Lithotrophs are a diverse group of organisms using aninorganic substrate (usually of mineral origin) to obtainreducing equivalents for use inbiosynthesis (e.g.,carbon dioxide fixation) or energy conservation (i.e.,ATP production) viaaerobic oranaerobic respiration.[1] Whilelithotrophs in the broader sense include photolithotrophs like plants,chemolithotrophs are exclusivelymicroorganisms; no knownmacrofauna possesses 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 worms orplastids, which are organelles within plant cells that may have evolved from photolithotrophic cyanobacteria-like organisms. Chemolithotrophs belong to the domainsBacteria andArchaea. 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 ancestor of 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 thecatabolism of organic compounds.
The term was suggested in 1946 byLwoff and collaborators.[3]
Lithotrophs consumereducedinorganiccompounds (electron donors).
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 CO2 is converted toglucose.[6] This group oforganisms includes 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 inprokaryotes and was first characterized by Ukrainian microbiologistSergei Winogradsky.[7]
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 CO2 and produce the energy the microorganism needs to survive. Sincechemosynthesis can 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 sulfide to elementalsulfur by ½O2 produces far less energy (50kcal/mol or 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]
There is a fairly large variation in the types of inorganic substrates that thesemicroorganisms can 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 (inaerobic bacteria), but a variety of other electron acceptors,organic and 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 whichmay use oxygen or nitrate as electron acceptors:
| Name | Examples | Source of electrons | Respiration electron acceptor |
|---|---|---|---|
| Iron bacteria | Acidithiobacillus ferrooxidans | Fe2+ (ferrous iron) →Fe3+ (ferric iron) + e−[21] | O 2 (oxygen) + 4H+ + 4e−→ 2H 2O[21] |
| Nitrosifying bacteria | Nitrosomonas | NH3 (ammonia) + 2H 2O → | 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] |
| Anammox bacteria | Planctomycetota | NH+ 4 (ammonium) | NO− 2 (nitrite) + 4H+ + 3e−→ 1/2N2 (nitrogen) + 2H |
| Thiobacillus denitrificans | Thiobacillus denitrificans | S0 (sulfur) + 4H 2O →SO2− 4 + 8H+ + 6e−[26] | NO− 3 (nitrate) + 6H+ + 5e−→ 1/2N2 (nitrogen) + 3H |
| 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− | SO2− 4 (sulfate) + 8H+ + 6e− → S0 |
| 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 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).
Lithotrophic bacteria cannot use, of course, their inorganic energy source as acarbon source for the synthesis of their cells. They choose one of three options:
In addition to this division, lithotrophs differ in the initial energy source which initiates ATP production:
Lithotrophs participate in many geological processes, such as the formation of soil and thebiogeochemical cycling ofcarbon,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 inendolith communities.[27]
A primary example of lithotrophs that contribute tosoil formation isCyanobacteria. This group of bacteria are nitrogen-fixing 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 of elements is an essential component of lithotrophs within microbial environments. For example, in thecarbon cycle, there are certain bacteria classified asphotolithoautotrophs that generate organic carbon from atmospheric carbon dioxide. Certainchemolithoautotrophic bacteria 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.
Lithotrophic microbes are responsible for the phenomenon known asacid mine drainage. Typically occurring in mining areas, this process concerns the active metabolism ofpyrites and 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.
It has been suggested thatbiominerals could be important indicators ofextraterrestrial life and 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 andbiotic reactions.[31]
On January 24, 2014,NASA reported that current studies by theCuriosity andOpportunityrovers on Mars will now be searching for evidence of ancient life, including abiosphere based onautotrophic,chemotrophic and/orchemolithoautotrophicmicroorganisms, as well as ancient water, includingfluvio-lacustrine environments (plains related to ancientrivers orlakes) that may have beenhabitable.[32][33][34][35] The search for evidence ofhabitability,taphonomy (related tofossils), andorganic carbon on the planetMars is now a primary NASA objective.[32][33]
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