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Microbial Metabolism > Anaerobic Respiration
Anaerobic Respiration
• Electron Donors and Acceptors in Anaerobic Respiration
• Nitrate Reduction and Denitrification
• Sulfate and Sulfur Reduction
• Methanogenesis
• Proton Reduction
• Anoxic Hydrocarbon Oxidation
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Microbial Metabolism > Anaerobic Respiration
Electron Donors and Acceptors in Anaerobic Respiration
• Both inorganic and organic compounds may be used as electron acceptors in
anaerobic respiration. Inorganic compounds include sulfate (SO42-), nitrate (NO3), and ferric iron (Fe3+). Organic compounds include DMSO.
• These molecules have a lower reduction potential than oxygen. Therefore, less
energy is formed per molecule of glucose in anaerobic versus aerobic conditions.
• The reduction of certain inorganic compounds by anaerobic microbes is often
ecologically significant.
Anaerobic Respiration
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Microbial Metabolism > Anaerobic Respiration
Nitrate Reduction and Denitrification
• Denitrification generally proceeds through a stepwise reduction of some
combination of the following intermediate forms: NO3− → NO2− → NO + N2O →
N2.
• Generally, several species of bacteria are involved in the complete reduction of
nitrate to molecular nitrogen, and more than one enzymatic pathway has been
identified in the reduction process.
• Complete denitrification is an environmentally significant process as some
intermediates of denitrification (nitric oxide and nitrous oxide) are significant
greenhouse gases that react with sunlight and ozone to produce nitric acid, a
component of acid rain.
The role of soil bacteria in the Nitrogen cycle
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Microbial Metabolism > Anaerobic Respiration
Sulfate and Sulfur Reduction
• Sulfate reduction is a vital mechanism for bacteria and archaea living in oxygendepleted, sulfate-rich environments.
• Sulfate reducers may be organotrophic, using carbon compounds, such as lactate
and pyruvate as electron donors, or lithotrophic, and use hydrogen gas (H2) as an
electron donor.
• Before sulfate can be used as an electron acceptor, it must be activated by ATPsulfurylase, which uses ATP and sulfate to create adenosine 5'-phosphosulfate
(APS).
• Sulfate-reducing bacteria can be traced back to 3.5 billion years ago and are
considered to be among the oldest forms of microorganisms, having contributed
Black sludge
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to the sulfur cycle soon after life emerged on Earth.
• Toxic hydrogen sulfide is one waste product of sulfate-reducing bactera, and is
the source of the rotten egg odor.
• Sulfate-reducing bacteria may be utilized for cleaning up contaminated soils.
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Microbial Metabolism > Anaerobic Respiration
Methanogenesis
• Carbon dioxide or acetic acid are the most commonly used electron acceptor in
methanogenesis.
• Microbes capable of producing methane are called methanogens. They have
been identified only from the domain Archaea - a group that is phylogenetically
distinct from eukaryotes and bacteria.
• The production of methane is an important and widespread form of microbial
metabolism. In most environments, it is the final step in the decomposition of
biomass.
• Methane is a major greenhouse gas. The average cow emits around 250 liters of
methane a day as a result of the the breakdown of cellulose by methanogens.
Methanogenesis of acetate
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Therefore, the large scale raising of cattle for meat is a considerable contributor to
global warming.
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Microbial Metabolism > Anaerobic Respiration
Proton Reduction
• In denitrification, protons are transported across the membrane by the initial
NADH reductase, quinones, and nitrous oxide reductase to produce the
electrochemical gradient critical for respiration.
• An electrochemical gradient represents one of the many interchangeable forms of
potential energy through which energy may be conserved. In biological
processes, the direction an ion moves by diffusion or active transport across a
membrane is determined by the electrochemical gradient.
• In mitochondria and chloroplasts, proton gradients are used to generate a
chemiosmotic potential that is also known as a proton motive force.
The Basics of Redox
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Microbial Metabolism > Anaerobic Respiration
Anoxic Hydrocarbon Oxidation
• Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon.
