Microbial metabolism

Source: Wikipedia, the free encyclopedia.

Microbial metabolism is the means by which a microbe obtains the energy and nutrients (e.g. carbon) it needs to live and reproduce. Microbes use many different types of metabolic strategies and species can often be differentiated from each other based on metabolic characteristics. The specific metabolic properties of a microbe are the major factors in determining that microbe's ecological niche, and often allow for that microbe to be useful in industrial processes or responsible for biogeochemical cycles.

Types

Flow chart to determine the metabolic characteristics of microorganisms
Main article: Primary nutritional groups

All microbial metabolisms can be arranged according to three principles:

1. How the organism obtains carbon for synthesizing cell mass:[1]

2. How the organism obtains

reducing equivalents
(hydrogen atoms or electrons) used either in energy conservation or in biosynthetic reactions:

3. How the organism obtains energy for living and growing:

In practice, these terms are almost freely combined. Typical examples are as follows:

Heterotrophic microbial metabolism

Some microbes are heterotrophic (more precisely chemoorganoheterotrophic), using organic compounds as both carbon and energy sources. Heterotrophic microbes live off of nutrients that they scavenge from living hosts (as

mineralization) requires several different organisms, with one breaking down the polymer into its constituent monomers, one able to use the monomers and excreting simpler waste compounds as by-products, and one able to use the excreted wastes. There are many variations on this theme, as different organisms are able to degrade different polymers and secrete different waste products. Some organisms are even able to degrade more recalcitrant compounds such as petroleum compounds or pesticides, making them useful in bioremediation
.

Biochemically,

terminal electron acceptor
that may be used. As discussed below, the use of terminal electron acceptors other than oxygen has important biogeochemical consequences.

Fermentation

Main article:
Fermentation (biochemistry)

Fermentation is a specific type of heterotrophic metabolism that uses

facultative anaerobes. To avoid the overproduction of NADH, obligately fermentative organisms usually do not have a complete citric acid cycle. Instead of using an ATP synthase as in respiration, ATP in fermentative organisms is produced by substrate-level phosphorylation where a phosphate group is transferred from a high-energy organic compound to ADP to form ATP. As a result of the need to produce high energy phosphate-containing organic compounds (generally in the form of Coenzyme A-esters) fermentative organisms use NADH and other cofactors to produce many different reduced metabolic by-products, often including hydrogen gas (H
2). These reduced organic compounds are generally small organic acids and alcohols derived from pyruvate, the end product of glycolysis. Examples include ethanol, acetate, lactate, and butyrate
. Fermentative organisms are very important industrially and are used to make many different types of food products. The different metabolic end products produced by each specific bacterial species are responsible for the different tastes and properties of each food.

Not all fermentative organisms use substrate-level

succinate fermentation by Propionigenium modestum and oxalate fermentation by Oxalobacter formigenes. These reactions are extremely low-energy yielding. Humans and other higher animals also use fermentation to produce lactate
from excess NADH, although this is not the major form of metabolism as it is in fermentative microorganisms.

Special metabolic properties

Methylotrophy

Methylotrophy refers to the ability of an organism to use

methyl amines, formaldehyde, and formate. Several other less common substrates may also be used for metabolism, all of which lack carbon-carbon bonds. Examples of methylotrophs include the bacteria Methylomonas and Methylobacter. Methanotrophs are a specific type of methylotroph that are also able to use methane (CH
4) as a carbon source by oxidizing it sequentially to methanol (CH
3OH), formaldehyde (CH
2O), formate (HCOO−
), and carbon dioxide CO2 initially using the enzyme methane monooxygenase. As oxygen is required for this process, all (conventional) methanotrophs are obligate aerobes. Reducing power in the form of quinones and NADH is produced during these oxidations to produce a proton motive force and therefore ATP generation. Methylotrophs and methanotrophs are not considered as autotrophic, because they are able to incorporate some of the oxidized methane (or other metabolites) into cellular carbon before it is completely oxidized to CO2 (at the level of formaldehyde), using either the serine pathway (Methylosinus, Methylocystis) or the ribulose monophosphate pathway (Methylococcus
), depending on the species of methylotroph.

