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Source of hydrogen in reduction of oxygen in electron transport?

Source of hydrogen in reduction of oxygen in electron transport?


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What's the source of hydrogen in the reduction of oxygen to water in metabolism?

Is this implying that half the hydrogen comes from NADH and the other half from lone protons?


The equation you've shown is similar to other cellular respiration equations that I've seen, in that, it is trying to simplify a pretty complex process.

My guess is that you're doing first year cell biology like I am, so you probably know that there is a difference between the pH of the mitochondrial matrix (7.8 - making it alkaline) and the inter membrane space (roughly that of the cytosol - usually about 7). The lower the pH of a solution, the greater the concentration of H+. My point is that the solutions on either side of the inner mitochondrial membrane both have a concentration of H+.

So even though, when NADH is oxidised, it loses a H+ to solution, thereby contributing to the pH of the mitochondrial matrix, what is the likelihood that that particular H+ will be the H+ that is part of the molecule that takes the electrons at the end of the electron transport chain? I think low.

Indirectly, NADH does contribute H+ to the overall pH of the mitochondrial matrix but it doesn't directly give the H+ to "half an oxygen gas molecule".

Check the video below which is from the publisher of the textbook my course uses.

http://www.garlandscience.com/garlandscience_resources/resource_detail.jsf?landing=student&resource_id=9780815344544_CH14_QTM02


Electron Transport Chain

The electron transport chain is a crucial step in oxidative phosphorylation in which electrons are transferred from electron carriers, into the proteins of the electron transport chain which then deposit the electrons onto oxygen atoms and consequently transport protons across the mitochondrial membrane. This excess of protons drives the protein complex ATP synthase, which is the final step in oxidative phosphorylation and creates ATP.


Proton Gradients in Reductive Metabolism

Biological energy is frequently stored and released by means of redox reactions, or the transfer of electrons. Reduction occurs when an oxidant gains an electron. Photosynthesis involves the reduction of carbon dioxide into sugars and the oxidation of water into molecular oxygen. The reverse reaction, respiration, oxidizes sugars (loses an electron) to produce carbon dioxide and water. As intermediate steps, the reduced carbon compounds are used to reduce nicotinamide adenine dinucleotide (NAD+), which then contributes to the creation of a proton gradient. This then drives the synthesis of adenosine triphosphate ( ATP ) and is maintained by the reduction of oxygen, or alternative receptors for anaerobic respiration. In animal cells, the mitochondria performs similar functions.

Figure: The Basics of Redox: In every redox reaction you have two halves: reduction and oxidation.

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 the mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force. This potential energy is used for the synthesis of ATP by phosphorylation. An electrochemical gradient has two components. First, the electrical component is caused by a charge difference across the lipid membrane. Second, a chemical component is caused by a differential concentration of ions across the membrane. The combination of these two factors determines the thermodynamically favorable direction for an ion&rsquos movement across a membrane. The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria, and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential or proton motive force.

In respiring bacteria under physiological conditions, ATP synthase, in general, runs in the opposite direction, creating ATP while using the proton motive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed oxidative phosphorylation. The same process takes place in the mitochondria, where ATP synthase is located in the inner mitochondrial membrane, so that F1 part sticks into the mitochondrial matrix where ATP synthesis takes place.

Cellular respiration (both aerobic and anaerobic) utilizes highly reduced species such as NADH and FADH2 to establish an electrochemical gradient (often a proton gradient) across a membrane, resulting in an electrical potential or ion concentration difference across the membrane. The reduced species are oxidized by a series of respiratory integral membrane proteins with sequentially increasing reduction potentials, the final electron acceptor being oxygen (in aerobic respiration) or another species (in anaerobic respiration). The membrane in question is the inner mitochondrial membrane in eukaryotes and the cell membrane in prokaryotes. A proton motive force or pmf drives protons down the gradient (across the membrane) through the proton channel of ATP synthase. The resulting current drives ATP synthesis from ADP and inorganic phosphate.

Proton reduction is important for setting up electrochemical gradients for anaerobic respiration. For example, 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. In organisms that use hydrogen as an energy source, hydrogen is oxidized by a membrane-bound hydrogenase causing proton pumping via electron transfer to various quinones and cytochromes. Sulfur oxidation is a two step process that occurs because energetically sulfide is a better electron donor than inorganic sulfur or thiosulfate, allowing for a greater number of protons to be translocated across the membrane.