• The majority of hydrocarbons occur naturally in crude oil, where decomposed
organic matter provides an abundance of carbon and hydrogen. The combustion
of hydrocarbons is the primary energy source for current civilizations.
• Anaerobic oxidation of methane (AOM) is a microbial process that occurs in
anoxic marine sediments. AOM is considered to be a very important process,
reducing the emission of methane (a greenhouse gas) from the ocean into the
atmosphere by up to 90%.
Contaminated soil
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Appendix
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Microbial Metabolism
Key terms
• anaerobic Without oxygen; especially of an environment or organism.
• anaerobic respiration metabolic reactions and processes that take place in the cells of organisms that use electron acceptors
other than oxygen
• anoxic Lacking oxygen.
• cofactor A substance, especially a coenzyme or a metal, that must be present for an enzyme to function.
• electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is
an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process.
• eutrophication The process of becoming eutrophic.
• facultative Not obligate; optional, discretionary or elective
• fermentation Any of many anaerobic biochemical reactions in which an enzyme (or several enzymes produced by a
microorganism) catalyses the conversion of one substance into another; especially the conversion (using yeast) of sugars to
alcohol or acetic acid with the evolution of carbon dioxide.
• lithotrophic Obtains electrons for respiration from inorganic substrates.
• methanethiol A colourless gas, a thiol with a smell like rotten cabbage, found naturally in plants and animals.
• methanotrophic The ability to metabolize methane as an only source of carbon and energy.
• organotrophic Obtains electrons for respiration from organic substrates.
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Microbial Metabolism
• phosphorylation The process of transferring a phosphate group from a donor to an acceptor; often catalysed by enzymes
• reduction A reaction in which electrons are gained and valence is reduced; often by the removal of oxygen or the addition of
hydrogen.
• syntrophic When one species lives off the products of another species.
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Microbial Metabolism
Contaminated soil
Microbes may be used to degrade toxic hydrocarbons in anaerobic environments.
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Microbial Metabolism
Methanogenesis of acetate
Acetate is broken down to methane by methanogenesis, a type of anaerobic respiration.
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Microbial Metabolism
Anaerobic Respiration
A molecule other than oxygen is used as the terminal electron acceptor in anaerobic respiration.
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Microbial Metabolism
Black sludge
The black color of this pond is due to metal sulfides that result from the action of sulfate-reducing bacteria.
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Microbial Metabolism
The Basics of Redox
In every redox reaction you have two halves: reduction and oxidation.
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Microbial Metabolism
The role of soil bacteria in the Nitrogen cycle
Denitrification is an important process in maintaining ecosystems. Generally, denitrification takes place in environments depleted of oxygen.
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Microbial Metabolism
Under anaerobic conditions, there is a lack of oxygen thus, the
inability to use oxygen as the terminal electron acceptor. Different
types of electron acceptors and donors that can be used for
anaerobic respiration and ATP formation include:
A) uranyl, ferric iron and nitrous oxide
B) arsenite, hydrogen sulfide and toluene
C) ferric iron, sulfate and phosphite
D) fumarate, manganous and uranium
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Microbial Metabolism
Under anaerobic conditions, there is a lack of oxygen thus, the
inability to use oxygen as the terminal electron acceptor. Different
types of electron acceptors and donors that can be used for
anaerobic respiration and ATP formation include:
A) uranyl, ferric iron and nitrous oxide
B) arsenite, hydrogen sulfide and toluene
C) ferric iron, sulfate and phosphite
D) fumarate, manganous and uranium
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Microbial Metabolism
The process of denitrification involves the reduction of nitrate. The
steps, in order, following nitrate reduction are:
A) formation of nitric oxide -> nitrous oxide -> nitrite -> dinitrogen
B) formation of nitrous oxide -> nitric oxide -> nitrite -> dinitrogen
C) formation of nitrite -> nitric oxide -> nitrous oxide -> dinitrogen
D) formation of nitrite -> dinitrogen -> nitric oxide -> nitrous oxide