In addition to aerobic methylotrophy, methane can also be oxidized anaerobically. This occurs by a consortium of sulfate-reducing bacteria and relatives of methanogenic Archaea working syntrophically (see below). Little is currently known about the biochemistry and ecology of this process.

Methanogenesis is the biological production of methane. It is carried out by methanogens, strictly anaerobic Archaea such as Methanococcus, Methanocaldococcus, Methanobacterium,

reductive acetyl-CoA pathway
to fix CO2 and obtain cellular carbon.

Syntrophy

Syntrophy, in the context of microbial metabolism, refers to the pairing of multiple species to achieve a

mineralization of these compounds. These reactions help prevent the excess sequestration of carbon
over geologic time scales, releasing it back to the biosphere in usable forms such as methane and CO2.

Aerobic respiration

Aerobic metabolism occurs in Bacteria, Archaea and Eucarya. Although most bacterial species are anaerobic, many are facultative or obligate aerobes. The majority of archaeal species live in extreme environments that are often highly anaerobic. There are, however, several cases of aerobic archaea such as Haiobacterium, Thermoplasma, Sulfolobus and Yymbaculum. Most of the known eukaryotes carry out aerobic metabolism within their mitochondria which is an organelle that had a symbiogenesis origin from prokarya . All aerobic organisms contain oxidases of the cytochrome oxidase super family, but some members of the Pseudomonadota (E. coli and Acetobacter) can also use an unrelated cytochrome bd complex as a respiratory terminal oxidase.[5]

Anaerobic respiration

While

facultative anaerobes
can use either oxygen or alternative terminal electron acceptors for respiration depending on the environmental conditions.

Most respiring anaerobes are heterotrophs, although some do live autotrophically. All of the processes described below are dissimilative, meaning that they are used during energy production and not to provide nutrients for the cell (assimilative). Assimilative pathways for many forms of anaerobic respiration are also known.

Denitrification – nitrate as electron acceptor

Main article: Denitrification

Denitrification is the utilization of

E. coli) only produce nitrate reductase and therefore can accomplish only the first reduction leading to the accumulation of nitrite. Others (e.g. 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 important 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 is used to reduce the amount of nitrogen released into the environment thereby reducing eutrophication. Denitrification can be determined via a nitrate reductase test
.

Sulfate reduction – sulfate as electron acceptor

Main article:
Sulfate-reducing bacteria

Dissimilatory sulfate reduction is a relatively energetically poor process used by many Gram-negative bacteria found within the Thermodesulfobacteriota, Gram-positive organisms relating to Desulfotomaculum or the archaeon Archaeoglobus. Hydrogen sulfide (H
2S) is produced as a metabolic end product. For sulfate reduction electron donors and energy are needed.

Electron donors

Many sulfate reducers are organotrophic, using carbon compounds such as lactate and pyruvate (among many others) as

phosphite (HPO−
3) as an electron donor[8] whereas others (e.g. Desulfovibrio sulfodismutans, Desulfocapsa thiozymogenes, 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 (SO2−
3), and thiosulfate (S
2O2−
3) to produce both hydrogen sulfide (H
2S) and sulfate (SO2−
4).[9]

Energy for reduction

All sulfate-reducing organisms are strict anaerobes. Because sulfate is energetically stable, before it can be metabolized it must first be activated by adenylation to form APS (adenosine 5’-phosphosulfate) thereby consuming ATP. The APS is then reduced by the enzyme APS reductase to form sulfite (SO2−
3) 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.

Acetogenesis – carbon dioxide as electron acceptor

Main article: Acetogenesis

Acetogenesis is a type of microbial metabolism that uses hydrogen (H
2) as an electron donor and carbon dioxide (CO2) as an electron acceptor to produce acetate, the same electron donors and acceptors used in methanogenesis (see above). Bacteria that can autotrophically synthesize acetate are called homoacetogens. Carbon dioxide reduction in all homoacetogens occurs by the acetyl-CoA pathway. This pathway is also used for carbon fixation by autotrophic sulfate-reducing bacteria and hydrogenotrophic methanogens. Often homoacetogens can also be fermentative, using the hydrogen and carbon dioxide produced as a result of fermentation to produce acetate, which is secreted as an end product.

