Bacteria and Archaea and the Cycles of Elements in the Environment (page 3)
(This chapter has 4 pages)
© Kenneth Todar, PhD
The Nitrogen Cycle
The nitrogen cycle is the most complex of the cycles of elements
make up biological systems. This is due to the importance and
of N in cellular metabolism, the diversity of types of nitrogen
and the existence of the element in so many forms. Procaryotes are
involved in the biological nitrogen cycle in three unique processes.
Nitrogen Fixation: this process converts N2 in the
atmosphere into NH3 (ammonia), which is assimilated into
acids and proteins. Nitrogen fixation occurs in many free-living
such as clostridia, azotobacters and cyanobacteria, and in symbiotic
such as Rhizobium and Frankia, which associate with
roots to form characteristic nodules. Biological nitrogen fixation is
most important way that N2 from the air enters into
N2 ----------------> 2 NH3 nitrogen
Anaerobic Respiration: this relates to the use of oxidized
of nitrogen (NO3 and NO2) as final electron
for respiration. Anaerobic respirers such as Bacillus and Pseudomonads
are common soil inhabitants that will use nitrate (NO3)
electron acceptor. NO3 is reduced to NO2
and then to a gaseous form of nitrogen such as N2 or N2O
(nitrous oxide). The process is called denitrification. (A
related process conducted by some Bacillus species, called dissimilatory
nitrate reduction reduces NO3 to ammonia (NH3),
but this is not considered denitrification.)
Denitrifying bacteria are typically facultative microbes that respire
is available by aerobic respiration. If O2 is unavailable
respiration, they will turn to the alternative anaerobic respiration
uses NO3. Since NO3 is a common and expensive
of fertilizer in soils, denitrification may not be so good for
and one rationale for tilling the soil is to keep it aerobic, thereby
nitrate fertilizer in the soil.
NO3 ----------------> NO2
The overall reactions of denitrification shown
above proceed through the formation of nitrous oxide (N2O).
A recent article by Wunsch an Zumft in Journal of Bacteriology, vol.
(2005), sheds new light on the process of denitrification. N2O
is a bacterial metabolite in the REVERSAL of Nitrogen fixation. The
atmospheric increase of N2O is a cause for concern, as noted
above (as a greenhouse gas, N2O has 300 times the heat
absorbing capacity as CO2). Denitrifying bacteria respire
N2O as an electron acceptor yielding N2
and the thereby provide a sink for N2O. This article
new insight into this process by identifying a membrane-bound protein
denitrifying bacteria called NosR, that is necessary for the expression
of N2O reductase from the nosZ gene. The NosR
has redox centers positioned on opposite sides of the cytoplasmic
which allows it to sustain whole-cell N2O respiration by
on N2O reductase.
Nitrification is a form of lithotrophic metabolism that is
the opposite of denitrification. Nitrifying bacteria such as Nitrosomonas
utilize NH3 as an energy source, oxidizing it to NO2,
while Nitrobacter will oxidize NO2 to NO3.
Nitrifying bacteria generally occur in aquatic environments and their
in soil fertility and the global nitrogen cycle is not well understood.
The Overall process of Nitrification
NH3 ----------------> NO2 (Nitrosomonas)
NO2 ----------------> NO3 (Nitrobacter)
A final important aspect of the nitrogen cycle that involves
though not exclusively, is decomposition of nitrogen-containing
compounds. Most organic nitrogen (in protein, for example) yields
(NH3) during the process of deamination. Fungi are
in decomposition, as well.
Plants, animals and protista, as well as the procaryotes, complete
nitrogen cycle during the uptake of the element for their own
Nitrogen assimilation is usually in the form of nitrate, an
group, or ammonia.
Figure 2. The Nitrogen Cycle
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