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The N-cycle

The N cycle with the major processes converting nitrogenous compounds, all mediated by microorganisms (anaerobic ammonium oxidation not depicted). Initials of members of this research program are placed at their respective major research focus, those with major expertise in environmental microbiology and/or functional genomics in green, ecosystems researchers in blue. Abbreviations: HMW-N: Nitrogen fixed in organic biomass, DNRA: dissimilatory Nitrate reduction.


Overall research plan and thematic focuses of the DK

Nitrogen (N) is the second most abundant element in living cells and the major constituent of our atmosphere. Its availability therefore can have direct impact on the rate of key ecosystem processes (Thornton et al., 2009 [1]). While N can be a limiting nutrient in many natural ecosystems (Elser et al., 2007 [2], LeBauer and Treseder, 2008 [3]) its global availability as reactive nitrogen species has more than doubled over the past few decades due to mineral fertilizer production. Already 10 years ago, 125 million tons of N2 were estimated to be fixed per year as fertilizers, by leguminous crops and by fossil fuel burning (Galloway et al. 2008 [4]). This is three times higher than the proposed planetary boundary for preventing unacceptable environmental changes (Rockström et al., 2009 [5]). This acceleration of the nitrogen cycle which is fueled by a globally increasing demand on food production and leads to increased emissions of nitrous oxide, a most potent greenhouse gas and threat to the atmospheric ozone layer (Montzka et al., 2011 [6], Ravishankara et al., 2009 [7]). On the other hand, N-transformations by microorganisms are routinely exploited in wastewater treatment plants to eliminate excess N from sewage (Daims and Wagner, 2010 [8]).

All major processes in the cycling of N are mediated by microorganisms (Klotz and Stein 2008[9]). But many basic metabolisms and the ecophysiology of N-transforming microorganisms are still poorly understood, as are their interdependences and trophic interactions with other complex organisms and their impact on microbial communities and ecosystems.

In this DK a faculty with extensive experience on different aspects of N cycling have gathered in a program that offers PhD education and research at the highest technical level on single-cell technologies, microbial physiology and environmental N transformations to educate a new generation of scientists that can better integrate microbial activities and their diversity on different scales: from the single cell, to populations and microbial communities up to the ecosystem level.


The research topics, which involved nitrogen cycling microorganisms engaging in N fixation and organic N cycling to N mineralization, nitrification and denitrification, were collected around two themes:

Theme 1: Eukaryote-Microbe Interaction

Theme 2: Microbial metabolic flexibility and niche differentiation

In both themes novel techniques were developed and applied, including single-cell based approaches, stable-isotope coupled methods and integrated functional genomics to allow deeper insights into the physiological versatility of N-transforming microorganisms and to be able to relate their activities to ecosystem functioning.

For further information, please see current projects.


[1]                            Thornton, P., Doney, S., Lindsay, K., Moore, J., Mahowald, N., Randerson, J., Fung, I., Lamarque, J., Feddema, J., Lee, Y., 2009. Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model. BIOgeosciences 6, 2099-2120.

[2]                            Elser, J., Bracken, M., Cleland, E., Gruner, D., Harpole, W., Hillebrand, H., Ngai, J., Seabloom, E., Shurin, J., Smith, J., 2007. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecology Letters 10, 1135-1142.

[3]                            LeBauer, D., Treseder, K., 2008. Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed. Ecology 89, 371-379.

[4]                            Galloway, J., Townsend, A., Erisman, J., Bekunda, M., Cai, Z., Freney, J., Martinelli, L., Seitzinger, S., Sutton, M., 2008. Transformation ofthenitrogencycle: recenttrends, questions, and potential solutions. Science 320, 889-892.

[5]                            Rockström, J., Steffen, W., Noone, K., Persson, A., Stuart Chapin III, F., Lambin, E., Lenton, T., Scheffer, M., Folke, C., Schellnhuber, H., Nykvist, B., de Wit, C., Hughes, T., van der Leeuw, S., Rodhe, H., Sörlin, S., Snyder, P., Costanza, R., Svedin, U., Falkenmark, M., Karlberg, L., Corell, R., Fabry, V., Hansen, J., Walker, B., Liverman, D., Richardson, K., Crutzen, P., Foley, J., 2009, A safe operating space for humanity. Nature 461, 472475.

[6]                            Montzka, S., Dlugokencky, E., Butler, J., 2011. Non-CO2 greenhouse gases and climate change. Nature 476, 43-50.

[7]                            Ravishankara, A., Daniel, J., Portmann, R., 2009. Nitrous oxide (N2O): The dominant ozone-depleting substance emitted in the 21st century. Science 326, 123-125.

[8]                            Daims, H., Wagner, M., 2010. The microbiology of nitrogen removal. In: The microbiology of activated sludge (Nielsen PH, Seviour RJ). IWA Publishing, London, UK.

[9]                            Klotz, M.G., Stein, L.Y., 2008. Nitrifier genomics and evolution of the nitrogen cycle. FEMS Microbiol Lett. 278(2); 146-56

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