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Lithotroph

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Lithotrophsare a diverse group of organisms using aninorganicsubstrate (usually of mineral origin) to obtainreducing equivalentsfor use inbiosynthesis(e.g.,carbon dioxide fixation) or energy conservation (i.e.,ATPproduction) viaaerobicoranaerobic respiration.[1]Whilelithotrophs in the broader senseinclude photolithotrophs like plants,chemolithotrophsare exclusivelymicroorganisms;no knownmacrofaunapossesses the ability to use inorganic compounds as electron sources. Macrofauna and lithotrophs can form symbiotic relationships, in which case the lithotrophs are called "prokaryotic symbionts". An example of this is chemolithotrophic bacteria ingiant tube wormsorplastids,which are organelles within plant cells that may have evolved from photolithotrophic cyanobacteria-like organisms. Chemolithotrophs belong to the domainsBacteriaandArchaea.The term "lithotroph" was created from the Greek terms 'lithos' (rock) and 'troph' (consumer), meaning "eaters of rock". Many but not all lithoautotrophs areextremophiles.

Thelast universal common ancestorof life is thought to be a chemolithotroph (due to its presence in the prokaryotes).[2]Different from a lithotroph is anorganotroph,an organism which obtains its reducing agents from thecatabolismof organic compounds.

History[edit]

The term was suggested in 1946 byLwoffand collaborators.[3]

Biochemistry[edit]

Lithotrophs consumereducedinorganiccompounds(electron donors).

Chemolithotrophs[edit]

A chemolithotroph is able to use inorganic reduced compounds in its energy-producing reactions.[4]: 155 [5]This process involves the oxidation of inorganic compounds coupled to ATP synthesis. The majority of chemolithotrophs arechemolithoautotrophs,able to fixcarbon dioxide(CO2) through theCalvin cycle,a metabolic pathway in which CO2is converted toglucose.[6]This group oforganismsincludes sulfur oxidizers,nitrifying bacteria,iron oxidizers, and hydrogen oxidizers.

The term "chemolithotrophy" refers to a cell's acquisition of energy from the oxidation of inorganic compounds, also known as electron donors. This form of metabolism is believed to occur only inprokaryotesand was first characterized by Ukrainian microbiologistSergei Winogradsky.[7]

Habitat of chemolithotrophs[edit]

The survival of these bacteria is dependent on the physiochemical conditions of their environment. Although they are sensitive to certain factors such as quality of inorganic substrate, they are able to thrive under some of the most inhospitable conditions in the world, such as temperatures above 110 degrees Celsius and below 2 pH.[8]The most important requirement for chemolithotropic life is an abundant source of inorganic compounds,[9]which provide a suitable electron donor in order to fix CO2and produce the energy the microorganism needs to survive. Sincechemosynthesiscan take place in the absence of sunlight, these organisms are found mostly around hydrothermal vents and other locations rich in inorganic substrate.

The energy obtained from inorganic oxidation varies depending on the substrate and the reaction. For example, the oxidation ofhydrogen sulfideto elementalsulfurby ½O2produces far less energy (50kcal/molor 210kJ/mol) than the oxidation of elemental sulfur tosulfate(150 kcal/mol or 627 kJ/mol) by 3/2 O2,.[10]The majority of lithotrophs fix carbon dioxide through the Calvin cycle, an energetically expensive process.[6]For some low-energy substrates, such asferrous iron,the cells must cull through large amounts of inorganic substrate to secure just a small amount of energy. This makes their metabolic process inefficient in many places and hinders them from thriving.[11]

Overview of the metabolic process[edit]

There is a fairly large variation in the types of inorganic substrates that thesemicroorganismscan use to produce energy. Sulfur is one of many inorganic substrates that can be used in different reduced forms depending on the specific biochemical process that a lithotroph uses.[12]The chemolithotrophs that are best documented are aerobic respirers, meaning that they use oxygen in their metabolic process. The list of these microorganisms that employ anaerobic respiration though is growing. At the heart of this metabolic process is an electron transport system that is similar to that of chemoorganotrophs. The major difference between these two microorganisms is that chemolithotrophs directly provide electrons to the electron transport chain, while chemoorganotrophs must generate their own cellular reducing power by oxidizing reduced organic compounds. Chemolithotrophs bypass this by obtaining their reducing power directly from the inorganic substrate or by the reverse electron transport reaction.[13]Certain specialized chemolithotrophic bacteria use different derivatives of the Sox system; a central pathway specific to sulfur oxidation.[12]This ancient and unique pathway illustrates the power that chemolithotrophs have evolved to use from inorganic substrates, such as sulfur.