In contrast, fermentation does not utilize an electrochemical gradient. Instead, it only uses substrate-level phosphorylation to produce ATP. The electron acceptor NAD+ is regenerated from NADH formed in oxidative steps of the fermentation pathway by the reduction of oxidized compounds. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. For example, in homofermentative lactic acid bacteria, NADH formed during the oxidation of glyceraldehyde-3-phosphate is oxidized back to NAD+ by the reduction of pyruvate to lactic acid at a later stage in the pathway. In yeast, acetaldehyde is reduced to ethanol.


Formation of Reactive Oxygen Species and Cellular Damage

Reactive oxygen species (ROS) are molecules containing an oxygen atom with an unpaired electron in its outer shell. As ROS are formed, they become very unstable due to the unpaired electron now residing in the outermost shell. The unstable forms of oxygen are sometimes called free radicals.

How do ROS actually get generated in cells? One way is via cellular respiration driven by the electron transport chain in the mitochondria. The electron transport chain is responsible for generating ATP, the main source of energy for a cell to function. A key molecule that helps “jump start” the electron transport chain, is NADH (or nicotinamide adenine dinucleotide), which serves as the electron donor (i.e., the H in the NADH). NADH is often referred to as a “coenzyme”, even though it is not an enzyme (a protein).

NADH is present in all cells–it is generated by many biochemical reactions. One way that NADH gets generated in large quantities is when alcohol is metabolized (or oxidized) to form acetaldehyde and then to acetic acid. During the metabolism of alcohol, the enzyme alcohol dehydrogenase (ADH) and NAD + convert alcohol to acetylaldehyde, generating NADH. A second enzyme, aldehyde dehydrogenase (ALDH) and NAD + convert acetaldehyde to acetic acid, generating even more NADH. In these reactions, the coenzyme NAD + is reduced to NADH (and alcohol and acetaldehyde are oxidized).

Review the oxidation of alcohol by alcohol dehydrogenase (ADH)

To learn more about the oxidation of alcohol by ADH, you can participate in a virtual reality game called “DiVE into Alcohol” at www.rise.duke.edu/dive-alcohol.

Now there is plenty of NADH available to “jump start” mitochondrial respiration. NADH moves from the cytosol into the mitochondria where it donates an electron to the electron transport chain. The electron transport chain consists of a group of proteins (and some lipids) that work together to pass electrons “down the line”. Finally in the presence of oxygen, ATP is formed, providing energy for many cellular functions.

However, some electrons can “escape” the electron transport chain and combine with oxygen to form a very unstable form of oxygen called a superoxide radical (O2•-). The superoxide radical is one of the reactive oxygen species (ROS).

The superoxide radical is a type of free radical. Free radicals have a lone electron in their outer electron orbital and they are very reactive molecules because they tend to donate single electrons (e-) or steal e- from other molecules. Free radicals can be destructive to cellular components. Free radicals often have a • shown to indicate the lone e-.

Our cells have ways to protect themselves from the damaging effects of these reactive molecules. For example, our cells are able to maintain low levels of the superoxide radicals with the help of the enzyme superoxide dismutase (SOD). SOD helps reduce superoxide to form hydrogen peroxide (H2O2), which is then converted (detoxified) by the enzyme catalase to water and O2.

However, sometimes the levels of superoxide rise, for example after alcohol exposure (which generates a lot of NADH). Thus, more hydrogen peroxide is formed and can’t be detoxified by the limited amount of catalase. Instead hydrogen peroxide becomes reduced by iron (Fe 2+ ) (normally present in cells), which donates an electron to produce the hydroxyl radical (•OH), a very nasty molecule. It is extremely reactive, and it’s a great oxidizing agent. The hydroxyl radical oxidizes cellular components such as lipids, proteins, and DNA by literally stealing an e- (associated with an H atom) from them, damaging cells.

Figure: The metabolism (i.e., oxidation) of alcohol produces NADH, which acts as an electron donor for the electron transport chain (molecules designated with roman numerals). Electrons (e-) that “leak out” of the electron transport chain (stars at I and III) combine with oxygen to produce superoxide radicals (O2•-). Through a series of reactions the superoxide radicals generate hydroxyl (OH•) radicals. Oxygen radicals are circled in red.


Complex III

The third complex is composed of cytochrome b, another Fe-S protein, Rieske center (2Fe-2S center), and cytochrome c proteins this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between different oxidation states: Fe + + (reduced) and Fe + + + (oxidized). The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes (cytochrome c is the acceptor of electrons from Q however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time).


