Nitrogen fixation is the conversion of nitrogen gas (N2) into ammonia (NH3), which dissolves in soil water to form ammonium ions (NH4+). Only certain prokaryotes can do this biologically.The reaction is catalysed by the enzyme nitrogenase, which reduces N2 to ammonia. It needs a lot of ATP and hydrogen, and nitrogenase is inactivated by oxygen, so it must work in anaerobic conditions.Mutualistic nitrogen-fixing bacteria
Rhizobium lives inside swellings called root nodules on the roots of leguminous plants (legumes), such as peas, beans and clover. The relationship is mutualistic:
• the plant receives ammonium, converted into amino acids;
• the bacteria receive carbohydrates (sugars from photosynthesis), which they use in respiration to make ATP, plus a protected habitat.The nodules contain leghaemoglobin, a pigment similar to haemoglobin, which binds oxygen. This keeps the oxygen concentration low enough for nitrogenase to work, while still supplying oxygen for the bacteria's aerobic respiration.Free-living nitrogen-fixing bacteria
Azotobacter lives freely in the soil and fixes nitrogen, using the ammonia to make its own amino acids. When these bacteria die and decompose, the nitrogen is released as ammonium ions.Farming link: growing legumes in crop rotation and ploughing them into the soil increases soil nitrogen, reducing the need for artificial fertilisers.
Rhizobium lives inside swellings called root nodules on the roots of leguminous plants (legumes), such as peas, beans and clover. The relationship is mutualistic:
• the plant receives ammonium, converted into amino acids;
• the bacteria receive carbohydrates (sugars from photosynthesis), which they use in respiration to make ATP, plus a protected habitat.The nodules contain leghaemoglobin, a pigment similar to haemoglobin, which binds oxygen. This keeps the oxygen concentration low enough for nitrogenase to work, while still supplying oxygen for the bacteria's aerobic respiration.Free-living nitrogen-fixing bacteria
Azotobacter lives freely in the soil and fixes nitrogen, using the ammonia to make its own amino acids. When these bacteria die and decompose, the nitrogen is released as ammonium ions.Farming link: growing legumes in crop rotation and ploughing them into the soil increases soil nitrogen, reducing the need for artificial fertilisers.