In chemolithotrophs, the compounds – theelectron donors– are oxidized in thecell,and the electrons are channeled into respiratory chains, ultimately producingATP.The electron acceptor can beoxygen(inaerobicbacteria), but a variety of other electron acceptors,organicand inorganic, are also used by variousspecies.Aerobic bacteria such as the nitrifying bacteria,Nitrobacter,use oxygen to oxidize nitrite to nitrate.[14]Some lithotrophs produce organic compounds from carbon dioxide in a process calledchemosynthesis,much as plants do inphotosynthesis.Plants use energy from sunlight to drive carbon dioxide fixation, but chemosynthesis can take place in the absence of sunlight (e.g., around ahydrothermal vent). Ecosystems establish in and around hydrothermal vents as the abundance of inorganic substances, namely hydrogen, are constantly being supplied via magma in pockets below the sea floor.[15]Other lithotrophs are able to directly use inorganic substances, e.g., ferrous iron, hydrogen sulfide, elemental sulfur, thiosulfate, or ammonia, for some or all of their energy needs.[16][17][18][19][20]

Here are a few examples of chemolithotrophic pathways, any of whichmayuse oxygen or nitrate as electron acceptors:

Name Examples Source of electrons Respiration electron acceptor
Iron bacteria Acidithiobacillus ferrooxidans Fe2+(ferrousiron) →Fe3+(ferriciron) + e−[21] O
2(oxygen) + 4H++ 4e−→ 2H
2O[21]
Nitrosifying bacteria Nitrosomonas NH3(ammonia) + 2H
2O →

NO−
2(nitrite) + 7H++ 6e−[22]

O
2(oxygen) + 4H++ 4e−→ 2H
2O[22]
Nitrifying bacteria Nitrobacter NO−
2(nitrite) + H
2O →NO−
3(nitrate) + 2H++ 2e−[23] 		O
2(oxygen) + 4H++ 4e−→ 2H
2O[23]
Chemotrophicpurple sulfur bacteria Halothiobacillaceae S2−
(sulfide) →S0
(sulfur) + 2e− 		O
2(oxygen) + 4H++ 4e−→ 2H
2O
Sulfur-oxidizing bacteria ChemotrophicRhodobacteraceae
andThiotrichaceae 		S0
(sulfur) + 4H
2O →SO2−
4(sulfate) + 8H++ 6e− 		O
2(oxygen) + 4H++ 4e−→ 2H
2O
Aerobichydrogen bacteria Cupriavidus metallidurans H2(hydrogen) → 2H++ 2e−[24] O
2(oxygen) + 4H++ 4e−→ 2H
2O[24]
Anammoxbacteria Planctomycetota NH+
4(ammonium)

→ 1/2N2(nitrogen) + 4H++ 3e−[25]

NO−
2(nitrite) + 4H++ 3e−→

1/2N2(nitrogen) + 2H
2O[25]

Thiobacillus denitrificans Thiobacillus denitrificans S0
(sulfur) + 4H
2O →SO2−
4+ 8H++ 6e−[26] 		NO−
3(nitrate) + 6H++ 5e−→

1/2N2(nitrogen) + 3H
2O[26]

Sulfate-reducing bacteria:Hydrogen bacteria Desulfovibrio paquesii H2(hydrogen) → 2H++ 2e−[24] SO2−
4+ 8H++ 6e−→S0
+ 4H
2O[24]
Sulfate-reducing bacteria:Phosphite bacteria Desulfotignum phosphitoxidans PO3−
3(phosphite) + H
2O →

PO3−
4(phosphate) + 2H++ 2e−

SO2−
4(sulfate) + 8H++ 6e−→

S0
(sulfur) + 4H
2O

Methanogens Archaea H2(hydrogen) → 2H++ 2e− CO2+ 8H++ 8e−→CH4(methane) + 2H
2O
Carboxydotrophic bacteria Carboxydothermus hydrogenoformans CO(carbon monoxide) + H
2O →CO2+ 2H++ 2e− 		2H++ 2e−→H
2(hydrogen)

Photolithotrophs[edit]

Photolithotrophs such as plants obtain energy from light and therefore use inorganic electron donors such as water only to fuel biosynthetic reactions (e. g., carbon dioxide fixation in lithoautotrophs).

Lithoheterotrophs versus lithoautotrophs[edit]

Lithotrophic bacteria cannot use, of course, their inorganic energy source as acarbonsource for the synthesis of their cells. They choose one of three options:

  • Lithoheterotrophs do not have the ability to fixcarbon dioxideand must consume additional organic compounds in order to break them apart and use their carbon. Only a few bacteria are fully lithoheterotrophic.
  • Lithoautotrophsare able to use carbon dioxide from theairas a carbon source, the same wayplantsdo.
  • Mixotrophswill take up and use organic material to complement their carbon dioxide fixation source (mix between autotrophy and heterotrophy). Many lithotrophs are recognized as mixotrophic in regard to their C-metabolism.