Growth Factors

Most of the organisms are capable of producing enzymes required for biochemical pathways by the presence of nutrients however, there are several organisms that lack specific enzymes required by the microbes. Therefore, they must obtain these constituents or their precursors from the environment. Organic compounds that are essential cell components or precursors of such components but cannot be synthesized by the organism are called growth factors. There are three major types of growth factors such as Amino acids, purines, and pyrimidines and Vitamins. Some microorganisms require many vitamins for example, Enterococcus faecalis needs eight different vitamins for growth. Other growth factors are also seen heme (from hemoglobin or cytochromes) is required by Haemophilus influenzae, and some mycoplasmas need cholesterol.

Understanding the growth factor requirements of microbes has important practical applications. Both microbes with known, specific requirements and those that produce large quantities of a substance (e.g., vitamins) are useful. Microbes with a specific growth factor requirement can be used in bioassays for the factor they need. The atypical assay is a growth-response assay, which allows the amount of growth factor in a solution to be determined.


The basics of the electron transport chain

The electron transport chain consists of four protein complexes, each of which has a specific function in transferring electrons from NADH and FADH₂ to oxygen.

  • Complex I picks up the electrons from NADH and reduces it to NAD + , as well as releases four hydrogen ions (H + ) from the mitochondrial matrix into the intermembrane space. This establishes an electrochemical gradient.
  • Complex II receives FADH₂, which bypasses the first complex and delivers electrons directly into the electron transport chain. Ubiquinone(Q), a carrier that can freely travel through the membrane, receives the electrons from complexes I and II and delivers them to complex III. This complex does not pump protons into the intermembrane space.
  • Complex III pumps out four hydrogen ions into the intermembrane space. It also passes electrons to cytochrome c, another carrier with the ability to move through the membrane, which transfers them to complex IV.
  • Complex IV receives electrons from cytochrome c and transfers them to oxygen – the final acceptor in the electron transport chain. Then, oxygen picks up two hydrogen ions from the surrounding medium and forms water. Note that without oxygen, the electron transport will stop working and ATP cannot be produced aerobically.

The released hydrogen ions produce an outward current, creating electrical potential across the mitochondrial membrane. The difference in current between the two sides of the membrane creates an electrochemical gradient, also known as a proton-motive force. Then, these ions diffuse from higher to lower concentration back into the mitochondrial matrix, carrying electrical potential.

This movement of hydrogen ions powers an enzyme called ATP synthase, which phosphorylates ADP and produces ATP. But before we jump too far ahead, let’s take a closer look at what happens in each complex.

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The oxidative phase of oxidative phosphorylation Oxidizes NADH & FADH₂ Aerobic process Takes place in the mitochondria Pumps out hydrogen ions into the intermembrane space Creates a hydrogen ion gradient

Complex I

Complex I consists of flavin mononucleotide (FMN) and an iron-sulfur (Fe-S) -containing protein. FMN is derived from vitamin B₂, also known as riboflavin. It is also one of many prosthetic groups in the electron transport chain.

A prosthetic group is a non-amino acid component required for the biological function of a protein. These non-peptide molecules can be organic (such as vitamins, sugars, or lipids) or inorganic (such as metal ions). Prosthetic groups also include coenzymes, because they are prosthetic groups of enzymes.

In complex I, the catalyzing enzyme is NADH dehydrogenase – one of our largest membrane-bound proteins consisting of 46 amino acid chains. This complex has two main functions transferring electrons from NADH to ubiquinone and pumping out hydrogen ions across the mitochondrial matrix into the intermembrane space.

This process begins when FMN strips NADH from its two electrons and delivers them down a chain of iron-sulfur clusters. Then, these electrons are placed on a ubiquinone molecule which carries them to the next complex in the electron transport chain. This process releases four protons across the membrane for every molecule of NADH, which establishes and maintains an electrochemical gradient that powers ATP synthase.

Complex II & Ubiquinone

Complex II, succinate dehydrogenase, plays an interesting role in mitochondrial metabolism. It is the same enzyme that was also used in the citric acid cycle to transform succinate into fumarate in the process of producing FADH₂. Together, succinate dehydrogenase and FADH₂ form a small complex that bypasses the first complex and transfers electrons directly into the electron transport chain. Unlike complex I, complex II does not pump out hydrogen ions into the intermembrane space.

