Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 29

a catabolic pathway, then it can become a 'food' for the organism. This is the
basis of bioremediation. The way in which energy is transferred from the 'food'
molecule to ATP may take two substantially different routes. One is cytoplasmic
synthesis of ATP which is the direct transfer of a phosphate group to ADP,
storing the energy of that reaction in chemical bonds. The other involves a fairly
complicated system involving transfer of electrons and protons, or hydrogen ions,
which originated from the oxidation of the 'food' at some stage during its passage
through the catabolic pathways. The final sink for the electrons and hydrogen
ions is oxygen, in the case of oxidative phosphorylation, to produce water. This
explains the need for good aeration in many of the processes of environmental
biotechnology, where organisms are using oxidative phosphorylation as their
main method for synthesising ATP. An example of this is the activated sludge
process in sewage treatment. However, many microbes are anaerobes, an example
being a class of archaea, the methanogens, which are obligate anaerobes in that
they will die if presented with an oxygenated atmosphere. This being the case,
they are unable to utilise the oxidative phosphorylation pathways and so instead,
operate an electron transport chain similar in principle, although not in detail.
It has as the ultimate electron and hydrogen sink, a variety of simple organic
compounds including acetic acid, methanol and carbon dioxide. In this case,
the end product is methane in addition to carbon dioxide or water depending
on the identity of the electron sink. These are the processes responsible for the
production of methane in an anaerobic digester which explains the necessity to
exclude air from the process.
Fermentation and respiration
The electrons derived from the catabolism of the carbon source are eventually
either donated to an organic molecule in which case the process is described as
fermentation, or donated to an inorganic acceptor by transfer along an electron
chain. This latter process is respiration and may be aerobic where the terminal
electron acceptor is oxygen, or anaerobic where the terminal electron acceptor is
other than oxygen such as nitrate, sulphate, carbon dioxide, sulphur or ferric ion.
Unfortunately, respiration is a term which has more than one definition. It may
also be used to describe a subset of the respiration processes mentioned above
to include only oxidation of organic material and where the ultimate electron
acceptor is molecular oxygen. This latter definition is the basis of Biological
Oxygen Demand (BOD), which is often used to characterise potential environ-
mental pollutants, especially effluents, being a measure of the biodegradable
material available for oxidation by microbes.
Fermentations
In modern parlance, there are many definitions of the term 'fermentation'. They
range from the broadest and somewhat archaic to mean any large scale culture of
micro organisms, to the very specific, meaning growth on an organic substance