Microorganisms, specifically nitrogen-fixing bacteria and archaea, can assimilate di-nitrogen gas N2 into an accessible form for living beings through a process called biological nitrogen fixation. This process converts atmospheric nitrogen N2 into ammonia NH3 , which can then be used by plants and other organisms for the synthesis of amino acids, nucleic acids, and other nitrogen-containing compounds.The key enzyme involved in biological nitrogen fixation is nitrogenase, which is a complex metalloenzyme that catalyzes the reduction of N2 to NH3. Nitrogenase consists of two main components: the dinitrogenase reductase also known as the Fe protein and the dinitrogenase also known as the MoFe protein . The reaction requires ATP and reducing equivalents electrons provided by the host organism.The overall reaction for nitrogen fixation is:N2 + 8H+ + 8e- + 16ATP 2NH3 + H2 + 16ADP + 16PiThe biochemical pathways involved in nitrogen metabolism to sustain the nitrogen balance in ecosystems are:1. Nitrogen fixation: As mentioned earlier, nitrogen-fixing bacteria and archaea convert atmospheric N2 into ammonia NH3 using the nitrogenase enzyme.2. Nitrification: This is a two-step process in which ammonia is first oxidized to nitrite NO2- by ammonia-oxidizing bacteria AOB or archaea AOA , and then nitrite is further oxidized to nitrate NO3- by nitrite-oxidizing bacteria NOB . The key enzymes involved in nitrification are ammonia monooxygenase AMO and nitrite oxidoreductase NXR .3. Assimilation: Plants and other organisms assimilate ammonia NH3 or nitrate NO3- into organic compounds, such as amino acids and nucleic acids, through the glutamine synthetase-glutamate synthase GS-GOGAT pathway or the glutamate dehydrogenase GDH pathway.4. Ammonification: Decomposition of organic nitrogen compounds by heterotrophic bacteria and fungi releases ammonia NH3 back into the environment, which can be used by plants or undergo nitrification.5. Denitrification: Denitrifying bacteria convert nitrate NO3- into gaseous nitrogen compounds, such as nitric oxide NO , nitrous oxide N2O , and dinitrogen gas N2 , under anaerobic conditions. The key enzymes involved in denitrification are nitrate reductase NAR , nitrite reductase NIR , nitric oxide reductase NOR , and nitrous oxide reductase N2OR .These biochemical pathways help maintain the nitrogen balance in ecosystems by cycling nitrogen between its various forms, ensuring that it is available for use by living organisms and preventing the accumulation of toxic nitrogen compounds.