Acidogenesis
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2008-07-03T20:54:56Z
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'''Acidogenesis''' represents the second stage in the four stages of [[anaerobic digestion]]:
* [[Hydrolysis]]: A chemical reaction where [[particulate]]s are solubilized and large [[polymer]]s converted into simpler [[monomer]]s;
* Acidogenesis: A biological reaction where simple monomers are converted into volatile [[fatty acid]]s;
* [[Acetogenesis]]: A biological reaction where volatile fatty acids are converted into [[acetic acid]], [[carbon dioxide]], and [[hydrogen]]; and
* [[Methanogenesis]]: A biological reaction where [[acetate]]s are converted into [[methane]] and carbon dioxide, while hydrogen is consumed.
Anaerobic digestion is a complex [[Biochemistry|biochemical]] process of [[Biology|biologically]]-mediated reactions by a consortium of [[microorganism]]s to convert [[organic compound]]s into methane and carbon dioxide. It is a stabilization process, reducing odor, [[pathogen]]s, and mass [[Redox|reduction]].
[[Hydrolysis|Hydrolytic]] [[bacteria]] form a variety of [[Redox|reduced]] end-products from the [[Fermentation (biochemistry)|fermentation]] of a given [[Substrate (biology)|substrate]]. One fundamental question which arises, concerns the [[Metabolism|metabolic]] features which control [[carbon]] and [[electron]] flow to a given reduced end-product during [[pure culture]], and mixed [[Methanogenesis|methanogenic]] [[Microbiological culture|cultures]] of hydrolytic bacteria. ''[[Thermoanaerobium brockii]]'' is a representative [[Thermophile|thermophilic]], hydrolytic bacterium, which ferments [[glucose]], via the [[Embden-Meyerhof pathway|Embden-Meyerhof Parnas Pathway]]. ''T. brockii'' is an atypical hetero-[[lactic acid]] bacterium because it forms molecular hydrogen ([[H2]]), in addition to lactic acid and [[ethanol]]. The reduced end-products of glucose fermentation are [[Enzyme|enzymatically]]-formed from [[pyruvate]], via the following mechanisms: lactate by [[Fructose bisphosphatase|fructose 1-6]] all-[[phosphate]] ([[fructose-6-phosphate|F6P]]) activated [[lactate dehydrogenase]]; H2 by pyruvate [[ferredoxin]] [[oxidoreductase]] and [[hydrogenase]]; and ethanol via [[NADH]]- and [[NADPH]]-linked [[alcohol dehydrogenase]] <ref>Marchaim, U. (1992). FAO Agricultural Services Bulletin – 95: [[Biogas]] process for sustainable development, FAO – Fodd and Agriculture Organization of the United Nations, ISBN 92 – 5 – 103126, http://www.fao.org, (1/9/2003)</ref> .
By its side, the acidogenic activity was found in the early 20th century, but it was not until mid-[['60s]] that the engineering of phases separation was assumed in order to improve the stability and [[Waste treatment|waste digesters treatment]] <ref>Alexiou, I.E. and Panter, K. (2004). A review of two phase applications to define best practice for the treatment of various waste streams. Anaerobic Digestion 10th World Congress, Sept. 2004. Montreal, Quebec, Canada</ref>. In this phase, complex [[molecule]]s ([[carbohydrate]]s, [[lipid]]s, [[protein]]s) are [[depolymerization|depolymerized]] into soluble compounds by hydrolytic enzymes ([[cellulase]]s, [[hemicellulase]]s, [[amylase]]s, [[lipase]]s and [[protease]]s). The hydrolyzed compounds are fermented into volatile fatty acids (acetate, [[propionate]], [[butyrate]], and [[lactic acid|lactate]]), neutral compounds (ethanol, [[methanol]]), [[ammonia]], hydrogen and carbon dioxide <ref>Cairó, J.J. and París, J.M. (1988). Microbiología de la digestión anaerobia, metanogénesis. 4o Seminario de Depuración Anaerobia de Aguas Residuales. Valladolid. F.F. Polanco, P.A. García y S. Hernándo. (Eds.) pp. 41-51</ref> <ref>Dinopolou, G., Rudd, T. and Lester, J.N. (1987). Anaerobic acidogenesis of a complex wastewater: I. The influence of operational parameters on reactor performance. Biotech. And Bioeng. 31: 958 – 968</ref> <ref>Laroche, M. (1983). Metabolisme intermediaire des acides gras volatils en fermentation methanique. These de Docteur – Ingenieur en Sciences Alimentaires_Fermentations. Institut National de la Recherche Agronomique, France</ref>
[[Acetogenesis]] is one of the main reactions of this stage, in this, the intermediary [[metabolite]]s produced are metabolized to acetate, hydrogen and carbonic gas by the three main groups of bacteria:
* [[homoacetogen]]s;
* [[syntrophe]]s; and
* [[sulphoreductor]]s.
For the [[acetic acid]] production are considered three kind of bacteria:
* ''[[Clostridium aceticum]]'';
* ''[[Acetobacter woodii]]''; and
* ''[[Clostridium termoautotrophicum]]''.
Winter y Wolfe, in 1979, demonstrated that ''A. wodii'' in syntrophic association with ''[[Methanosarcina]]'' produce methane and carbon dioxide from [[fructose]], instead of three molecules of acetate <ref>Winter, J.U. and Wolfe, R.S. (1979). Complete degradation of carbohydrates to CO<sub>2</sub> and methane by syntrophic cultures of Acetobacterium woodii y Methanosarcina barkeri. Arch. Microbiol. 121: 97 – 102</ref>. ''[[C. thermoaceticum]]'' and ''[[C. formiaceticum]]'' are able to reduce the carbonic gas to acetate, but they do not have [[hydrogenase]]s which inhabilite the hydrogen use, so they can produce three molecules of acetate from fructose. Acetic acid is equally a co-[[metabolite]] of the organic substrates fermentation ([[sugars]], [[glycerol]], [[lactic acid]], etc.) by diverse groups of microorganisms which produce different acids: *[[Propionate|propionic]] bacteria ([[propionate]] + acetate);
* ''[[Clostridium]]'' (butyrate + acetate);
* [[Enterobacteria]] (acetate + lactate); and
* Hetero-fermentative bacteria (acetate, propionate, butyrate, [[valerate]], etc.).
==References==
<references/>
[[Category:Anaerobic digestion]]
[[Category:Bacteriology]]
[[Category:Microbiology]]
[[cs:Acidogeneze]]
[[de:Acidogenese]]
[[fr:Acidogénèse]]