methanogenesis biochemistry and microbiology

Anaerobic fermentation or digestion is carried out by complex bacterial populations which, under very precise environmental conditions (redox potential ORP in the region of -250 mV, pH close to neutral), will constitute stable associations. This fermentation normally takes place in nature when a high concentration of organic matter (OM) is maintained in a state of anaerobiosis: marshes, lake sediment, digestive tract …

The methane fermentation of the OM can occur in "cold" (10-25 °C), mesophilic (30-40°C) or thermophilic (> 50 °C) ecosystems.

The various ways in which soluble or particulate organic matter degrades under anaerobic conditions can be outlined as follows (figure 38):

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hydrolysis and acidogenesis stages – (1)

These stages involve a very wide range of heterotrophic species: exclusively or optionally anaerobic.

This initial phase produces a mixture of volatile fatty acids (VFA): acetic, lactic, propionic, butyric…, neutral compounds (ethanol), gas products (CO₂ and H₂) and ammonium.

These micro-organisms frequently have a generation phase that is shorter than those responsible for subsequent phases. Therefore, by their activity, they can lead to a lowering of the pH and can block the development of populations required for later stages.

acetogenesis stage – (2)

This stage is produced from the reduced metabolites of stage 1 and is carried out by proton reducing bacteria that, therefore, produce hydrogen and acetate, for example:

methanogenesis stages (in the strict sense) – (3)

There are two major methanogenesis routes.One, from the H₂/CO₂ couple produces H₂O the other, termed acetoclastic that breaks acetate down into CO₂ and CH₄. In sludge treatment for example, 70% of methane is produced via the acetoclastic route.

Other sources of carbon such as methanol, formaldehyde and methylamines can also be used by methane producing micro-organisms.