Geoscience Reference
In-Depth Information
increases in the current and future concentrations of CO
2
in the
atmosphere, and consequently in the ocean, would not directly affect
marine primary production. However, experiments on natural
plankton assemblages enclosed in mesocosms (large bags of several
cubic meters floating in the ocean) artificially kept under three
different CO
2
partial pressures showed that the amount of carbon fixed
by phytoplankton increased in response to increased pCO
2
[RIE 07].
Hence it is possible that an increase in CO
2
concentration in the ocean
could directly affect the functioning of marine ecosystems, but the
mechanism responsible for this increase remains unknown.
There is another major process by which organic matter is
synthesized in the ocean, i.e. chemosynthesis. In this case, organisms
obtain the energy required for the synthesis of organic matter from
inorganic chemical compounds, instead of light as in photosynthesis.
These organisms are often found in environments without light, for
example in hydrothermal systems on the seafloor.
A second group of biological processes involved in the short
organic carbon cycle is respiration and aerobic decomposition of
organic matter by oxidation [
13
]. With these processes, organisms can
use the energy stored in organic matter, with accessory production of
CO
2
and water:
C
6
H
12
O
6
+ 6 O
2
→
energy + 6 CO
2
+ 6 H
2
O
[4.13]
In equation [4.13], the oxygen atoms in the H
2
O molecules (to the
right) come from the O
2
molecules and not from the sugar (to the left),
which only supplies the hydrogen atoms. ∑CO
2
resulting from
the respiration and aerobic decomposition of organic matter in surface
waters or the underlying intermediate waters returns to the surface
within a few years, from where it may escape into the atmosphere if
pCO
2(ocean)
> pCO
2(atmosphere)
. On the contrary, ∑CO
2
that is released
into deep ocean waters may be sequestered there for several centuries,
within the deep ocean circulation (see section 2.1.2 in Chapter 2 of
[MON 14b]).
