These molecules have a lower reduction potential than oxygen; thus, less energy is formed per molecule of glucose in anaerobic versus aerobic conditions. Anaerobic Respiration : A molecule other than oxygen is used as the terminal electron acceptor in anaerobic respiration.
Many different types of electron acceptors may be used for anaerobic respiration. Nitrate, like oxygen, has a high reduction potential. This process is widespread, and used by many members of Proteobacteria. It is also used in Gram-positive organisms related to Desulfotomaculum or the archaeon Archaeoglobus.
Sulfate reduction requires the use of electron donors, such as the carbon compounds lactate and pyruvate organotrophic reducers , or hydrogen gas lithotrophic reducers.
Some unusual autotrophic sulfate-reducing bacteria, such as Desulfotignum phosphitoxidans, can use phosphite HPO 3 — as an electron donor. Acetogenesis is a type of microbial metabolism that uses hydrogen H 2 as an electron donor and carbon dioxide CO 2 as an electron acceptor to produce acetate, the same electron donors and acceptors used in methanogenesis.
Electron flow in these organisms is similar to those in electron transport, ending in oxygen or nitrate, except that in ferric iron-reducing organisms the final enzyme in this system is a ferric iron reductase. Since some ferric iron-reducing bacteria e. Organic compounds may also be used as electron acceptors in anaerobic respiration. In anaerobic respiration, denitrification utilizes nitrate NO 3 — as a terminal electron acceptor in the respiratory electron transport chain.
Denitrification is a widely used process; many facultative anaerobes use denitrification because nitrate, like oxygen, has a high reduction potential. Denitrification is a microbially facilitated process involving the stepwise reduction of nitrate to nitrite NO 2 — nitric oxide NO , nitrous oxide N 2 O , and, eventually, to dinitrogen N 2 by the enzymes nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase. Protons are transported across the membrane by the initial NADH reductase, quinones and nitrous oxide reductase to produce the electrochemical gradient critical for respiration.
Some organisms e. Others e. Paracoccus denitrificans or Pseudomonas stutzeri reduce nitrate completely. Complete denitrification is an environmentally significant process because 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. Denitrification is also important in biological wastewater treatment, where it can be used to reduce the amount of nitrogen released into the environment, thereby reducing eutrophication.
Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen is depleted and bacteria respire nitrate as a substitute terminal electron acceptor.
Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in anaerobic environments where oxygen consumption exceeds the oxygen supply and where sufficient quantities of nitrate are present. These environments may include certain soils and groundwater, wetlands, oil reservoirs, poorly ventilated corners of the ocean, and in sea floor sediments.
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. Denitrification is performed primarily by heterotrophic bacteria e. Paracoccus denitrificans , although autotrophic denitrifiers have also been identified e.
Generally, several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway have been identified in the reduction process.
Rhizobia are soil bacteria with the unique ability to establish a N 2 -fixing symbiosis on legume roots. When faced with a shortage of oxygen, some rhizobia species are able to switch from O 2 -respiration to using nitrates to support respiration.
The direct reduction of nitrate to ammonium dissimilatory nitrate reduction can be performed by organisms with the nrf- gene. This is a less common method of nitrate reduction than denitrification in most ecosystems.
Other genes involved in denitrification include nir nitrite reductase and nos nitrous oxide reductase , which are possessed by such organisms as Alcaligenes faecalis , Alcaligenes xylosoxidans , Pseudomonas spp , Bradyrhizobium japonicum , and Blastobacter denitrificans. 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 H 2 as an electron donor.
Some unusual autotrophic sulfate-reducing bacteria e. Before sulfate can be used as an electron acceptor, it must be activated. 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. 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. 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. 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.
Black sludge : The black color of this pond is due to metal sulfides that result from the action of sulfate-reducing bacteria. 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. Methanogenesis is a form of anaerobic respiration that uses carbon as a electron acceptor and results in the production of methane. Methanogenesis, or biomethanation, is a form of anaerobic respiration that uses carbon as the terminal electron acceptor, resulting in the production of methane.
