Lithotroph

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.

Biochemistry

Lithotrophs consume reduced compounds (rich in electrons).

Chemolithotrophs

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)
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

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)

Lithoheterotrophs versus lithoautotrophs

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:

• 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.

Chemolithotrophs versus photolithotrophs

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 use light as energy source. These bacteria are photosynthetic; photolithotrophic bacteria are found in the purple bacteria (e. g., Chromatiaceae), green bacteria (Chlorobiaceae and Chloroflexi) 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 chemolithotrophic metabolism in the dark.

Geological significance

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.

See also

• Organotroph
• Endolith

References

1. ^ http://faculty.weber.edu/mzwolinski/IntroMicro/1113Units/Unit%203/Litotrophs%20and%20Phototrophs.pdf
2. ^ Jorge G. Ibanez; Margarita Hernandez-Esparza, Carmen Doria-Serrano, Mono Mohan Singh (2007). Environmental Chemistry: Fundamentals. Springer. p. 156. ISBN 9780387260617. http://books.google.com/?id=6sPU95tqqn4C&pg=PA156&dq=sulfur+oxidizers+%22elemental+sulfur%22&q=sulfur%20oxidizers%20%22elemental%20sulfur%22. 
3. ^ Lengeler, Joseph W.; Drews, Gerhart; Schlegel, Hans Günter (1999). Biology of the Prokaryotes. Georg Thieme Verlag. p. 249. ISBN 9783131084118. http://books.google.com/?id=MiwpFtTdmjQC&pg=PA249&dq=sulfur+oxidizers+%22elemental+sulfur%22&q=sulfur%20oxidizers%20%22elemental%20sulfur%22. 
4. ^ Reddy, K. Ramesh; DeLaune, Ronald D. (2008). Biogeochemistry of Wetlands: Science and Applications. CRC Press. p. 466. ISBN 9781566706780. http://books.google.com/?id=8yLE_tMMTl8C&pg=PA466&dq=sulfur+oxidizers+%22elemental+sulfur%22&q=sulfur%20oxidizers%20%22elemental%20sulfur%22. 
5. ^ Canfield, Donald E.; Kristensen, Erik; Thamdrup, Bo (2005). Aquatic Geomicrobiology. Elsevier. p. 285. ISBN 9780120261475. http://books.google.com/?id=yrJxfG0_WyYC&pg=PA285&dq=iron+oxidizer#v=onepage&q=iron%20oxidizer.