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Microbial Metabolism
The process of denitrification involves the reduction of nitrate. The
steps, in order, following nitrate reduction are:
A) formation of nitric oxide -> nitrous oxide -> nitrite -> dinitrogen
B) formation of nitrous oxide -> nitric oxide -> nitrite -> dinitrogen
C) formation of nitrite -> nitric oxide -> nitrous oxide -> dinitrogen
D) formation of nitrite -> dinitrogen -> nitric oxide -> nitrous oxide
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Microbial Metabolism
In an anaerobic soil environment, Rhizobia are a unique soil
bacteria which can switch from O2-respiration to N2-fixation. The
ability to perform N2-fixation is important when:
A) complete denitrification does not occur and there is production of acid
rain
B) there is an increase in plant roots and an excess amount of nitrogen
C) oxygen consumption exceeds oxygen supply
D) there is contamination of the soil by sewage, thus, increases in
nitrogen
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Microbial Metabolism
In an anaerobic soil environment, Rhizobia are a unique soil
bacteria which can switch from O2-respiration to N2-fixation. The
ability to perform N2-fixation is important when:
A) complete denitrification does not occur and there is production of acid
rain
B) there is an increase in plant roots and an excess amount of nitrogen
C) oxygen consumption exceeds oxygen supply
D) there is contamination of the soil by sewage, thus, increases in
nitrogen
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Microbial Metabolism
Which of the following are steps involved in sulfate reduction?
A) AMP is turned into ADP using ATP
B) Sulfate is activated by ATP-sulfurylase to create adenosine 5'phosphosulfate (APS)
C) Adenosine 5'-phosphosulfate (APS) is reduced by APS reductase to
form sulfite and AMP
D) All of the choices
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Microbial Metabolism
Which of the following are steps involved in sulfate reduction?
A) AMP is turned into ADP using ATP
B) Sulfate is activated by ATP-sulfurylase to create adenosine 5'phosphosulfate (APS)
C) Adenosine 5'-phosphosulfate (APS) is reduced by APS reductase to
form sulfite and AMP
D) All of the choices
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Microbial Metabolism
If an anaerobic environment suddenly halts the process of
methanogenesis, what would be the effect?
A) Removal of hydrogen, small organics and carbon dioxide would occur
B) There would be increased decomposition of biomass
C) Methane would continue to accumulate without being broken down
D) Accumulation of carbon in the form of fermentation products
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Microbial Metabolism
If an anaerobic environment suddenly halts the process of
methanogenesis, what would be the effect?
A) Removal of hydrogen, small organics and carbon dioxide would occur
B) There would be increased decomposition of biomass
C) Methane would continue to accumulate without being broken down
D) Accumulation of carbon in the form of fermentation products
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Microbial Metabolism
In denitrification, NADH reductase, quinones and nitrous oxide
reductase play major roles in preparing for respiration. This is
done by:
A) driving ATP synthesis by maintaining the reduction of oxgyen
B) regulating substrate-level phosphorylation for ATP production
C) pumping protons across the membrane to establish an
electrochemical gradient
D) translocating protons across the membrane via sulfur oxidation
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Microbial Metabolism
In denitrification, NADH reductase, quinones and nitrous oxide
reductase play major roles in preparing for respiration. This is
done by:
A) driving ATP synthesis by maintaining the reduction of oxgyen
B) regulating substrate-level phosphorylation for ATP production
C) pumping protons across the membrane to establish an
electrochemical gradient
D) translocating protons across the membrane via sulfur oxidation
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Microbial Metabolism
In a case of a crude oil spill into a marine environment, which of
the following would be considered an innate mechanism that
could aid in the elimination process of crude oil?
A) The presence of anoxic marine sediments that carry out anaerobic
oxidation of methane
B) The presence of microbial communities that are majority anaerobic
hydrocarbon-degrading bacteria
C) The presence of microbial communites that are majority sulfatereducing bacteria
D) The presence of microbial communites that work in syntrophy
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Microbial Metabolism
In a case of a crude oil spill into a marine environment, which of
the following would be considered an innate mechanism that
could aid in the elimination process of crude oil?
