Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 34

Bacterial electron transport systems, denitrification and methanogenesis
As previously mentioned, the term respiration is applied to many processes.
Without further specification it is usually used to mean the consumption of
molecular oxygen, by reduction to water in the case of the electron transport
discussed above, or by oxidation of an organic molecule to produce carbon diox-
ide and serine in the case of photorespiration, discussed later in this chapter. Thus
the term anaerobic respiration seems a contradiction. It does however describe
fundamentally the same process of electron transfer to a final acceptor which
although inorganic, in this case is not oxygen. An example of such an electron
acceptor is nitrate which is converted to nitrite. This is a toxic substance, and
so many bacteria have the facility to convert nitrite to nitrogen gas. This overall
series of reactions is described as denitrification and is the basis of the pro-
cess by which denitrifying bacteria such as members of the Pseudomonas and
Bacillus genera are able to reduce nitrate and nitrite levels down to consent val-
ues during sewage treatment. Such bacteria have different components in their
electron transport chain in comparison with mitochondria, which have the neces-
sary enzymatic activities to carry out these processes. Like mitochondrial electron
transport, denitrification can be associated with synthesis of ATP although with
much reduced efficiency.
Other examples of terminal electron acceptors are firstly sulphate, in which
case one of the final products is elemental sulphur. This process is carried out
by the obligate anaerobe, Desulfovibrio and members of the archaean genus
Archaeglobus . Another anaerobe, Alkaliphilus transvaalensis , an extreme alka-
liphile, growing at a pH of 8.5-12.5, isolated from an ultra-deep gold mine in
South Africa, can use elemental sulphur, thiosulphate or fumarate as an additional
electron acceptor (Takai et al ., 2001). Secondly, carbon dioxide may be the final
electron acceptor in which case one of the final products is methane. This pro-
cess is also carried out by obligate anaerobes, in this case, the methanogens, all
of which are archaeans and are responsible for methane production in anaerobic
digesters and landfill sites. Again, it functions on much the same principles as the
other chains mentioned above but has a different set of cofactors which are most
unusual. For both of the above obligate anaerobes, anaerobic respiration is an
important mechanism of ATP synthesis. It is less efficient than aerobic respiration
due to the smaller drop in electro-potential between sulphate or carbon dioxide
and NADH compared with the difference between NADH and oxygen, and so
less energy is available to be released during electron transport and consequently
less ATP is synthesised per mole of NADH entering the pathway. Anaerobic
respiration is however more efficient than fermentation and so is the route of
choice for ATP synthesis for an anaerobe.
The energy balance sheet between substrate level and electron transport
linked ATP synthesis
An approximate comparison may be made between the efficiency with respect
to energy production, of ATP synthesis by substrate level phosphorylation and