A chemolithotroph is an organism that is able to use inorganic reduced compounds as a source of energy. This mode of metabolism is known as chemolithotrophy.
Chemolithotrophy was discovered by Winogradsky while studying the microorganisms involved in the oxidation of sulfur compounds.
Chemolithotrophy is found only in prokaryotes and is widely distributed among Bacteria and Archaea. The spectrum of inorganic compounds that can be used as electron donors by chemolithotrophs is rather broad (H2S, S0, S2O 3 2− , H2, Fe2+, NO2 −or NH3). Some microorganisms are rather specific regarding the inorganic substrates they can use to generate energy, while others are able to use different compounds (versatile). The best characterized chemolithotrophs are aerobic respirers, which use oxygen as the electron acceptor, although the list of chemolithotrophs capable of employing anaerobic respiration is increasing rapidly. Chemolithotrophs have electron transport systems similar to those of chemoorganotrophs, which are used for the generation of a proton motive force. The only difference is that chemolithotrophs donate electrons directly to the electron transport chain, while chemoorganotrophs must generate cellular reducing power (NADH) from the oxidation of reduced organic compounds, which are then used to donate electrons to the electron transport system. This proton motive force is used to generate ATP or any cellular functions that might require this type of energy (active transport, movement, etc). An important distinction between chemolithotrophs and chemoorganotrophs is their source of carbon. Chemoorganotrophs use organic compounds as both energy and carbon sources, while chemolithotrophs are generally autotrophs (with few exceptions, known as mixotrophs, that use reduced organic compounds as a source of carbon). Chemolithotrophs can obtain the reducing power needed to assimilate CO2 directly from the inorganic substrate (only H2 oxidizers) or by the reverse electron transport reaction (the rest of chemolithotrophs), in this case using proton motive force as a source of energy.
Hydrogen is a common product of geochemical reactions and microbial metabolism, and a number of chemolithotrophs are able to use it as an electron donor in energy metabolism. A wide variety of anaerobic H2-oxidizing Bacteria and Archaea are known, differing in the electron acceptor they use (nitrate, sulfate, ferric iron, etc).
The most common sulfur compounds used as electron donors are hydrogen sulfide (H2S), elemental sulfur (S0), and thiosulfate (S2O 3 2− ). The final product of sulfur oxidation is sulfate (SO 4 2− ), although an intermediate step is the formation of elemental sulfur, which in some cases is stored as an alternative source of energy. One of the products of sulfur oxidation reaction is the generation of protons (H+), consequently one result of the oxidation of reduced sulfur compounds is the acidification of the environment by the production of sulfuric acid.
The aerobic oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) is an energy-yielding reaction, used by some prokaryotes to conserve energy. Only a small amount of energy is generated by this reaction, thus iron-oxidizing microorganisms must oxidize large amounts of reduced iron to grow. Ferrous iron is oxidized very rapidly in the presence of oxygen, while it is very stable at acidic conditions. This is probably the reason why many iron-oxidizing microorganisms are acidophilic. Despite the instability of ferrous iron at neutral pH, there are a number of iron-oxidizing bacteria that can thrive at circumneutral pH. Some anoxigenic phototrophic bacteria can use ferrous iron as a source of environmental reducing power. Recently it has been shown that some denitrifying bacteria can anaerobically respire (oxidize) reduced iron. The use of ferrous iron to obtain energy is widely distributed in nature, a property that was ignored until recently, due to thermodynamic considerations.
The most common nitrogen compounds used as electron donors for energy conservation are ammonia (NH3) and nitrite (NO2−). Both compounds can be oxidized aerobically by chemolithotrophic nitrifying bacteria. Some nitrifying microorganisms oxidize ammonia to nitrite, while another group oxidizes nitrite to nitrate. The complete oxidation of ammonia requires the concerted activity of these two types of microorganisms. A special case of nitrogen-oxidizing microorganisms corresponds to those capable of carrying out the anoxic oxidation of ammonia, a process known as anamox. In this case the electron acceptor is nitrite, and the product of the metabolic reaction in addition to proton motive force is the generation of N2. This metabolic reaction is carried out by a special type of microorganisms belonging to the Planctomycetes phylum of Bacteria.
Due to their metabolic properties, chemolithotrophs are of astrobiological interest and also critical elements of the biogeochemical cycles.