Microbes and Metabolism 4518 molecules of ATP and 12 molecules of NADPH. Since photophosphorylation
is driven by a proton gradient established during electron flow after illumination,
there is not a stoichiometric relationship between the number of photons exciting
the systems and the amount of ATP produced. However, it is now established that
for every eight photons incident on the two photosystems, four for each system,
one molecule of oxygen is released, two molecules of NADP+are reduced to
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 degradation 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 byNitrosomas, 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 typically occurs in trickling bed filters which have, over
time, become populated with aNitrosomasandNitrobacteralong with the usual
flora and fauna which balance this ecosystem. Denitrification may then occur to
release atmospheric nitrogen or the nitrate ion, released byNitrobacter,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.