Acetogenesis is the stage when the

Acetogenesis is the stage when the products of the hydrolysis are processed to hydrogen,

carbon dioxide, formate and acetate. This pathway occurs naturally in well balanced

methanogenic systems. However, in practice, there are cases of electron or hydrogen

accumulation (e.g. when methanogenesis is inhibited) when numerous other fermentation

products may be formed (e. g. propionate, butyrate, lactate, succinate, and alcohols) as a

mechanism to remove the excess electrons or hydrogen. Organisms that convert these

fermentation products to acetate, generally exhibit obligate proton- reducing metabolism

and are obligatory dependent on the hydrogen removal as referenced in Archives of Env.

Protection. Because of this the acetogenic bacteria are also called obligatory hydrogen-
producing acetogens (OHPAs).

Despite the significant importance of synthrophs, the knowledge of their taxonomic

position, diversity and physiology is insufficient, mainly because of the difficulties in

isolating them. Several important proton-reducing syntrophic bacteria such as butyrate-
oxidizers, propionate- oxidizers and even acetate-oxidizers have been successfully isolated

and cultured from methanogenic communities in recent years as referenced in Archives of

Env. Protection. Thermophilic acetate-oxydizing syntroph, Thermacetogenium phaetum,

was isolated and characterized by Hattori et al. (2000). The first described syntrophic

propionate-oxidizing bacterium is Syntrophobacter wolinii, followed by two other

Syntrophobacter species. Syntrophus aciditrophicus, isolated by Jackson et al. (1999), is a

universal syntroph oxidizing fatty acids and benzoate. Smithella propionica, which was

isolated by Liu et al. (1999) is an organism that produces much less acetate from

propionate than the Syntrophobacter strains, and besides acetate it produces small amount

of butyrate. Thermophilic propionate- oxidizing bacteria have also been described, and two

of these have been obtained in pure culture so far: Pelotomaculum thermopropionicum

strain SI, and Desulfotomaculum thermobenzoicum, subsp. Thermosintrophicum. Finally,

Sekiguchi et al. (2000) isolated a thermophilic butyrate-oxydizer capable of oxidizing

saturated fatty acids with four to ten carbon atoms.

3.3 Methanogenic microorganisms;

The main route of methane production is through a syntrophic relationship between

acetate-oxidizing bacteria and hydrogen-utilizing methanogenic Archea. The acetoclastic

and hydrogenotrophic methanogens contribute 70% and 30%, respectively, to the methane

production in industrial wastewater treatment.

Numerous methanogens have been isolated and described so far, but the studies, mainly

based on 16S rDNA cloning analyses, suggest that the most commonly found methanogens

genera, in the biogas reactors, are Methanobacterium, Methanothermobacter (formerly

Methanobacterium), Methanobrevibacter, Methanosarcina, and Methanosaeta (formerly

Methanotrix) as referenced in Archives of Env. Protection.

Among the acetoclastic methanogenic organisms, Methanosarcina and Methanosaeta

species has been reported to be dominated in large-scale mesophilic and thermophilic

digesters treating wastewater and sewage sludge. Its dominance comes mainly due to its

wide tolerance for environmental factors such as nutrients and temperature (Palmisano &

Barlaz 1996).

3.4 Interactions between different microbial consortia in the AD

reactors

As mentioned previously the anaerobic methanogenesis is a process that evolves at least

four different groups of anaerobic microorganisms. Each group contains diverse

microorganisms responsible for different metabolic tasks. Distinguishing characteristic of

this anaerobic consortium is that different species of anaerobic microorganisms degrade

one organic compound interactively, sharing energy and carbon sources from the

compound (Sekiguchi et al. 2001).

These organisms have developed specific kind of interdependent relationship called

syntrophy, special kind of symbiotic cooperation of mutual dependence of the partner

bacteria with respect to energy limitation where neither partner can exist without the

other and together they exhibit a metabolic activity that neither one could accomplish on

its own. In this unique cooperation between two metabolically different types of

microorganisms they depend on each other for degradation of a certain substrate for

energetic reasons (Schink 1997).

This unique cooperation between the MOs involved in the methanogenesis has evolved due

to the need to utilize the energy obtained from the electron donor substrate more

efficiently. The overall reaction anaerobic degradation is a reaction with very low energy

yield comparing to the aerobic degradation. The main reason is that the electron acceptor

in this case is the carbon dioxide and not oxygen like in the aerobic degradation. Carbon in

the carbon dioxide is in the most highly oxidized state with a COD: C ratio of z