Other inorganic electron acceptors

Ferric iron (Fe3+
) is a widespread anaerobic terminal electron acceptor both for autotrophic and heterotrophic organisms. 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. Model organisms include Shewanella putrefaciens and Geobacter metallireducens. Since some ferric iron-reducing bacteria (e.g. G. metallireducens) can use toxic hydrocarbons such as toluene as a carbon source, there is significant interest in using these organisms as bioremediation agents in ferric iron-rich contaminated aquifers
.

Although ferric iron is the most prevalent inorganic electron acceptor, a number of organisms (including the iron-reducing bacteria mentioned above) can use other

inorganic ions in anaerobic respiration. While these processes may often be less significant ecologically, they are of considerable interest for bioremediation, especially when heavy metals or radionuclides
are used as electron acceptors. Examples include:

Organic terminal electron acceptors

A number of organisms, instead of using inorganic compounds as terminal electron acceptors, are able to use organic compounds to accept electrons from respiration. Examples include:

TMAO is a chemical commonly produced by fish, and when reduced to TMA produces a strong odor. DMSO is a common marine and freshwater chemical which is also odiferous when reduced to DMS. Reductive dechlorination is the process by which chlorinated organic compounds are reduced to form their non-chlorinated endproducts. As chlorinated organic compounds are often important (and difficult to degrade) environmental pollutants, reductive dechlorination is an important process in bioremediation.

Chemolithotrophy

Chemolithotrophy
is a type of metabolism where energy is obtained from the oxidation of inorganic compounds. Most chemolithotrophic organisms are also autotrophic. There are two major objectives to chemolithotrophy: the generation of energy (ATP) and the generation of reducing power (NADH).

Hydrogen oxidation

Main article:
Hydrogen oxidizing bacteria

Many organisms are capable of using hydrogen (H
2) as a source of energy. While several mechanisms of anaerobic hydrogen

oxidation have been mentioned previously (e.g. sulfate reducing- and acetogenic bacteria), the chemical energy of hydrogen can be used in the aerobic Knallgas reaction:[10]

2 H2 + O2 → 2 H2O + energy

In these organisms, hydrogen is oxidized by a membrane-bound

Ralstonia eutropha), often inhabit oxic-anoxic interfaces in nature to take advantage of the hydrogen produced by anaerobic fermentative organisms while still maintaining a supply of oxygen.[11]

Sulfur oxidation

Sulfur oxidation involves the oxidation of reduced sulfur compounds (such as sulfide H
2S), inorganic sulfur (S), and thiosulfate (S
2O2−
3) to form

thermodynamic gradient to produce NADH. Biochemically, reduced sulfur compounds are converted to sulfite (SO2−
3) and subsequently converted to sulfate (SO2−
4) by the enzyme sulfite oxidase.[12] Some organisms, however, accomplish the same oxidation using a reversal of the APS reductase system used by sulfate-reducing bacteria (see above). In all cases the energy liberated is transferred to the electron transport chain for ATP and NADH production.[12] In addition to aerobic sulfur oxidation, some organisms (e.g. Thiobacillus denitrificans
) use nitrate (NO−
3) as a terminal electron acceptor and therefore grow anaerobically.

Ferrous iron (Fe2+
) oxidation

Further information: Acidophiles in acid mine drainage

archaeon Ferroplasma. These microbes oxidize iron in environments that have a very low pH and are important in acid mine drainage. The second type of microbes oxidize ferrous iron at near-neutral pH. These micro-organisms (for example Gallionella ferruginea, Leptothrix ochracea, or Mariprofundus ferrooxydans) live at the oxic-anoxic interfaces and are microaerophiles. The third type of iron-oxidizing microbes are anaerobic photosynthetic bacteria such as Rhodopseudomonas,[13] which use ferrous iron to produce NADH for autotrophic carbon dioxide fixation. Biochemically, aerobic iron oxidation is a very energetically poor process which therefore requires large amounts of iron to be oxidized by the enzyme rusticyanin
to facilitate the formation of proton motive force. Like sulfur oxidation, reverse electron flow must be used to form the NADH used for carbon dioxide fixation via the Calvin cycle.

