There is very little nitrogen in mineral form, but the atmosphere contains up to 78 million kg of gaseous nitrogen, N2
over every hectare of land. It is very stable and represents 79 % of the atmosphere. Even though it is essential for protein synthesis, nitrogen in its gaseous form cannot be assimilated by most living beings.
and blue algae transforming N2
into ammonia (NH3
) usable by plants and other bacteria (arrow pointing from NH3
to a plant). Thanks to the food chain, nitrogen then becomes a constituent of animals. Excrement, carcasses, and dead plants are waste products that contain organic nitrogen. These wastes are transformed into ammonia (NH3
) by bacteria that participate in the decomposition process. Nitrification is the process by which ammonia is transformed by nitrifying bacteria (e.g. Nitrosomonas
into nitrite (NO2-
) and then into nitrate (NO3-
) by Nitrobacters
. These nitrates can then be transformed into N2
, which is returned into the atmosphere through a denitrification process carried out by many bacterial species such as Paracoccus denitrificans
. In this way, nitrogen circulates in the biosphere and gets transformed from one organism to the next. Nitrates (NO3-
) also fertilize plants, which absorb them through their roots and use them to synthesize amino acids and then proteins.
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