New Pathways and Processes in the Global Nitrogen Cycle

Our understanding of the players and pathways of the global nitrogen cycle has advanced substantially over recent years with discoveries of several new groups of organisms and new types of metabolism. This review focuses on recently discovered processes that add new functionality to the nitrogen cycle and on the organisms that perform these functions. The processes include denitrification and other dissimilatory nitrogen transformations in eukaryotes, anaerobic ammonium oxidation, and anaerobic methane oxidation with nitrite. Of these, anaerobic ammonium oxidation coupled to nitrite reduction by anammox bacteria has been well documented in natural environments and constitutes an important sink for fixed nitrogen. Benthic foraminifera also contribute substantially to denitrification in some sediments, in what potentially represents an ancestral eukaryotic metabolism. The ecophysiology of the novel organisms and their interactions with classical types of nitrogen metabolism are important for understanding the nitrogen cycle and its tight links to the cycling of carbon today, in the past, and in the future.