The nitrogen cycle
The nitrogen cycle
Nitrogen is required for the biosynthesis of many compounds occurring in organisms and, unlike phosphorus – another element often limiting growth in many environments – can take on many oxidation states, ranging from -3 to +5. Consequently, nitrogen compounds can act as either electron donors (chemolithotrophy) or electron acceptors (anaerobic respiration). The nitrogen cycle starts with nitrogen fixation, the reduction of nitrogen gas to ammonium. Nitrogen fixation is carried out only by prokaryotes, mainly some cyanobacteria and heterotrophic bacteria. The ammonium resulting from nitrogen fixation is quickly used by many organisms for biosynthesis, being preferred over nitrate as a nitrogen source. It is also oxidized aerobically by chemolithoautotrophic bacteria and archaea during the first step of nitrification. The second step, nitrite oxidation, is carried out by other microbes not involved in ammonia oxidation, resulting in the formation of nitrate, which can then be reduced to nitrogen gas or nitrous oxide during denitrification, or to ammonium. Nitrogen gas is also released by anaerobic oxidation of ammonium, which is carried out by bacteria in the Planctomycetes phylum. The anaerobic ammonium oxidation pathway seems most important in producing nitrogen gas in deep oceanic sediments receiving low fluxes of organic material. Another gas in the nitrogen cycle – nitrous oxide – is a greenhouse gas produced by ammonia-oxidizing microbes. Most models indicate that the global nitrogen cycle is in balance, with losses from nitrogen gas production equalling gains via nitrogen fixation.
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