Ubiquinone (Q) is a compound that ties complexes I and II to complex III. It is lipid-soluble and can freely move through the hydrophobic core of the mitochondrial membrane. After ubiquinone is reduced to ubiquinol (QH₂), it releases its electrons to the next complex in the electron transport chain.

Ubiquinone receives electrons from two sources:

Since the electrons provided by FADH₂ bypass the first complex, they do not participate in pumping out hydrogen ions into the intermembrane space. Because the total ATP yield is directly related to the number of hydrogen ions released, fewer ATP molecules are generated from FADH₂.

Complex III

Complex III is also known as Q-cytochrome c oxidoreductase. It consists of three subunits:

  1. Cytochrome c, which contains a single prosthetic heme group.
  2. Cytochrome b, which contains two prosthetic heme groups.
  3. The Rieske center, which contains the 2Fe-2S center.

Cytochrome proteins contain a prosthetic heme group. A heme molecule is similar to hemoglobin but carries electrons instead of oxygen. This also means that the iron ion at its core is reduced and oxidized as it donates and accepts electrons. Thus, it fluctuates between oxidized (Fe 3+ ) and reduced (Fe 2+ ) states during the electron transport process. Because the heme molecules in the cytochromes are bound to different proteins, they have different characteristics. This is also what makes each complex of the electron transport chain.

Cytochrome c is a unique electron transport protein because it is not a part of a larger complex. Thus, it is free to diffuse through the inner mitochondrial membrane. Cytochrome c is the acceptor of electrons from ubiquinone. However, while Q carries electrons in pairs, cytochrome c can only accept one of them at a time. This process of transferring electrons from ubiquinol to cytochrome c is called the Q cycle.

In addition to passing electrons to cytochrome c for transport to the fourth complex, complex III also pumps out four hydrogen ions across the mitochondrial membrane.

Complex IV

Complex IV, also known as cytochrome c oxidase, is a multiunit structure that transfers electrons from cytochrome c to oxygen. It is composed of three cytochrome proteins c, a, and a3. The complex contains two heme groups (one in each cytochromes a and a3) as well as three copper ions (two CuA and one CuB in cytochrome a3).

These cytochromes hold an oxygen molecule between the iron and copper ions. After the oxygen is reduced, it picks up two hydrogen ions from the surrounding medium and produces water (H₂O).

Complex IV also pumps out two hydrogen ions out of the mitochondrial matrix into the intermembrane space, further contributing to the electrochemical gradient. This gradient is used in the process of chemiosmosis to synthesize ATP.


Electron Transport in the Energy Cycle of the Cell

The eukaryotic cell's most efficient path for production of vital ATP is the aerobic respiration that takes place in the mitochondria. After glycolysis, the pyruvate product is taken into the mitochondia and is further oxidized in the TCA cycle. This cycle deposits energy in the reduced coenzymes which transfer that energy through what is called the electron transport chain.

The energy given to the electrons of the reduced coenzyme NADH and to succinate by the TCA cycle is transferred in small steps in the inner membrane of the mitochondrion through a chain of five protein complexes. These small oxidation steps accomplish the conversion of ADP to the energy currency molecule ATP. This series of coupled reactions is often referred to as oxidative phosphorylation.

The energy used in the electron transport chain pumps protons across the inner mitochondrial membrane from the inner matrix to the intermembrane space, producing a strong hydrogen concentration gradient. This process was called chemiosmosis by its discover, Peter Mitchell. This difference in proton concentration produces both an electrical potential and a pH potential across the membranes. The protein complex ATP synthase then makes use of this membrane potential to accomplish the phosphorylation of ADP to ATP.


Contents

Anaerobic cellular respiration and fermentation generate ATP in very different ways, and the terms should not be treated as synonyms. Cellular respiration (both aerobic and anaerobic) utilizes highly reduced chemical compounds such as NADH and FADH2 (for example produced during glycolysis and the citric acid cycle) to establish an electrochemical gradient (often a proton gradient) across a membrane. This results in an electrical potential or ion concentration difference across the membrane. The reduced chemical compounds are oxidized by a series of respiratory integral membrane proteins with sequentially increasing reduction potentials, with the final electron acceptor being oxygen (in aerobic respiration) or another chemical substance (in anaerobic respiration). A proton motive force drives protons down the gradient (across the membrane) through the proton channel of ATP synthase. The resulting current drives ATP synthesis from ADP and inorganic phosphate.

