Lithotroph

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A lithotroph is an organism which 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] Lithotrophs are exclusively microbes or plants; macrofauna do not possess the capability 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 either to the domain Bacteria or Archaea. The term "Lithotroph" is created from the terms 'lithos' (rock) and 'troph' (consumer). It literally is the "eaters of rock." Many lithoautotrophs are extremophiles, but this is not universally so.

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

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 a 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 crucial 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.

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

Iron bacteria oxidize ferrous iron (Fe²⁺) into ferric iron (Fe³⁺)
Nitrifying bacteria oxidize ammonia into nitrite or, alternatively, nitrite into nitrate.
Purple sulfur bacteria and some chemolithotrophs oxidize sulfide into sulfur. Here oxygen is the electron acceptor.
Sulfur bacteria use oxidized sulfur compounds to produce sulfide. They also can grow on a number of oxidized or partly oxidized sulfur compounds (e. g. sulfate, thiosulfate, thionates, polysulfides, sulfite). Here sulfur is the electron acceptor.
Hydrogen bacteria oxidize hydrogen to water.
Carboxydotrophic bacteria oxidise carbon monoxide to carbon dioxide.

In the following examples, compounds other than oxygen is used as electron acceptors:

Methanogens are Archaea capable of oxidising hydrogen at the cost of carbon dioxide reduction to methane.
Thiobacillus denitrificans is one of many known sulfur bacteria oxidizing reduced sulfur compounds with nitrate instead of oxygen.
The recently discovered Anammox bacteria oxidise ammonia with nitrite as electron acceptor to produce nitrogen gas.
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:

Lithoheterotrophs do not have the possibility to fix carbon dioxide and must consume additional organic compounds in order to break them apart and use their carbon. Only few bacteria are fully heterolithotrophic.
Lithoautotrophs are able to use carbon dioxide from the air as carbon source, the same way plants do.
Mixotrophs will take up and utilise organic material to complement their carbon dioxide fixation source (mix between autotrophy and heterotrophy). Many lithotrophs are recognised as mixotrophic in regard of their C-metabolism.

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 electrons derived from the inorganic donors also need to be chanelled 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 use light as energy source. These bacteria are photosynthetic; photolithotrophic bacteria are found in the purple bacteria (e. g. Chromatiaceae), green bacteria (Chlorobiaceae and Chloroflexaceae) and Cyanobacteria. The electrons obtained from the electron donors (purple and green bacteria oxidize sulfide, sulfur, sulfite, iron or hydrogen; Cyanobacteria extract reducing equivalents from water, i. e. oxidise water to oxygen) are not used for ATP production (as long as there is light); they are used in biosynthetic reactions. Some photolithotrophs shift over to chemolithotropic metabolism in the dark.