Environmental Engineering Reference
In-Depth Information
BOX 2.2
CHEMOSYNTHESIS
Chemosynthesis exploits chemical energy
to convert inorganic carbon compounds
into organic matter, in contrast with photo-
synthesis, which exploits the energy of light
to produce organic matter. Chemosynthetic
reactions are carried out by prokaryotic micro-
organisms, principally bacteria and archaea
(referred to as “bacteria” in the following).
Energy is produced in chemosynthetic reac-
tions from oxidizing reduced compounds.
There are a variety of chemosynthetic bacteria
that carry out these reactions including
nitrifying bacteria (oxidizing NH
4
or NO
2
),
sulfur bacteria (oxidizing H
2
S, S, and other
sulfur compounds), hydrogen bacteria (oxi-
dizing H
2
), methane bacteria (oxidizing
CH
4
), iron and manganese bacteria (oxidizing
reduced iron and manganese compounds),
and carbon monoxide bacteria (oxidizing
CO). This is not an exhaustive list and
new modes of chemosynthesis as well as
new chemosynthetic bacteria are still being
discovered.
Chemosynthetic reactions often occur at
the interface of aerobic and anaerobic
environments where the end-products of
anaerobic decomposition as well as oxygen
are available. Thus, these reactions are
most often apparent in soils and sediments
where oxygen is depleted. For example,
methane is produced by anaerobic bacteria
that convert fermentative end-products like
acetate to methane. Methane builds up in
anaerobic zones of soils, sediments, and
stratified water columns. Methane-oxidiz-
ing bacteria grow at
chemosynthetic reaction of methane oxida-
tion is:
4H
1
and 2 e
2
The energy generated by this reaction is
represented by the reducing power of the
hydrogen ions and electrons produced. These
are coupled to biochemical reactions used to
fix inorganic carbon. For methane-oxidizing
bacteria the initial organic compound pro-
duced in the coupled oxidation-reduction
reaction is formaldehyde (HCHO), which is
a precursor for further organic synthesis.
Understanding of chemosynthetic pro-
cesses is still advancing with new findings
that reveal reactions in environments where
they were not previously believed to occur.
For example, methane oxidation can occur
in anaerobic environments where microbes
use sulfate or nitrate to oxidize methane.
Raghoebarsing et al. (2006)
studied a fresh-
water canal polluted with high concentra-
tions of agricultural runoff and documented
anaerobic methane oxidation conforming to
the following reaction:
CH
4
1
O
2
-
CO
2
1
5CO
2
1
4N
2
8NO
3
1
8H
1
-
5CH
4
1
14 H
2
O
The microbial community found in the
canal was able to use methane and nitrate as
a sole energy source and did not require
oxygen to convert methane to carbon diox-
ide. The energy gained from this reaction is
used to fuel growth (via the fixed CO
2
).
Further research is likely to bring to light
novel mechanisms by which microbes use
chemicals in their environment as energy
sources to fix carbon.
1
the interface of the
aerobic
anaerobic zone exploiting methane
that moves out of the anaerobic area. The
