Chemistry Reference
In-Depth Information
reduce CO
2
to organic matter in the absence of light (i.e., they are chemoautotrophs). N
2
O, a greenhouse gas,
3
is
a by-product of this process, and nitrification by marine and terrestrial organisms is an important source of
atmospheric N
2
O.
In the absence of oxygen, a third set of opportunistic microbes uses NO
3
and NO
2
as electron acceptors in the
anaerobic oxidation of organic matter. Nitrate reduction is coupled to the anaerobic oxidation of organic carbon
producing either NH
4
in a process known as dissimilatory nitrate reduction to ammonium (DNRA) or, more
commonly, N
2
gas during denitrification (
Figure 18.5
). Organisms which carry out denitrification include
representatives of many bacteria and archaea, as well as some eukaryotes. Four metalloenzymes are involved in
denitrification: nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase. N
2
Oisan
obligate intermediate (
Figure 18.5
), and some ultimately escapes to the atmosphere, making denitrification
another important source of this greenhouse gas from both marine and terrestrial environments.
An alternative bacterial route from NO
3
to N
2
, where NH
4
oxidation is coupled to NO
2
reduction in a process
called anammox (anaerobic ammonium oxidation), dominates N
2
production in many marine environments, but,
unlike classical denitrification, it does not lead to the production of N
2
O. Together, denitrification and anammox
close the nitrogen cycle by returning N
2
gas back to the atmosphere.
Sulfur and Selenium
Since the early atmosphere of our planet was essentially characterised by low redox potentials, it is highly
probable that sulfur was a very important element. Hydrogen sulfide was likely present at mM concentrations in
aqueous solution, and transition metal sulfides were probably among the first biocatalysts. Although some bacteria
can synthesise sulfur-containing organic compounds directly from elemental sulfur or from sulfite, most organ-
isms acquire sulfur from sulfate. Just as CO
2
and N
2
must undergo fixation in order to be utilised, sulfate utilisation
requires metabolic activation to a form that can readily undergo reduction. In plants and bacteria, this involves the
condensation of sulfate with ATP to form APS (adenosine-5
0
-phosphosulfate), which is further phosphorylated to
give PAPS, which has an additional
phosphate in the 3
0
-position (
Figure 18.6
). PAPS is then used in bacteria both as an activated form of sulfate, both
for sulfation reactions and as a substrate for sulfate reduction. It is initially reduced to sulfite (SO
2
3
) in a reaction
FIGURE 18.6
Structure of PAPS.
3. N
2
O is currently the third of the 'greenhouse gases', after carbon dioxide and methane in importance. While not as abundant as carbon
dioxide, it is 300 times more potent in its ability to warm the planet.
