixation is biologically mediated, the rates and spatial distribution of this process are directly related to the sensitivities of the organisms carrying out this process. Related to our work on NO₃^-, we can compare the δ¹⁵N of the sinking flux with the δ¹⁵N of newly fixed nitrogen to estimate how much N₂ fixation is occurring.

Where?

Isotopic evidence indicates N₂ fixation is occuring at significant rates in the Western Tropical South Pacific, but at much lower rates in the Eastern Tropical South Pacific, and the Sargasso Sea. A current project is evalauting the importance of N₂ fixation and NO₃^- in the Gulf of Mexico, especially as they may support the food web of larval bluefin tuna.

Publications

Knapp, A.N., K.M. McCabe, O. Grosso, N. Leblond, T. Moutin, and S. Bonnet, Distribution and rates of nitrogen fixation in the western tropical South Pacific Ocean constrained by nitrogen isotope budgets, 2018, Biogeosciences, 15, 2619-2628, doi:10.5194/bg-2619-2018

Marconi, D., D. Sigman, K. Casciotti, E. Campbell, M. Weigand, S. Fawcett, A. Knapp, P. Rafter, B. Ward, and G. Haug, Tropical dominance of N2 fixation in the North Atlantic Ocean, 2017, Global Biogeochemical Cycles, 31, doi:10.1002/2016GB005613

Knapp, A.N., S.E. Fawcett, A. Martinez-Garcia, N. Leblond, T. Moutin, and S. Bonnet, Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments, 2016, Biogeosciences, 13, 4645-4657, doi:10.5194/bg-13-4645-2016

Knapp, A.N., K.L. Casciotti, W.M. Berelson, M.G. Prokopenko, and D.G. Capone, Low rates of nitrogen fixation in the eastern tropical South Pacific surface waters, 2016, Proceedings of the National Academy of Sciences, 113, 4398-4403, doi:10.1073/pnas.1515641113

Berelson, W.M., W. Haskell, M.G. Prokopenko, A.N. Knapp, D. Hammond, N. Rollins, D.G. Capone, 2015, Biogenic rain and benthic remineralization in the Eastern Tropical South Pacific, Deep-Sea Research I, 99, 23-34, doi:10.1016/j.dsr.2014.12.006.

Knapp, A.N., 2012, The sensitivity of marine N2 fixation to dissolved inorganic nitrogen, Frontiers in Microbiology, 3:374. doi: 10.3389/fmicb.2012.00374

Knapp, A.N., J. Dekaezemacker, S. Bonnet, J.A. Sohm, and D.G. Capone, 2012, Sensitivity of Trichodesmium and Crocosphaera abundance and N2 fixation rates to varying NO3- and PO43- concentrations in batch cultures, Aquatic Microbial Ecology, 66, 223-236, doi:10.3354/ame01577

Knapp, A.N., P.J. DiFiore, C. Deutsch, D.M. Sigman, and F. Lipschultz, 2008, Nitrate isotopic composition between Bermuda and Puerto Rico Implications for N2 fixation in the Atlantic Ocean, Global Biogeochem. Cycles, 22, GB3014.

Capone, D.G., and A.N. Knapp, 2007, A marine nitrogen cycle fix?, Nature, 445, 159-160

Knapp, A.N., D.M. Sigman, and F. Lipschultz, 2005, N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic Time-series Study site, Global Biogeochem. Cycles, 19, GB1018, doi:10.1029/2004GB002320

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