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At Acetogenesis the carbonic acids are converted to acetate, H2 and CO2. Just these substances
can be changed directly into methane by methanogenic bacteria. This process is also called "anaerobic oxidation",
because electrons in the form of H2 are released.
The releasing of butyric acid into acetate is shown in Figure 5. This reaction works at a low partial
pressure of hydrogen.

Figure 5: reaction equation of butyric acid into acetate

Whether there is an energy gain or energy usage depends on the partial pressure of hydrogen.
If it is at 10^-4^ bar or lower, then there is an energy gain at the most important acetogene reactions.
The gain is also dependent on the substrate. While ethanol is metabolized at a partial pressure of 10^-2^
bar and energy gain, propionate cannot be fermented with gain at <10^-4^ bar. This dependency is shown in Figure 6.

Figure 6: addiction of energy gain of acetogenic reactions of partial pressure of hydrogen.

On energetic sight the acetogenic reactions take place at the expense of the methanogenic reactions.
The "thermodynamic window" of the degradation of propionic acid is shown in the hatched part of Figure 6.
It starts at a partial pressure of hydrogen at 10^-6^ bar and ends at 10^-4^ bar. If the window is
enlarged it can be seen that the energy gaining degradation of butyrate starts beneath 10^-3^ bar.
The energy gaining degradation of ethanol is marginal, inhibited at a partial pressure of hydrogen of
about one bar, as well.
The acetogenic bacteria are obligate H2 producers, but they only survive at very low
H2 concentrations. Due to that, they live in symbiosis with H2 consuming organisms, generally
methane bacteria, to hold up the low partial pressure of hydrogen.

During acetogenesis acetate is generated primarily, which is the source material for generating methane.
Therefore carbonic acids are metabolized. Acetate can also be formed out of glucose or saccharose
by separating hydrogen and carbon dioxide. The reaction equations are shown in Figure 7 and Figure 8.

Figure 7: reaction equation of saccharose to acetate, hydrogen and carbon dioxide

Figure 8: reaction equation of glucose to acetate, hydrogen and carbon dioxide

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