partially endergonic (i.e. only possible with energy input), since bacteria alone are not
capable of sustaining that type of reaction.
Methane-producing bacteria, involved in
the third step, decompose compounds with
a low molecular weight. For example, they
utilize hydrogen, carbon dioxide and acetic
acid to form methane and carbon dioxide.
Under natural conditions, methane
producing microorganisms occur to the
extent that anaerobic conditions are
provided, e.g. under water (for exemple in
marine sediments), in ruminant stomaches
and in marshes. They are obligatory
anaerobic and very sensitive to
environmental changes. In contrast to the
acidogenic and acetogenic bacteria, the
methanogenic bacteria belong to the
archaebacter genus, i.e. to a group of
bacteria with a very heterogeneous
morphology and a number of common
biochemical and molecular-biological
properties that distinguish them from all
other bacterial general. The main
difference lies in the makeup of the
bacteria’s cell walls.
Figure 3: Various types of methanogenic bacteria.
The spherically shaped bacteria are of the
methanosarcina genus; the long, tubular ones are
methanothrix bacteria, and the short, curved rods
are bacteria that catabolize furfural and sulfates.
The total length of the broken bar at top left,
which serves as a size reference, corresponds to
Source: Production and Utilization of Biogas in Rural
Areas of Industrialized and Developing Countries,
Schriftenreihe der gtz, No. 97, p. 55
Symbiosis of bacteria
Methane- and acid-producing bacteria act in a symbiotical way. On the one hand, acid-
producing bacteria create an atmosphere with ideal parameters for methane-producing
bacteria (anaerobic conditions, compounds with a low molecular weight). On the other hand,
methane-producing microorganisms use the intermediates of the acid-producing bacteria.
Without consuming them, toxic conditions for the acid-producing microorganisms would
In practical fermentation processes the metabolic actions of various bacteria all act in
concert. No single bacteria is able to produce fermentation products alone.