Lithotroph

Lithotrophs are a diverse group of organisms using 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 microorganisms; no known macrofauna possesses the ability to use 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 but not all lithoautotrophs are extremophiles. Lithotrophs are a diverse group of organisms using 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 microorganisms; no known macrofauna possesses the ability to use 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 but not all lithoautotrophs are extremophiles. Different from a lithotroph is an organotroph, an organism which obtains its reducing agents from the catabolism of organic compounds. The term was suggested in 1946 by Lwoff and collaborators. Lithotrophs consume reduced inorganic compounds (rich in electrons). A chemolithotroph (named after the process of chemolithotrophy) 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. 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 microbiologist Sergei Winogradsky. 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 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. 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 utilized 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 high electronegativity of oxygen and resulting large energy gains makes it ideal for use as a Terminal Electron Acceptor (TEA). 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 utilize 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 utilize from inorganic substrates, such as sulfur.