Lithotroph 3525665 221989455 2008-06-27T00:51:08Z Sabedon 2083035 /* See also */ [[Category:Microbiology]] {{Moresources|date=February 2008}} 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.<ref>http://faculty.weber.edu/mzwolinski/IntroMicro/1113Units/Unit%203/Litotrophs%20and%20Phototrophs.pdf</ref> Lithotrophs are exclusively [[microbe]]s or [[plant]]s; macro[[fauna]] 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 [[Redox|reduced]] [[Chemical compounds|compounds]] (rich in [[electron]]s). In chemolithotrophs, the compounds - the '''electron donors''' - are oxidized in the [[cell (biology)|cell]], and the electrons are channeled into respiratory chains, ultimately producing [[Adenosine triphosphate|ATP]]. The electron acceptor can be [[oxygen]] (in [[aerobic]] bacteria), but a variety of other electron acceptors, [[organic compound|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 [[chemical element|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]] [[oxidation|oxidize]] ferrous iron (Fe<sup>2+</sup>) into ferric iron (Fe<sup>3+</sup>) * [[Nitrifying bacteria]] oxidize [[ammonia]] into [[nitrite]] or, alternatively, nitrite into [[nitrate]]. * Purple sulfur [[bacteria]] and some [[chemolithotroph]]s 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. * '''[[Lithoautotroph]]s''' are able to use [[carbon dioxide]] from the [[air]] as carbon source, the same way [[plant]]s do. * '''[[Mixotroph]]s''' 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 [[photosynthesis|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. The opposite of lithotroph is [[organotroph]] - an organism which gets its energy from the break up of organic compounds. == References == {{Reflist}} == See also == * [[Endolith]] [[Category:Soil biology]] [[Category:Metabolism]] [[Category:Microbiology]] [[cs:Litotrofie]] [[de:Lithotrophie]] [[et:litotroof]] [[fr:Lithotrophie]] [[nl:Lithotroof]] [[uk:Літотрофи]]