Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 20

Acetogenesis is the third phase of anaerobic digestion in which simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen.

The acid-producing bacteria, involved in the second step, convert the intermediates of fermenting bacteria into acetic acid (CH₃COOH), hydrogen (H₂) and carbon dioxide (CO₂). These bacteria are facultatively anaerobic and can grow under acid conditions. To produce acetic acid, the bacteria need oxygen and carbon. For this, they use the oxygen solved in the solution or bounded-oxygen whereby the acid-producing bacteria create an anaerobic condition which is essential for the methane producing microorganisms. Moreover, these bacteria reduce the compounds with a low molecular weight into alcohols, organic acids, amino acids, carbon dioxide, hydrogen sulfide and traces of methane.

From a chemical standpoint, this process is partially endergonic (i.e., only possible with energy input), since bacteria alone are not capable of sustaining that type of reaction. An endergonic reaction is a chemical reaction in which total amount of energy is a loss – it takes more energy to initiate the reaction than the energy produced by the reaction and, thus, the total energy is a negative net result.

An acetogen is a microorganism that generates acetate (CH₃COO⁻) as an end product of anaerobic respiration (fermentation) and can produce, in most cases, acetate as the end product) from two molecules of carbon dioxide (CO₂) and four molecules of molecular hydrogen (H₂). This process (acetogenesis) is different from acetate fermentation, although both occur in the absence of molecular oxygen (O₂) and produce acetate.

Acetogens are found in a variety of habitats, generally those that are anaerobic (lack oxygen). Thus, acetogenesis is a process through which acetate is produced from carbon dioxide and an electron source (such as hydrogen and carbon monoxide) by anaerobic bacteria. In this reaction, carbon dioxide is reduced to carbon monoxide and formic acid (HCO₂H) or directly into a formyl group, the formyl group is reduced to a methyl group and then combined with the carbon monoxide and Coenzyme A produce acetyl-CoA. Two specific enzymes participate on the carbon monoxide side of the pathway: (i) CO-dehydrogenase, which catalyzes the reduction of the carbon dioxide and (ii) acetyl CoA synthase, which combines the resulting carbon monoxide with a methyl group to give acetyl-CoA.

The key aspects of the acetogenic pathway are several reactions that include the reduction of carbon dioxide to carbon monoxide and the attachment of the carbon monoxide to a methyl group. The first process is catalyzed by specific enzymes (carbon monoxide dehydrogenase enzymes) and the coupling of the methyl group (provided by methylcobalamin) and the carbon monoxide is catalyzed by acetyl CoA synthetase.

The accumulation of hydrogen can inhibit the metabolism of the acetogenic bacteria and present knowledge suggests that hydrogen may be a limiting feedstock for methanogens. This assumption is based on the fact that addition of hydrogen-producing bacteria to the natural biogas-producing consortium increases the daily biogas production. At the end of the degradation chain, two groups of methanogenic bacteria produce methane from acetate or hydrogen and carbon dioxide. These bacteria are strict anaerobes and require a lower redox potential for growth than most other anaerobic bacteria.

See also: Acidogenesis, Anaerobic Digestion, Methanogenesis.