The
methanogenic bacteria
possess the capability of deriving energy for their progressive growth by certain particular oxidizing
chemical entities, for instance : hydrogen (H
2), formic acid (HCOOH) ; and actually exert their ‘action’
by making use of the electrons thus produced to reduce ultimately carbon-dioxide (CO
2) to give rise to
the formation of methane (CH
4) gas :
CO
2 + 4H2
⎯⎯⎯→
CH4 + 2H2
O
Carbon Hydrogen Methane Water
dioxide
It has been observed that certain genera specifically may grow as
autotrophs* — thereby utilizing
hydrogen and carbon dioxide as exclusive sources of carbon as well as energy ; whereas some others
do need several additional components, for instance : organic-sulphur compounds, amino acids, acetic
acid, and vitamins. Interestingly, a plethora of species actually grow quite abundantly and aggressively
in a
complex media viz., comprising of yeast extract in comparison to inorganic-salts containing media.
Coenzymes** :
There are at least two uncommon coenzymes that invariably occur in all methanogenic
bacteria (methanogens)
that have not been noticed in other varieties of microorganisms.
Examples :
Following are two typical examples of methanogenic coenzymes :
(
a) Coenzyme M — directly involved in methyl transfer reactions, and
(
b) Coenzyme F420 — a flavin-like chemical entity intimately involved in the anaerobic electron
transport system
of these microorganisms. It has the ability to fluoresce when exposed
to UV light ; and, therefore, its presence may be detected by visualizing the organisms
via a
fluorescence microscope
conveniently (also used for its critical identification and examination).
Differentiation of Methanogens :
The genera of methanogens i.e., the methane-producing bacteria
may be clearly differentiated exclusively based upon their
morphology*** and Gram reaction****.
However, the glaring distinct differences occurring in the cell-wall composition have been duly observed
to correlate specifically with these genera.
Importantly, the cell walls of
two genera essentially consist of pseudomurein, that prominently
differs from eubacterial
peptidoglycan by the following two distinct structural features, namely :
(
a) substitution of N-acetyltalosaminuronic acid for N-acetylmuramic acid, and
(
b) presence of tetrapeptide composed totally of L-amino acids, having glutamic acid attached
duly at the C-terminal end.
Habitats :
Interestingly, the methanogenic bacteria most commonly found in anaerobic habitats
that are eventually rich organic matter which ultimately produced by
nonmethanogenic bacteria via
fermentation to yield H
2 and CO2. A few such common as well as vital habitats are, namely : marine
sediments, swamps, marshes, pond and lake mud, intestinal tract of humans (GIT) and animals, rumen
of cattle (
e.g., cow, buffalow, sheep, pig, goat etc.), and anaerobic sludge digesters in sewage-treatment
plants.
Figure. 2.6 [A and B] depicts the diagramatic sketch of the cells commonly observed in various
kinds of methanogenic organisms (
viz., methane-producing bacteria).
Figure 2.6 [A] evidently shows the typical cells of
Methanosarcina barkeri and Methanospirillum
hungatei
representing ideally the methane-producing bacteria.
Figure 2.6 [B] likewise illustrates the characteristic cells of
Methanobacterium thermoautotrophicumand Methanobacterium ruminantium designating the methanogens.