Geoscience Reference
In-Depth Information
Equation [4.13] also describes the combustion (i.e. burning) of
organic matter, where the energy produced is heat. In fact, respiration
is a specific type of combustion where metabolic cycles or processes
of living organisms strictly control the release of energy from organic
matter, whereas burning is not naturally controlled. Because
most of the vegetation biomass removed from forests is burnt in the
short or medium term, the human activity of deforestation [
14
]
accelerates the transfer of carbon from the continental biosphere to the
atmosphere.
Two types of anaerobic respiration are mentioned later (section
4.4.2), namely nitrate respiration or denitrification and sulfate
reduction (or respiration) (arrow [
14
] in Figure 4.4). Denitrification is
explained in more detail in section 4.5.2.
A third group of biological processes are involved in the short
organic carbon cycle, i.e. fermentation, which takes place in an
anaerobic environment (i.e. in the absence of O
2
). In the example
below, fermentation uses the energy contained in organic matter, and
produces two inorganic substances, i.e. CO
2
and CH
4
[
15
]:
C
6
H
12
O
6
→
3 CO
2
+ 3 CH
4
+ energy
[4.14]
There are several types of fermentation. For example, during the
production of alcoholic drinks, fermentation produces two molecules
of ethanol (C
2
H
5
OH) and two molecules of CO
2
from one molecule
of glucose (C
6
H
12
O
6
). In the water column, fermentation takes place
in the digestive tract (anaerobic environment) of various organisms,
including zooplankton and fish, where it contributes to food digestion.
The different processes above are coupled with variations in
atmospheric CO
2
and CH
4
on time scales of several years to decades.
Organisms that synthesize organic matter from organic compounds
are called autotrophs and those which feed on existing organic
matter are called heterotrophs; some organisms, called mixotrophs, are
capable of using the two different sources for nutrition. All organisms -
autotrophs, heterotrophs or mixotrophs - respire.