Chemolithotrophs versus photolithotrophs[edit]

In addition to this division, lithotrophs differ in the initial energy source which initiates ATP production:

  • Chemolithotrophs use the above-mentioned inorganic compounds for aerobic or anaerobic respiration. The energy produced by the oxidation of these compounds is enough for ATP production. Some of the electrons derived from the inorganic donors also need to be channeled into biosynthesis. Mostly, additional energy has to be invested to transform these reducing equivalents to the forms and redox potentials needed (mostly NADH or NADPH), which occurs by reverse electron transfer reactions.
  • Photolithotrophs uselightas their energy source. These organisms arephotosynthetic;examples ofphotolithotrophic bacteriaarepurple bacteria(e. g.,Chromatiaceae), green bacteria (ChlorobiaceaeandChloroflexota), and "Cyanobacteria".Purple and green bacteria oxidize sulfide, sulfur, sulfite, iron or hydrogen. Cyanobacteria and plants extract reducing equivalents from water, i.e., they oxidize water to oxygen. The electrons obtained from the electron donors are not used for ATP production (as long as there is light); they are used in biosynthetic reactions. Some photolithotrophs shift over to chemolithotrophic metabolism in the dark.

Geological significance[edit]

Lithotrophs participate in many geological processes, such as the formation of soil and thebiogeochemical cyclingofcarbon,nitrogen,and otherelements.Lithotrophs also associate with the modern-day issue ofacid mine drainage.Lithotrophs may be present in a variety of environments, including deep terrestrial subsurfaces, soils, mines, and inendolithcommunities.[27]

Soil formation[edit]

A primary example of lithotrophs that contribute tosoil formationisCyanobacteria.This group of bacteria are nitrogen-fi xing photolithotrophs that are capable of using energy from sunlight and inorganic nutrients from rocks asreductants.[27]This capability allows for their growth and development on native, oligotrophic rocks and aids in the subsequent deposition of their organic matter (nutrients) for other organisms to colonize.[28]Colonization can initiate the process of organic compounddecomposition:a primary factor for soil genesis. Such a mechanism has been attributed as part of the early evolutionary processes that helped shape the biological Earth.

Biogeochemical cycling[edit]

Biogeochemical cyclingof elements is an essential component of lithotrophs within microbial environments. For example, in thecarbon cycle,there are certain bacteria classified asphotolithoautotrophsthat generate organic carbon from atmospheric carbon dioxide. Certainchemolithoautotrophicbacteria can also produce organic carbon, some even in the absence of light.[28]Similar to plants, these microbes provide a usable form of energy for organisms to consume. On the contrary, there are lithotrophs that have the ability toferment,implying their ability to convert organic carbon into another usable form.[29]Lithotrophs play an important role in the biological aspect of theiron cycle.These organisms can use iron as either an electron donor, Fe(II) → Fe(III), or as an electron acceptor, Fe (III) → Fe(II).[30]Another example is thecycling of nitrogen.Many lithotrophic bacteria play a role in reducing inorganic nitrogen (nitrogen gas) to organic nitrogen (ammonium) in a process callednitrogen fixation.[28]Likewise, there are many lithotrophic bacteria that also convert ammonium into nitrogen gas in a process calleddenitrification.[27]Carbon and nitrogen are important nutrients, essential for metabolic processes, and can sometimes be the limiting factor that affects organismal growth and development. Thus, lithotrophs are key players in both providing and removing these important resource.

Acid mine drainage[edit]

Lithotrophic microbes are responsible for the phenomenon known asacid mine drainage.Typically occurring in mining areas, this process concerns the active metabolism ofpyritesand other reduced sulfur components tosulfate.One example is the acidophilic bacterial genus,A. ferrooxidans,that useiron(II) sulfide(FeS2) to generatesulfuric acid.[29]The acidic product of these specific lithotrophs has the potential to drain from the mining area via water run-off and enter the environment.

Acid mine drainage drastically alters the acidity (pH values of 2–3) and chemistry of groundwater and streams, and may endanger plant and animal populations downstream of mining areas.[29]Activities similar to acid mine drainage, but on a much lower scale, are also found in natural conditions such as the rocky beds of glaciers, in soil and talus, on stone monuments and buildings and in the deep subsurface.

Astrobiology[edit]

It has been suggested thatbiomineralscould be important indicators ofextraterrestrial lifeand thus could play an important role in the search for past or present life on the planetMars.[5]Furthermore,organic components(biosignatures) that are often associated with biominerals are believed to play crucial roles in both pre-biotic andbioticreactions.[31]

On January 24, 2014,NASAreported that current studies by theCuriosityandOpportunityroverson Mars will now be searching for evidence of ancient life, including abiospherebased onautotrophic,chemotrophicand/orchemolithoautotrophicmicroorganisms,as well as ancient water, includingfluvio-lacustrine environments(plainsrelated to ancientriversorlakes) that may have beenhabitable.[32][33][34][35]The search for evidence ofhabitability,taphonomy(related tofossils), andorganic carbonon the planetMarsis now a primary NASA objective.[32][33]

See also[edit]

References[edit]

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External links[edit]

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