Lithotroph

Lithotrophs are a diverse group of organisms using an inorganic substrate to obtain reducing equivalents for use in biosynthesis or energy conservation via aerobic or anaerobic respiration. While lithotrophs in the broader sense include photolithotrophs like plants, chemolithotrophs are exclusively microorganisms; no known macrofauna possesses the ability to use inorganic compounds as electron 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. Chemolithotrophs belong to the domains Bacteria and Archaea. The term "lithotroph" was created from the Greek terms 'lithos' and 'troph', meaning "eaters of rock". Many but not all lithoautotrophs are extremophiles.
The last universal common ancestor of life is thought to be a chemolithotroph. Different from a lithotroph is an organotroph, an organism which obtains its reducing agents from the catabolism of organic compounds.

History

The term was suggested in 1946 by Lwoff and collaborators.

Biochemistry

Lithotrophs consume reduced inorganic compounds.

Chemolithotrophs

A chemolithotroph is able to use inorganic reduced compounds in its energy-producing reactions. This process involves the oxidation of inorganic compounds coupled to ATP synthesis. The majority of chemolithotrophs are chemolithoautotrophs, able to fix carbon dioxide through the Calvin cycle, a metabolic pathway in which CO₂ is converted to glucose. This group of organisms includes sulfur oxidizers, nitrifying bacteria, iron oxidizers, and hydrogen oxidizers.
The term "chemolithotrophy" 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 Ukrainian microbiologist Sergei Winogradsky.

Habitat of chemolithotrophs

The survival of these 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 inorganic compounds, which provide a suitable electron donor in order to fix CO₂ and produce the energy the microorganism needs 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 by ½O₂ produces far less energy than the oxidation of elemental sulfur to sulfate by 3/2 O₂,. The majority of lithotrophs fix carbon dioxide through the Calvin cycle, an energetically expensive process. For some low-energy 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. Sulfur is one of many inorganic substrates that can be used in different reduced forms depending on the specific biochemical process that a lithotroph uses. 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. Certain specialized chemolithotrophic bacteria use different derivatives of the Sox system; a central pathway specific to sulfur oxidation. This ancient and unique pathway illustrates the power that chemolithotrophs have evolved to use from inorganic substrates, such as sulfur.
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, but a variety of other electron acceptors, organic and inorganic, are also used by various species. Aerobic bacteria such as the nitrifying bacteria, Nitrobacter, use oxygen to oxidize nitrite to nitrate. 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, but chemosynthesis can take place in the absence of sunlight. Ecosystems establish in and around hydrothermal vents as the abundance of inorganic substances, namely hydrogen, are constantly being supplied via magma in pockets below the sea floor. Other lithotrophs are able to directly use inorganic substances, e.g., ferrous iron, hydrogen sulfide, elemental sulfur, thiosulfate, or ammonia, for some or all of their energy needs.
Here are a few examples of chemolithotrophic pathways, any of which may use oxygen or nitrate as electron acceptors:

Name	Examples	Source of electrons	Respiration electron acceptor
Iron bacteria	Acidithiobacillus ferrooxidans	Fe²⁺ → Fe³⁺ + e⁻	O + 4H⁺ + 4e⁻→ 2HO
Nitrosifying bacteria	Nitrosomonas	NH₃ + 2HO → NO + 7H⁺ + 6e⁻	O + 4H⁺ + 4e⁻ → 2HO
Nitrifying bacteria	Nitrobacter	NO + HO → NO + 2H⁺ + 2e⁻	O + 4H⁺ + 4e⁻ → 2HO
Chemotrophic purple sulfur bacteria	Halothiobacillaceae	S → S + 2e⁻	O + 4H⁺ + 4e⁻→ 2HO
Sulfur-oxidizing bacteria	Chemotrophic Rhodobacteraceae and Thiotrichaceae	S + 4HO → SO + 8H⁺ + 6e⁻	O + 4H⁺ + 4e⁻→ 2HO
Aerobic hydrogen bacteria	Cupriavidus metallidurans	H₂ → 2H⁺ + 2e⁻	O + 4H⁺ + 4e⁻→ 2HO
Anammox bacteria	Planctomycetota	NH → 1/2N₂ + 4H⁺ + 3e⁻	NO + 4H⁺ + 3e⁻→ 1/2N₂ + 2HO
Thiobacillus denitrificans	Thiobacillus denitrificans	S + 4HO → SO + 8H⁺ + 6e⁻	NO + 6H⁺ + 5e⁻→ 1/2N₂ + 3HO
Sulfate-reducing bacteria: Hydrogen bacteria	Desulfovibrio paquesii	H₂ → 2H⁺ + 2e⁻	SO + 8H⁺ + 6e⁻ → S + 4HO
Sulfate-reducing bacteria: Phosphite bacteria	Desulfotignum phosphitoxidans	PO + HO → PO + 2H⁺ + 2e⁻	SO + 8H⁺ + 6e⁻ → S + 4HO
Methanogens	Archaea	H₂ → 2H⁺ + 2e⁻	CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2HO
Carboxydotrophic bacteria	Carboxydothermus hydrogenoformans	CO + HO → CO₂ + 2H⁺ + 2e⁻	2H⁺ + 2e⁻ → H

Photolithotrophs

Photolithotrophs such as plants obtain energy from light and therefore use inorganic electron donors such as water only to fuel biosynthetic reactions.

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 ability 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 lithoheterotrophic.
Lithoautotrophs are able to use carbon dioxide from the air as a carbon source, the same way plants do.
Mixotrophs will take up and use organic material to complement their carbon dioxide fixation source. Many lithotrophs are recognized as mixotrophic in regard to 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, which occurs by reverse electron transfer reactions.
Photolithotrophs use light as their energy source. These organisms are photosynthetic; examples of photolithotrophic bacteria are purple bacteria, green bacteria, and "Cyanobacteria". Purple and green bacteria oxidize sulfide, sulfur, sulfite, iron or hydrogen. Cyanobacteria and plants extract reducing equivalents from water, i.e., they oxidize water to oxygen. The electrons obtained from the electron donors are not used for ATP production ; they are used in biosynthetic reactions. Some photolithotrophs shift over to chemolithotrophic metabolism in the dark.