Bacterial and Archaeal diversity in nitrogen cycle: recent discoveries and future challenges

Nitrogen is an essential component of all living organisms. A typical bacterial cell for example contains about 12-15% nitrogen by dry weight as components of proteins, amino-sugars, nucleic acids and several other constituents of the cell. In nature, the nitrogen occurs mainly in the lithosphere and as inert gas in the atmosphere. Only a small part is found in the biosphere either in reduced forms as ammonium-ammonia and amine groups or in many oxidized forms from nitric oxide to nitrates, covering a wide range of oxidation states. Reduction-oxidation reactions between the different stages of oxidation offer the potential of energy-generation by microorganisms. Our understanding of the nitrogen cycle is however far from complete with respect to the microorganisms that are involved: new processes and players in the cycle evolve and are just beginning to be investigated and understood. For example, the discovery of anaerobic ammonia oxidation (anammox) by bacteria belonging to Planctomycetales dates back to the early 1990's; the cultivation of mesophilic Archaea able of performing chemoautrophic nitrification as Bacteria occurred only in 2005. Besides terrestrial, marine, estuarine and freshwater environments are differently investigated and known with respect to the nitrogen cycle. Over the past hundred years, human activity has dramatically altered the global nitrogen cycle in several ways, for instance by increasing inputs of inorganic and organic nitrogen through severe fertilization, by releasing nitrogen oxides in the atmosphere by industrial combustion of fossil fuels, by acidifying soils, streams and lakes, etc... Consequentially, human activities are altering the tendency of the processes of the nitrogen cycle to balance each other in natural ecosystems. This suggests that important knowledge on the nitrogen cycle is still missing and, even more, that classical knowledge may need a reassessment, finally leading to a better understanding of the overall nitrogen cycle. In this perspective, studies on the microorganisms involved in the nitrogen cycle appeared to be noteworthy.