Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 45

NADPH and approximately three molecules of ATP are synthesised. Since this
may leave the dark reactions slightly short of ATP for carbohydrate synthesis, it
is postulated that photosystem 2 passes through one extra cycle thus producing
additional ATP molecules with no additional NADPH.
The Nitrogen Cycle
Nitrogen is constantly taken, or fixed, from the atmosphere, oxidised to a form
able to be utilised by plants and some bacteria, to be subsumed into metabolic
pathways, and through the various routes described above is then excreted into
the environment as reduced nitrogen where it may be reoxidised by bacteria or
released back into the atmosphere as nitrogen gas. These combined processes are
known collectively as the nitrogen cycle.
The previous discussions have referred to the release of nitrogen during degra-
dation of proteins and nucleic acid bases, either in the form of ammonia, the
ammonium ion, urea or uric acid. The fate of all these nitrogen species is to be
oxidised to nitrite ion by Nitrosomas, a family of nitrifying bacteria. The nitrite
ion may be reduced and released as atmospheric nitrogen, or further oxidised
to nitrate by a different group of nitrifying bacteria, Nitrobacter. The process of
conversion from ammonia to nitrate is sometimes found as a tertiary treatment
in sewage works to enable the nitrate consent to be reached. The process typ-
ically occurs in trickling bed filters which have, over time, become populated
with a Nitrosomas and Nitrobacter along with the usual flora and fauna which
balance this ecosystem. Denitrification may then occur to release atmospheric
nitrogen or the nitrate ion, released by Nitrobacter, may be taken up by plants
or some species of anaerobic bacteria where it is reduced to ammonium ion and
incorporated into amino acids and other nitrogen-carbon containing compounds.
To complete the cycle, atmospheric nitrogen is then fixed by nitrifying bacteria,
either free living in the soil or in close harmony with plants as described earlier
in this chapter.
Closing Remarks
The underpinning biochemistry and natural cycles described in this chapter form
the basis of all environmental biotechnological interventions, and a thorough
appreciation of them is an essential part of understanding the practical applica-
tions which make up most of the rest of this work.
References
Allison, D.G., Gilbert, P., Lappin-Scott, H. and Wilson, M. (2000) Community
structure and co-operation in biofilms, Fifty-Ninth Symposium of the Society for
General Microbiology held at the University of Exeter , Cambridge University
Press, Cambridge, pp. 215-256, September 2000.