Fermentation, in contrast, does not utilize an electrochemical gradient. Fermentation instead only uses substrate-level phosphorylation to produce ATP. The electron acceptor NAD + is regenerated from NADH formed in oxidative steps of the fermentation pathway by the reduction of oxidized compounds. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. For example, in homofermentative lactic acid bacteria, NADH formed during the oxidation of glyceraldehyde-3-phosphate is oxidized back to NAD + by the reduction of pyruvate to lactic acid at a later stage in the pathway. In yeast, acetaldehyde is reduced to ethanol to regenerate NAD + .

There are two important anaerobic microbial methane formation pathways, through carbon dioxide / bicarbonate (HCO3 − ) reduction (respiration) or acetate fermentation. [3]

Anaerobic respiration is a critical component of the global nitrogen, iron, sulfur, and carbon cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. The biogeochemical cycling of these compounds, which depends upon anaerobic respiration, significantly impacts the carbon cycle and global warming. Anaerobic respiration occurs in many environments, including freshwater and marine sediments, soil, subsurface aquifers, deep subsurface environments, and biofilms. Even environments, such as soil, that contain oxygen also have micro-environments that lack oxygen due to the slow diffusion characteristics of oxygen gas.

An example of the ecological importance of anaerobic respiration is the use of nitrate as a terminal electron acceptor, or dissimilatory denitrification, which is the main route by which fixed nitrogen is returned to the atmosphere as molecular nitrogen gas. [4] The denitrification process is also very important in host-microbe interactions. Similar to mitochondria in oxygen-respiring microorganisms, some single-cellular anaerobic ciliates use denitrifying endosymbionts to gain energy. [5] Another example is methanogenesis, a form of carbon-dioxide respiration, that is used to produce methane gas by anaerobic digestion. Biogenic methane is used as a sustainable alternative to fossil fuels. On the negative side, uncontrolled methanogenesis in landfill sites releases large volumes of methane into the atmosphere, where it acts as a powerful greenhouse gas. [6] Sulfate respiration produces hydrogen sulfide, which is responsible for the characteristic 'rotten egg' smell of coastal wetlands and has the capacity to precipitate heavy metal ions from solution, leading to the deposition of sulfidic metal ores. [7]

Dissimilatory denitrification is widely used in the removal of nitrate and nitrite from municipal wastewater. An excess of nitrate can lead to eutrophication of waterways into which treated water is released. Elevated nitrite levels in drinking water can lead to problems due to its toxicity. Denitrification converts both compounds into harmless nitrogen gas. [8]

Anaerobic Denitrification (ETC System)

English: The model above shows the process of anaerobic respiration through denitrification, which uses nitrogen (in the form of nitrate, NO3 − ) as the electron acceptor. NO3 − goes through respiratory dehydrogenase and reduces through each step from the ubiquinose through the bc1 complex through the ATP synthase protein as well. Each reductase loses oxygen through each step so that the final product of anaerobic respiration is N2.

1. Cytoplasm
2. Periplasm Compare to the aerobic electron transport chain.

Specific types of anaerobic respiration are also critical in bioremediation, which uses microorganisms to convert toxic chemicals into less-harmful molecules to clean up contaminated beaches, aquifers, lakes, and oceans. For example, toxic arsenate or selenate can be reduced to less toxic compounds by various anaerobic bacteria via anaerobic respiration. The reduction of chlorinated chemical pollutants, such as vinyl chloride and carbon tetrachloride, also occurs through anaerobic respiration.

Anaerobic respiration is useful in generating electricity in microbial fuel cells, which employ bacteria that respire solid electron acceptors (such as oxidized iron) to transfer electrons from reduced compounds to an electrode. This process can simultaneously degrade organic carbon waste and generate electricity. [9]


  • Sulfate reduction is a vital mechanism for bacteria and archaea living in oxygen-depleted, 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 ATP -sulfurylase, which uses ATP and sulfate to create adenosine 5&prime-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 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.
  • lithotrophic: Obtains electrons for respiration from inorganic substrates.
  • organotrophic: Obtains electrons for respiration from organic substrates.

Sulfate reduction is a type of anaerobic respiration that utilizes sulfate as a terminal electron acceptor in the electron transport chain. Compared to aerobic respiration, sulfate reduction is a relatively energetically poor process, though it is a vital mechanism for bacteria and archaea living in oxygen-depleted, sulfate-rich environments.

