Acidogenesis
Encyclopedia
Acidogenesis represents the second stage in the four stages of anaerobic digestion
:
Anaerobic digestion is a complex biochemical
process of 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 reduction
.
Hydrolytic
bacteria
form a variety of reduced
end-products from the fermentation
of a given substrate
. One fundamental question that arises concerns the metabolic
features that control carbon
and electron
flow to a given reduced end-product during pure culture and mixed methanogenic
cultures
of hydrolytic bacteria. Thermoanaerobium brockii is a representative thermophilic
, hydrolytic bacterium, which ferments glucose
, via the 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 enzymatically
formed from pyruvate, via the following mechanisms: lactate by fructose 1-6 all-phosphate
(F6P
) activated lactate dehydrogenase
; H2 by pyruvate ferredoxin
oxidoreductase
and hydrogenase
; and ethanol via NADH- and NADPH-linked alcohol dehydrogenase
.
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 digesters treatment
. In this phase, complex molecule
s (carbohydrate
s, lipid
s, and protein
s) are depolymerized
into soluble compounds by hydrolytic enzymes (cellulase
s, hemicellulases, amylase
s, lipase
s and protease
s). The hydrolyzed compounds are fermented into volatile fatty acids (acetate, propionate
, butyrate
, and lactate
), neutral compounds (ethanol, methanol
), ammonia
, hydrogen and carbon dioxide.
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:
For the acetic acid
production are considered three kind of bacteria:
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. 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: *propionic
bacteria (propionate
+ acetate);
Anaerobic digestion
Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. It is used for industrial or domestic purposes to manage waste and/or to release energy....
:
- HydrolysisHydrolysisHydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
: A chemical reaction where particulates are solubilized and large polymerPolymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s converted into simpler monomerMonomerA monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
s; - Acidogenesis: A biological reaction where simple monomers are converted into volatile fatty acidFatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s; - AcetogenesisAcetogenesisAcetogenesis is a process through which acetate is produced by anaerobic bacteria from a variety of energy and carbon sources. The different bacterial species that are capable of acetogenesis are collectively termed acetogens.-Biochemistry:The precursor to acetic acid is the thioester acetyl CoA...
: A biological reaction where volatile fatty acids are converted into acetic acidAcetic acidAcetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
, carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, and hydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
; and - MethanogenesisMethanogenesisMethanogenesis or biomethanation is the formation of methane by microbes known as methanogens. Organisms capable of producing methane have been identified only from the domain Archaea, a group phylogenetically distinct from both eukaryotes and bacteria, although many live in close association with...
: A biological reaction where acetateAcetateAn acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
s are converted into methaneMethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
and carbon dioxide, while hydrogen is consumed.
Anaerobic digestion is a complex biochemical
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
process of biologically
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
mediated reactions by a consortium of microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
s to convert organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s into methane and carbon dioxide. It is a stabilization process, reducing odor, pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s, and mass reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
.
Hydrolytic
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
form a variety of reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
end-products from the fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
of a given substrate
Substrate (biology)
In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
. One fundamental question that arises concerns the metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
features that control carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
flow to a given reduced end-product during pure culture and mixed methanogenic
Methanogenesis
Methanogenesis or biomethanation is the formation of methane by microbes known as methanogens. Organisms capable of producing methane have been identified only from the domain Archaea, a group phylogenetically distinct from both eukaryotes and bacteria, although many live in close association with...
cultures
Microbiological culture
A microbiological culture, or microbial culture, is a method of multiplying microbial organisms by letting them reproduce in predetermined culture media under controlled laboratory conditions. Microbial cultures are used to determine the type of organism, its abundance in the sample being tested,...
of hydrolytic bacteria. Thermoanaerobium brockii is a representative thermophilic
Thermophile
A thermophile is an organism — a type of extremophile — that thrives at relatively high temperatures, between 45 and 122 °C . Many thermophiles are archaea...
, hydrolytic bacterium, which ferments glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, via the Embden-Meyerhof Parnas Pathway. T. brockii is an atypical hetero-lactic acid
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
bacterium because it forms molecular hydrogen (H2), in addition to lactic acid and ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
. The reduced end-products of glucose fermentation are enzymatically
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
formed from pyruvate, via the following mechanisms: lactate by fructose 1-6 all-phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
(F6P
Fructose 6-phosphate
Fructose 6-phosphate is fructose sugar phosphorylated on carbon 6 . The β-D-form of this compound is very common in cells. The vast majority of glucose and fructose entering a cell will become converted to this at some point...
