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A lithotroph is a subgroup of chemotrophic organism. Lithotrophs use an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation (i.e., ATP production) via aerobic or anaerobic respiration. 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 giant tube 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" was created from the Greek terms 'lithos' (rock) and 'troph' (consumer), meaning "eaters of rock". Many lithoautotrophs are extremophiles, but this is not universally so.
A chemolithotroph (named after the process of chemolithotropy) is able to use inorganic reduced compounds as a source of energy. This process is accomplished through oxidation and ATP synthesis. The majority of chemolithotrophs are able to fix carbon dioxide (CO2) through the Calvin cycle, a metabolic pathway in which carbon enters as CO2 and leaves as glucose. For some substrates, the cells must cull through large amounts of inorganic substrate to secure just a small amount of energy. This makes their metabolic process inefficient in many places and hinders them from thriving. This group of organisms includes sulfur oxidizers, nitrifying bacteria, iron oxidizers, and hydrogen oxidizers.
The term "chemolithotropy" refers to a cell’s acquisition of energy from the oxidation of inorganic compounds, also known as electron donors. This form of metabolism is believed to occur only in prokaryotes and was first characterized by Russian microbiologist Sergei Winogradsky.
Habitat of Chemolithotrophs
The survival of these prokaryotic bacteria is dependent on the physiochemical conditions of their environment. Although they are sensitive to certain factors such as quality of inorganic substrate, they are able to thrive under some of the most inhospitable conditions in the world, such as temperatures above 110 degrees Celsius and below 2 pH. The most important requirement for chemolithotropic life is an abundant source of rich inorganic compounds. These compounds are crucial for chemolithotrophs because they provide a suitable energy source/electron donor from which the microorganisms can fix CO2 and produce the energy they need to survive. Since chemosynthesis can take place in the absence of sunlight, these organisms are found mostly around hydrothermal vents and other locations rich in inorganic substrate.
The energy obtained from inorganic oxidation varies depending on the substrate and the reaction. For example, the oxidation of hydrogen sulfide to elemental sulfur produces far less energy (50.1 kcal/mol or 210.4 kJ/mol) than the oxidation of elemental sulfur to sulfate (149.8 kcal/mol or 629.2 kJ/mol). The majority of lithotrophs fix carbon dioxide through the Calvin cycle, an energetically expensive process. For some substrates, such as ferrous iron, the cells must cull through large amounts of inorganic substrate to secure just a small amount of energy. This makes their metabolic process inefficient in many places and hinders them from thriving.
Overview of the Metabolic Process
There is a fairly large variation in the types of inorganic substrates that these microorganisms can use to produce energy. The chemolithotrophs that are best documented are aerobic respirers, meaning that they use oxygen in their metabolic process. The list of these microorganisms that employ anaerobic respiration though is growing. At the heart of this metabolic process is an electron transport system that is similar to that of chemoorganotrophs. The major difference between these two microorganisms is that chemolithotrophs directly provide electrons to the electron transport chain, while chemoorganotrophs must generate their own cellular reducing power by oxidizing reduced organic compounds. Chemolithotrophs bypass this by obtaining their reducing power directly from the inorganic substrate or by the reverse electron transport reaction.
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. Some lithotrophs produce organic compounds from carbon dioxide in a process called chemosynthesis, much as plants do in photosynthesis. Plants use energy from sunlight to drive carbon dioxide fixation, since both water and carbon dioxide are low in energy. By contrast, the hydrogen compounds used in chemosynthesis are high in energy, so chemosynthesis can take place in the absence of sunlight (e.g., around a hydrothermal vent). 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.
Here are a few examples of chemolithotrophic pathways, any of which may use oxygen, sulfur or other molecules as electron acceptors:
Photolithotrophs obtain energy from light and therefore use inorganic electron donors only to fuel biosynthetic reactions (e. g., carbon dioxide fixation in lithoautotrophs).
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 a 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. Purple and green bacteria oxidize sulfide, sulfur, sulfite, iron or hydrogen. Cyanobacteria extract reducing equivalents from water, i.e., they oxidise water to oxygen. The electrons obtained from the electron donors 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.
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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