Lithotroph

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A lithotroph is an organism which uses an inorganic substrate (usually of mineral origin) to obtain energy. Lithoautotrophs are exclusively microbes; macrofauna do not possess the capability to utilize inorganic compounds as energy sources. Most lithoautotrophs belong to the domain Bacteria. The term "Lithotroph" is created from the terms 'lithos' (rock) and 'troph' (consumer); literally, it may be read "eaters of rock." Many lithoautotrophs are extremophiles, but this is not universally so.

Lithotrophs consume reduced compounds (rich in electrons). The compounds - the electron donors - are broken up in the cell, and the electrons are channeled into 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.

Lithoautotrophs participate in many geological processes, such as the weathering of parent material (bedrock) to form soil, as well as biogeochemical cycling of sulfur, potassium, and other elements. They may be present in the deep terrestrial subsurface (they have been found well over a 3km 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, lithoautotrophs play a crucial role in the maintainence of life on Earth.

Lithoautotrophic 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 sulfites, which form potentially toxic sulfuric acid when dissolved in water. Acid mine drainage drastically alters the acidity and chemistry of groundwater and streams, and may endanger plant and animal populations. Activity similar to acid mine drainage, but on a much lower scale, is also found in natural conditions such as the rocky beds of glaciers, in soil and talus, and in the deep subsurface.

Here are a few examples of lithotrophic pathways, all of which may use oxygen as electron acceptor:

Iron bacteria oxidize ferrous iron (Fe²⁺) into ferric iron (Fe³⁺)
Nitrifying bacteria oxidize ammonia into nitrite or, alternatively, nitrite into nitrate.
Sulfur bacteria oxidize sulfide into sulfur or, alternatively, sulfur into sulfate.
Hydrogen bacteria oxidize hydrogen into water.

In the following example, a compound other than oxygen is used as electron acceptor:

Phosphite bacteria oxidize phosphite into phosphate. They use sulfate as electron acceptor, and reduce it into sulfide.

Lithotrophic bacteria cannot use, of course, their inorganic energy source as a carbon source for the synthesis of their cells, because the above-mentioned electron donors contain no carbon. They choose one of two options:

Heterolithotrophs must consume additional organic compounds in order to break them apart and use their carbon. Only few bacteria are heterolithotrophic.
Autolithotrophs are able to use carbon dioxide from the air as carbon source, the same way plants do. (also known as lithoautotrophs.)

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

Chemolithotrophs use the above-mentioned inorganic compounds. The energy produced by the oxidation of these compounds is enough for ATP production.
Photolithotrophs use light. These bacteria are photosynthetic; the almost only photolithotrophic bacteria are purple bacteria. The energy obtained from oxidation reactions (they oxidize sulfide, sulfite, iron or hydrogen) is not enough for ATP production; the rest of the energy comes from light.

The opposite of lithotroph is organotroph - an organism which gets its energy from the break up of organic compounds.

Lithotroph

See also