The Nitrogen Cycle

• Nitrogen: essential component of proteins, amino acids, DNA

• Different forms of N: dinitrogen gas (N2), ammonium (NH4+), nitrite (NO2-), nitrate (NO3-); only few bacteria and cyanobacteria can access N2 directly (nitrogen fixation)

• Bacteria convert different forms of inorganic nitrogen:nitrogen fixation: N2 ? NH4+nitrification: NH4+ ? NO2- ? NO3-denitrification: NO3- ? N2 or N2O

• Animals excrete nitrogen as NH4+

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