) activated lactate dehydrogenase
Lactate dehydrogenase
Lactate dehydrogenase is an enzyme present in a wide variety of organisms, including plants and animals.Lactate dehydrogenases exist in four distinct enzyme classes. Two of them are cytochrome c-dependent enzymes, each acting on either D-lactate or L-lactate...
; H2 by pyruvate ferredoxin
Ferredoxin
Ferredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...
oxidoreductase
Oxidoreductase
In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule to another...
and hydrogenase
Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....
; and ethanol via NADH- and NADPH-linked alcohol dehydrogenase
Alcohol dehydrogenase
Alcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide...
.
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 digesters treatment
Waste treatment
Waste treatment refers to the activities required to ensure that waste has the least practicable impact on the environment. In many countries various forms of waste treatment are required by law.-Solid waste treatment:...
. In this phase, complex molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s (carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
s, lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s, and protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s) are depolymerized
Depolymerization
Depolymerization is the process of converting a polymer into a monomer or a mixture of monomers.Thioglycolysis, thiolysis and phloroglucinolysis are reactions used to study condensed tannins by means of their depolymerisation. Thioglycolysis is also used to study lignin....
into soluble compounds by hydrolytic enzymes (cellulase
Cellulase
400px|thumb|right|alt = Colored dice with checkered background|Ribbon representation of the Streptomyces lividans beta-1,4-endoglucanase catalytic domain - an example from the family 12 glycoside hydrolases...
s, hemicellulases, amylase
Amylase
Amylase is an enzyme that catalyses the breakdown of starch into sugars. Amylase is present in human saliva, where it begins the chemical process of digestion. Food that contains much starch but little sugar, such as rice and potato, taste slightly sweet as they are chewed because amylase turns...
s, lipase
Lipase
A lipase is an enzyme that catalyzes the formation or cleavage of fats . Lipases are a subclass of the esterases.Lipases perform essential roles in the digestion, transport and processing of dietary lipids in most, if not all, living organisms...
s and protease
Protease
A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....
s). The hydrolyzed compounds are fermented into volatile fatty acids (acetate, propionate
Propionate
The propanoate or propionate ion is C2H5COO− .A propanoic or propionic compound is a salt or ester of propanoic acid....
, butyrate
Butyrate
Butyrate is the traditional name for the conjugate base of butanoic acid . The formula of the butanoate ion is C4H7O2-. The archaic name is used as part of the name of butyrates or butanoates, or esters and salts of butyric acid, a short chain fatty acid...
, and lactate
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
), neutral compounds (ethanol, methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
), ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, hydrogen and carbon dioxide.
Acetogenesis
Acetogenesis
Acetogenesis is a process through which acetate is produced by anaerobic bacteria from a variety of energy and carbon sources. The different bacterial species that are capable of acetogenesis are collectively termed acetogens.-Biochemistry:The precursor to acetic acid is the thioester acetyl CoA...
is one of the main reactions of this stage, in this, the intermediary metabolite
Metabolite
Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...
s produced are metabolized to acetate, hydrogen and carbonic gas by the three main groups of bacteria:
- homoacetogens;
- syntrophes; and
- sulphoreductors.
For the acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
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
Methanosarcina
Methanosarcina are the only known anaerobic methanogens to produce methane using all three known metabolic pathways for methanogenesis. Most methanogens make methane from carbon dioxide and hydrogen gas. Some others utilize acetate in the acetoclastic pathway...
produce methane and carbon dioxide from fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
, instead of three molecules of acetate. C. thermoaceticum and C. formiaceticum are able to reduce the carbonic gas to acetate, but they do not have hydrogenase
Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....
s which inhabilite the hydrogen use, so they can produce three molecules of acetate from fructose. Acetic acid is equally a co-metabolite
Metabolite
Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...
of the organic substrates fermentation (sugars, glycerol
Glycerol
Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...
, lactic acid
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
, etc.) by diverse groups of microorganisms which produce different acids: *propionic
Propionate
The propanoate or propionate ion is C2H5COO− .A propanoic or propionic compound is a salt or ester of propanoic acid....
bacteria (propionate
Propionate
The propanoate or propionate ion is C2H5COO− .A propanoic or propionic compound is a salt or ester of propanoic acid....
+ acetate);
- ClostridiumClostridiumClostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster or spindle...
(butyrate + acetate); - Enterobacteria (acetate + lactate); and
- Hetero-fermentative bacteria (acetate, propionate, butyrate, valerateValerateA valerate is a salt or ester of valeric acid. The valerate ion is C4H9COO− .Many steroid-based pharmaceuticals, for example ones based on betamethasone or hydrocortisone, include the steroid as the valerate ester.-Examples:* Sodium valerate, Na* Methyl valerate, CH3...
, etc.).