Nitrification

Nitrification is the process by which ammonia (NH
3) is converted to nitrate (NO−
3). Nitrification is actually the net result of two distinct processes: oxidation of ammonia to nitrite (NO−
2) by nitrosifying bacteria (e.g. Nitrosomonas) and oxidation of nitrite to nitrate by the nitrite-oxidizing bacteria (e.g. Nitrobacter). Both of these processes are extremely energetically poor leading to very slow growth rates for both types of organisms. Biochemically, ammonia oxidation occurs by the stepwise oxidation of ammonia to hydroxylamine (NH
2OH) by the enzyme ammonia monooxygenase in the cytoplasm, followed by the oxidation of hydroxylamine to nitrite by the enzyme hydroxylamine oxidoreductase in the periplasm.

Electron and proton cycling are very complex but as a net result only one proton is translocated across the membrane per molecule of ammonia oxidized. Nitrite oxidation is much simpler, with nitrite being oxidized by the enzyme nitrite oxidoreductase coupled to proton translocation by a very short electron transport chain, again leading to very low growth rates for these organisms. Oxygen is required in both ammonia and nitrite oxidation, meaning that both nitrosifying and nitrite-oxidizing bacteria are aerobes. As in sulfur and iron oxidation, NADH for carbon dioxide fixation using the Calvin cycle is generated by reverse electron flow, thereby placing a further metabolic burden on an already energy-poor process.

In 2015, two groups independently showed the microbial genus Nitrospira is capable of complete nitrification (Comammox).[14][15]

Anammox

Anammox stands for anaerobic ammonia oxidation and the organisms responsible were relatively recently discovered, in the late 1990s.

Brocadia anammoxidans") and involves the coupling of ammonia oxidation to nitrite reduction. As oxygen is not required for this process, these organisms are strict anaerobes. Amazingly, hydrazine (N
2H
4 – rocket fuel) is produced as an intermediate during anammox metabolism. To deal with the high toxicity of hydrazine, anammox bacteria contain a hydrazine-containing intracellular organelle called the anammoxasome, surrounded by highly compact (and unusual) ladderane lipid membrane. These lipids are unique in nature, as is the use of hydrazine as a metabolic intermediate. Anammox organisms are autotrophs although the mechanism for carbon dioxide fixation is unclear. Because of this property, these organisms could be used to remove nitrogen in industrial wastewater treatment processes.[17] Anammox has also been shown to have widespread occurrence in anaerobic aquatic systems and has been speculated to account for approximately 50% of nitrogen gas production in the ocean.[18]

Manganese oxidation

In July 2020 researchers report the discovery of

chemolithoautotrophic bacterial culture that feeds on the metal manganese after performing unrelated experiments and named its bacterial species Candidatus Manganitrophus noduliformans and Ramlibacter lithotrophicus.[19][20][21]

Phototrophy

Main article:
Phototrophic prokaryotes

Many microbes (phototrophs) are capable of using light as a source of energy to produce

Chloroflexus), or the heliobacteria (Low %G+C Gram positives). In addition to these organisms, some microbes (e.g. the Archaeon Halobacterium or the bacterium Roseobacter, among others) can utilize light to produce energy using the enzyme bacteriorhodopsin, a light-driven proton pump. However, there are no known Archaea that carry out photosynthesis.[23]

As befits the large diversity of photosynthetic bacteria, there are many different mechanisms by which light is converted into energy for metabolism. All photosynthetic organisms locate their

carotenoids, allowing them to take advantage of different portions of the electromagnetic spectrum and thereby inhabit different niches. Some groups of organisms contain more specialized light-harvesting structures (e.g. phycobilisomes
in Cyanobacteria and chlorosomes in Green sulfur and non-sulfur bacteria), allowing for increased efficiency in light utilization.

