Geoscience Reference
In-Depth Information
replenishes itself with nitrate from the atmospheric reservoir and
readjusts the proportion of nitrate/phosphate to biological demand.
1.5.2.
When the ocean has an excess of nitrate
A regulation would not be possible if an inverse process did not
exist, consisting of emptying the ocean of some of its nitrate in favor
of the atmospheric N
2
.
In the interior ocean, the organic matter coming from the surface is
broken down, mainly by the aerobic and anaerobic respirations. The
aerobic respiration, whether it is that of organized multicellular life
forms (plant and animal) or that of monocellular life forms (protists,
bacteria, archaea), uses O
2
to oxidize the organic matter and finally
releases organic nitrogen in the form of rapidly oxidized nitrate,
ammonium NH
3
being only an intermediate.
In the current ocean, there exist a few anoxic environments,
permanent or transient, which facilitate anaerobic bacterial respiration.
These are always environments where the replenishing of oxygen, by the
circulation of water (ventilation) or by molecular diffusion, is
insufficient to compensate for the flux in oxygen consumed by the
breaking down of organic matter. These environments therefore
extend the anoxic perimeter until the two fluxes are in equilibrium.
Such conditions, which were dominant in the Earth's distant past
(before the accumulation of oxygen), are found today in certain
sediments and certain masses of water in the global ocean subject to
significant fluxes of organic matter and insufficient ventilation (layers
of water at intermediate depth with a minimum oxygen level and
certain waters at the bottom in contact with sediment). On the periphery
of the anoxic zones, where nitrate enters by diffusion, occurs, among
others, an anaerobic respiration called denitrification, which is carried
out by specialized bacteria. It transforms nitrate into oxides of
nitrogen (NO or NO
2
) or N
2
nitrogen, all these products being in gas
form in the conditions at the surface of the Earth, so transferable to the