The carbon is sourced from a small number of low molecular weight organic compounds, such as carbon dioxide, acetic acid, formic acid formate , methanol, methylamines, dimethyl sulfide, and methanethiol.
The two best described pathways of methanogenesis use carbon dioxide or acetic acid as the terminal electron acceptor:. Methanogenesis of acetate : Acetate is broken down to methane by methanogenesis, a type of anaerobic respiration. The biochemistry of methanogenesis is relatively complex.
It involves the coenzymes and cofactors F, coenzyme B, coenzyme M, methanofuran, and methanopterin. 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 — though many live in close association with anaerobic bacteria. The production of methane is an important and widespread form of microbial metabolism, and in most environments, it is the final step in the decomposition of biomass.
During the decay process, electron acceptors such as oxygen, ferric iron, sulfate, and nitrate become depleted, while hydrogen H2 , carbon dioxide, and light organics produced by fermentation accumulate. During advanced stages of organic decay, all electron acceptors become depleted except carbon dioxide, which is a product of most catabolic processes.
It is not depleted like other potential electron acceptors. Only methanogenesis and fermentation can occur in the absence of electron acceptors other than carbon. Fermentation only allows the breakdown of larger organic compounds, and produces small organic compounds. Methanogenesis effectively removes the semi-final products of decay: hydrogen, small organics, and carbon dioxide. Without methanogenesis, a great deal of carbon in the form of fermentation products would accumulate in anaerobic environments.
Methanogenesis also occurs in the guts of humans and other animals, especially ruminants. In the rumen, anaerobic organisms, including methanogens, digest cellulose into forms usable by the animal. Without these microorganisms, animals such as cattle would not be able to consume grass. The useful products of methanogenesis are absorbed by the gut. Methane is released from the animal mainly by belching eructation.
The average cow emits around liters of methane per day. Some, but not all, humans emit methane in their flatus! Some experiments even suggest that leaf tissues of living plants emit methane, although other research indicates that the plants themselves do not actually generate methane; they are just absorbing methane from the soil and then emitting it through their leaf tissues.
There may still be some unknown mechanism by which plants produce methane, but that is by no means certain. Therefore, the methane produced by methanogenesis in livestock is a considerable contributor to global warming. Methanogenesis can also be beneficially exploited. It is the primary pathway that breaks down organic matter in landfills which can release large volumes of methane into the atmosphere if left uncontrolled , and can be used to treat organic waste and to produce useful compounds.
Biogenic methane can be collected and used as a sustainable alternative to fossil fuels. Anaerobic respiration utilizes highly reduced species — such as a proton gradient — to establish electrochemical membrane gradients. 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. It lets organisms live in places where there is little or no oxygen. Such places include deep water , soil, and the digestive tracts of animals such as humans see Figure below. Another advantage of anaerobic respiration is its speed. It produces ATP very quickly. For example, it lets your muscles get the energy they need for short bursts of intense activity see Figure below.
Aerobic respiration, on the other hand, produces ATP more slowly. The muscles of these hurdlers need to use anaerobic respiration for energy. It gives them the energy they need for the short-term, intense activity of this sport. Aerobic vs. Advantages of Aerobic Respiration A major advantage of aerobic respiration is the amount of energy it releases. Advantages of Anaerobic Respiration One advantage of anaerobic respiration is obvious. Summary Aerobic respiration produces much more ATP than anaerobic respiration.
Anaerobic respiration occurs more quickly than aerobic respiration. Explore More Use this resource to answer the questions that follow. What is the significance of oxygen during cellular respiration? Which is more efficient: aerobic or anaerobic respiration? What is the difference in ATP production between aerobic and anaerobic respiration? Why was anaerobic respiration sufficient when it first evolved?
0コメント