Biochemically, anoxygenic photosynthesis is very different from oxygenic photosynthesis. Cyanobacteria (and by extension, chloroplasts) use the Z scheme of electron flow in which electrons eventually are used to form NADH. Two different reaction centers (photosystems) are used and proton motive force is generated both by using cyclic electron flow and the quinone pool. In anoxygenic photosynthetic bacteria, electron flow is cyclic, with all electrons used in photosynthesis eventually being transferred back to the single reaction center. A proton motive force is generated using only the quinone pool. In heliobacteria, Green sulfur, and Green non-sulfur bacteria, NADH is formed using the protein ferredoxin, an energetically favorable reaction. In purple bacteria, NADH is formed by reverse electron flow due to the lower chemical potential of this reaction center. In all cases, however, a proton motive force is generated and used to drive ATP production via an ATPase.

Most photosynthetic microbes are autotrophic, fixing carbon dioxide via the Calvin cycle. Some photosynthetic bacteria (e.g. Chloroflexus) are photoheterotrophs, meaning that they use organic carbon compounds as a carbon source for growth. Some photosynthetic organisms also fix nitrogen (see below).

Nitrogen fixation

Main article: Nitrogen fixation

Nitrogen is an element required for growth by all biological systems. While extremely common (80% by volume) in the atmosphere, dinitrogen gas (N
2) is generally biologically inaccessible due to its high activation energy. Throughout all of nature, only specialized bacteria and Archaea are capable of nitrogen fixation, converting dinitrogen gas into ammonia (NH
3), which is easily assimilated by all organisms.[25] These prokaryotes, therefore, are very important ecologically and are often essential for the survival of entire ecosystems. This is especially true in the ocean, where nitrogen-fixing cyanobacteria are often the only sources of fixed nitrogen, and in soils, where specialized symbioses exist between legumes and their nitrogen-fixing partners to provide the nitrogen needed by these plants for growth.

Nitrogen fixation can be found distributed throughout nearly all bacterial lineages and physiological classes but is not a universal property. Because the enzyme nitrogenase, responsible for nitrogen fixation, is very sensitive to oxygen which will inhibit it irreversibly, all nitrogen-fixing organisms must possess some mechanism to keep the concentration of oxygen low. Examples include:

  • heterocyst formation (cyanobacteria e.g. Anabaena) where one cell does not photosynthesize but instead fixes nitrogen for its neighbors which in turn provide it with energy
  • root nodule symbioses (e.g.
    leghaemoglobin
  • anaerobic lifestyle (e.g. Clostridium pasteurianum)
  • very fast metabolism (e.g. Azotobacter vinelandii)

The production and activity of nitrogenases is very highly regulated, both because nitrogen fixation is an extremely energetically expensive process (16–24 ATP are used per N
2 fixed) and due to the extreme sensitivity of the nitrogenase to oxygen.

See also

References

  1. ISBN 978-1319017637
  2. ^ Tang, K.-H., Tang, Y. J., Blankenship, R. E. (2011). "Carbon metabolic pathways in phototrophic bacteria and their broader evolutionary implications" Frontiers in Microbiology 2: Atc. 165. http://dx.doi.org/10.3389/micb.2011.00165
  3. ^ "Chemolithotrophy | Boundless Microbiology".
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  10. ^ "knallgas reaction". Oxford Reference. Retrieved August 19, 2017.
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  19. ^ "Bacteria with a metal diet discovered in dirty glassware". phys.org. Retrieved 16 August 2020.
  20. ^ Woodyatt, Amy. "Bacteria that eats metal accidentally discovered by scientists". CNN. Retrieved 16 August 2020.
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Further reading

  • Madigan, Michael T.; Martinko, John M. (2005). Brock Biology of Microorganisms. Pearson Prentice Hall.
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