A lithotroph is an organism that uses an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation via aerobic or anaerobic respiration.[1] Known chemolithotrophs are exclusively microbes; No known macrofauna possesses the ability to utilize inorganic compounds as energy sources. Macrofauna and lithotrophs can form symbiotic relationships, in which case the lithotrophs are called "prokaryotic symbionts." An example of this is chemolithotrophic bacteria in deep sea worms or plastids, which are organelles within plant cells that may have evolved from photolithotrophic cyanobacteria-like organisms. Lithotrophs belong to either the domain Bacteria or the domain Archaea. The term "Lithotroph" is created from the terms 'lithos' (rock) and 'troph' (consumer), meaning the "eaters of rock." Many lithoautotrophs are extremophiles, but this is not universally so.
Different from a lithotroph is an organotroph, an organism which gets its reducing agents from the catabolism of organic compounds.
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Lithotrophs consume reduced compounds (rich in electrons).
In chemolithotrophs, the compounds - the electron donors - are oxidized in the cell, and the electrons are channeled into respiratory chains, ultimately producing ATP. The electron acceptor can be oxygen (in aerobic bacteria), but a variety of other electron acceptors, organic and inorganic, are also used by various species. Unlike water, the hydrogen compounds used in chemosynthesis are high in energy. Other lithotrophs are able to directly utilize inorganic substances, e.g., iron, hydrogen sulfide, elemental sulfur, or thiosulfate, for some or all of their energy needs.[2][3][4][5]
Here are a few examples of chemolithotrophic pathways, any of which may use oxygen, sulfur or other molecules as electron acceptors:
| Name | Examples | Source of energy and electrons | Respiration electron acceptor |
|---|---|---|---|
| Iron bacteria | Acidithiobacillus ferrooxidans | Fe2+ (ferrous iron) → Fe3+ (ferric iron) + e- | O 2 (oxygen) → H 2O (water) |
| Nitrosifying bacteria | Nitrosomonas | NH3 (ammonia) → NO− 2 (nitrite) + e- |
O 2 (oxygen) → H 2O (water) |
| Nitrifying bacteria | Nitrobacter | NO− 2 (nitrite) → NO− 3 (nitrate) + e- |
O 2 (oxygen) → H 2O (water) |
| Chemotrophic purple sulfur bacteria | Halothiobacillaceae | S2− (sulfide) → S0 (sulfur) + e- |
O 2 (oxygen) → H 2O (water) |
| Sulfur-oxidizing bacteria | Chemotrophic Rhodobacteraceae and Thiotrichaceae |
S0 (sulfur) → SO2− 4 (sulfate) + e- |
O 2 (oxygen) → H 2O (water) |
| Aerobic hydrogen bacteria | Cupriavidus metallidurans | H2 (hydrogen) → H2O (water) + e- | O 2 (oxygen) → H 2O (water) |
| Anammox bacteria | Planctomycetes | NH3 (ammonia) → N 2 (nitrogen) + e- |
NO− 2 (nitrite) |
| Thiobacillus denitrificans | Thiobacillus denitrificans | S0 (sulfur) → SO2− 4 (sulfate) + e- |
NO− 3 (nitrate) |
| Sulfate-reducing bacteria: Hydrogen bacteria | H2 (hydrogen) → H2O (water) + e- | Sulfate (SO2− 4) |
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| Sulfate-reducing bacteria: Phosphite bacteria | Desulfotignum phosphitoxidans | PO3− 3 (phosphite) → PO3− 4 (phosphate) + e- |
Sulfate (SO2− 4) |
| Methanogens | Archaea | H2 (hydrogen) → H2O (water) + e- | CO2 (carbon dioxide) |
| Carboxydotrophic bacteria | Carboxydothermus hydrogenoformans | carbon monoxide (CO) → carbon dioxide (CO2) + e- | H 2O (water) → H 2 (hydrogen) |
Photolithotrophs obtain energy from light and therefore use inorganic electron donors only to fuel biosynthetic reactions (e. g., carbon dioxide fixation in lithoautotrophs). (See navigation box below)
Lithotrophic bacteria cannot use, of course, their inorganic energy source as a carbon 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 weathering of parent material (bedrock) to form soil, as well as biogeochemical cycling of sulfur, nitrogen, and other elements. They may be present in the deep terrestrial subsurface (they have been found well over 3 km below the surface of the planet), in soils, and in endolith communities. As they are responsible for the liberation of many crucial nutrients, and participate in the formation of soil, lithotrophs play a critical role in the maintenance of life on Earth.
Lithotrophic microbial consortia are responsible for the phenomenon known as acid mine drainage, whereby energy-rich pyrites and other reduced sulfur compounds present in mine tailing heaps and in exposed rock faces is metabolized to form sulfates, thereby forming potentially toxic sulfuric acid. Acid mine drainage drastically alters the acidity and chemistry of groundwater and streams, and may endanger plant and animal populations. 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.
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