A) The presence of anoxic marine sediments that carry out anaerobic
oxidation of methane
B) The presence of microbial communities that are majority anaerobic
hydrocarbon-degrading bacteria
C) The presence of microbial communites that are majority sulfatereducing bacteria
D) The presence of microbial communites that work in syntrophy
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Microbial Metabolism
Attribution
• Wikipedia. "Microbial metabolism." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Microbial_metabolism
• Wikipedia. "Sulfate-reducing bacteria." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Sulfate-reducing_bacteria
• Wikipedia. "lithotrophic." CC BY-SA 3.0 http://en.wikipedia.org/wiki/lithotrophic
• Wikipedia. "organotrophic." CC BY-SA 3.0 http://en.wikipedia.org/wiki/organotrophic
• Wikipedia. "Redox." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Redox
• Wikipedia. "Redox." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Redox
• Wikipedia. "Electrochemical gradient." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Electrochemical_gradient
• Wikipedia. "Anaerobic respiration." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Anaerobic_respiration
• Wikipedia. "Microbial metabolism." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Microbial_metabolism
• Wikipedia. "Microbial metabolism." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Microbial_metabolism
• Wiktionary. "phosphorylation." CC BY-SA 3.0 http://en.wiktionary.org/wiki/phosphorylation
• Wikipedia. "Geobacter." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Geobacter
• Wikipedia. "Hydrocarbon." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Hydrocarbon
• Wikipedia. "Anaerobic oxidation of methane." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Anaerobic_oxidation_of_methane
• Wikipedia. "Anoxic." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Anoxic
• Wikipedia. "Microbial biodegradation." CC BY-SA 3.0
http://en.wikipedia.org/wiki/Microbial_biodegradation#Anaerobic_biodegradation_of_pollutants
• Wikipedia. "methanotrophic." CC BY-SA 3.0 http://en.wikipedia.org/wiki/methanotrophic
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Microbial Metabolism
• Wikipedia. "syntrophic." CC BY-SA 3.0 http://en.wikipedia.org/wiki/syntrophic
• Wiktionary. "anoxic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/anoxic
• Wikipedia. "Sulfate-reducing bacteria." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Sulfate-reducing_bacteria
• Wikipedia. "Anaerobic respiration." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Anaerobic_respiration
• Wikipedia. "Methanogenesis." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Methanogenesis
• Wikipedia. "Microbial metabolism." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Microbial_metabolism
• Wiktionary. "fermentation." CC BY-SA 3.0 http://en.wiktionary.org/wiki/fermentation
• Wiktionary. "cofactor." CC BY-SA 3.0 http://en.wiktionary.org/wiki/cofactor
• Wiktionary. "methanethiol." CC BY-SA 3.0 http://en.wiktionary.org/wiki/methanethiol
• Wikipedia. "Microbial metabolism." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Microbial_metabolism
• Wikipedia. "Denitrification." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Denitrification
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/eutrophication
• Wiktionary. "facultative." CC BY-SA 3.0 http://en.wiktionary.org/wiki/facultative
• Wikipedia. "electron acceptor." CC BY-SA 3.0 http://en.wikipedia.org/wiki/electron%20acceptor
• Wikipedia. "Anaerobic respiration." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Anaerobic_respiration
• Wikipedia. "Microbial metabolism." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Microbial_metabolism
• Wikibooks. "Structural Biochemistry/Anaerobic Respiration (Fermentation)." CC BY-SA 3.0
http://en.wikibooks.org/wiki/Structural_Biochemistry/Anaerobic_Respiration_(Fermentation)
• Wikipedia. "anaerobic respiration." CC BY-SA 3.0 http://en.wikipedia.org/wiki/anaerobic%20respiration
• Wiktionary. "reduction." CC BY-SA 3.0 http://en.wiktionary.org/wiki/reduction
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Microbial Metabolism
• Wiktionary. "anaerobic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/anaerobic
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