Many sulfate reducers are organotrophic, using carbon compounds, such as lactate and pyruvate (among many others) as electron donors, while others are lithotrophic, and use hydrogen gas (H2) as an electron donor. Some unusual autotrophic sulfate-reducing bacteria (e.g., Desulfotignum phosphitoxidans) can use phosphite (HPO 3- ) as an electron donor, whereas others (e.g., Desulfovibrio sulfodismutans, Desulfocapsa thiozymogenes, and Desulfocapsa sulfoexigens) are capable of sulfur disproportionation (splitting one compound into two different compounds, in this case an electron donor and an electron acceptor) using elemental sulfur (S0), sulfite (SO3 2&minus ), and thiosulfate (S2O3 2&minus ) to produce both hydrogen sulfide (H2S) and sulfate (SO4 2&minus ).

Before sulfate can be used as an electron acceptor, it must be activated. This is done by the enzyme ATP-sulfurylase, which uses ATP and sulfate to create adenosine 5&prime-phosphosulfate (APS). APS is subsequently reduced to sulfite and AMP. Sulfite is then further reduced to sulfide, while AMP is turned into ADP using another molecule of ATP. The overall process, thus, involves an investment of two molecules of the energy carrier ATP, which must to be regained from the reduction.

All sulfate-reducing organisms are strict anaerobes. Because sulfate is energetically stable, it must be activated by adenylation to form APS (adenosine 5&prime-phosphosulfate) to form APS before it can be metabolized, thereby consuming ATP. The APS is then reduced by the enzyme APS reductase to form sulfite (SO3 2&minus ) and AMP. In organisms that use carbon compounds as electron donors, the ATP consumed is accounted for by fermentation of the carbon substrate. The hydrogen produced during fermentation is actually what drives respiration during sulfate reduction.

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 to the sulfur cycle soon after life emerged on Earth. Sulfate-reducing bacteria are common in anaerobic environments (such as seawater, sediment, and water rich in decaying organic material) where they aid in the degradation of organic materials. In these anaerobic environments, fermenting bacteria extract energy from large organic molecules the resulting smaller compounds (such as organic acids and alcohols) are further oxidized by acetogens, methanogens, and the competing sulfate-reducing bacteria.

Many bacteria reduce small amounts of sulfates in order to synthesize sulfur-containing cell components this is known as assimilatory sulfate reduction. By contrast, sulfate-reducing bacteria reduce sulfate in large amounts to obtain energy and expel the resulting sulfide as waste this is known as &ldquodissimilatory sulfate reduction. &rdquo Most sulfate-reducing bacteria can also reduce other oxidized inorganic sulfur compounds, such as sulfite, thiosulfate, or elemental sulfur (which is reduced to sulfide as hydrogen sulfide).

Toxic hydrogen sulfide is one waste product of sulfate-reducing bacteria its rotten egg odor is often a marker for the presence of sulfate-reducing bacteria in nature. Sulfate-reducing bacteria are responsible for the sulfurous odors of salt marshes and mud flats. Much of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides. These metal sulfides, such as ferrous sulfide (FeS), are insoluble and often black or brown, leading to the dark color of sludge. Thus, the black color of sludge on a pond is due to metal sulfides that result from the action of sulfate-reducing bacteria.

Figure: Black sludge: The black color of this pond is due to metal sulfides that result from the action of sulfate-reducing bacteria.

Some sulfate-reducing bacteria play a role in the anaerobic oxidation of methane (CH4+ SO4 2- &rarr HCO3&ndash + HS&ndash + H2O). An important fraction of the methane formed by methanogens below the seabed is oxidized by sulfate-reducing bacteria in the transition zone separating the methanogenesis from the sulfate reduction activity in the sediments.This process is also considered a major sink for sulfate in marine sediments. In hydrofracturing fluids used to frack shale formations to recover methane (shale gas), biocide compounds are often added to water to inhibit the microbial activity of sulfate-reducing bacteria in order to avoid anaerobic methane oxidation and to minimize potential production loss.

Sulfate-reducing bacteria often create problems when metal structures are exposed to sulfate-containing water. The interaction of water and metal creates a layer of molecular hydrogen on the metal surface. Sulfate-reducing bacteria oxidize this hydrogen, creating hydrogen sulfide, which contributes to corrosion. Hydrogen sulfide from sulfate-reducing bacteria also plays a role in the biogenic sulfide corrosion of concrete, and sours crude oil.

Sulfate-reducing bacteria may be utilized for cleaning up contaminated soils some species are able to reduce hydrocarbons, such as benzene, toluene, ethylbenzene, and xylene. Sulfate-reducing bacteria may also be a way to deal with acid mine waters.