Geoscience Reference
In-Depth Information
Fixation of N
2
(gas) by specialized cyanobacteria [
1
], which
produces ammonium (NH
4
+
):
2N
2
(gas) + 3 CH
2
O + 3 H
2
O + 4 H
+
→ 4 NH
4
+
+ 3 CO
2
[4.17]
where CH
2
O represents organic matter. This reaction is catalyzed by
the enzyme nitrogenase. The NH
4
+ is used to synthesize organic
matter, which brings nitrogen of atmospheric origin into the food web.
In the sea, the most remarkable N
2
fixing bacteria are cyanobacteria of
the genus
Trichodesmium
, which can form filaments of a several cells,
which in turn can join to form colonies with diameters of 1 to 10 mm.
Filaments or colonies of
Trichodesmium
may span tens or even
thousands of square kilometers at the surface of warm seas. In the soil,
N
2
fixing cyanobacteria are frequently found in symbiosis with plants,
in particular in the roots of leguminous plants.
The presence of N
2
fixing bacteria in the roots of leguminous
plants explains why farmers traditionally alternate between crops of
legumes, cereals, oleaginous plants and other varieties (called crop
rotation), which periodically enriches the soil with symbiotic nitrogen.
Today, this practice has been replaced by the addition of synthetic
nitrogenous fertilizers to soils, although some countries encourage
returning to crop rotation. Artificial fertilizers have themselves
replaced previously used natural fertilizers consisting of guano, i.e.
excrements of marine birds and bats, rich in nitrogen and phosphorus.
In order to understand the importance of large guano deposits prior to
the advent of artificial fertilizers, one must remember that the
ownership of such deposits caused wars between Chile, Bolivia and
Peru in the 19th
Century. Due to its key role in global agriculture,
guano was the oil of that era.
From the ocean surface, organic matter continually sinks toward
deep waters, hence the name marine snow [
2
]. Nitrogen contained in
the sinking organic matter is regenerated at depth (see the relevant
stages of ammonification and nitrification, below) and nutrients thus
produced are returned to the surface by upward movements of water
