Alithoautotroph is an organism that derives energy from reactions ofreduced compounds ofmineral (inorganic) origin.[1] Two types of lithoautotrophs are distinguished by their energy source; photolithoautotrophs derive their energy from light, while chemolithoautotrophs (chemolithotrophs or chemoautotrophs) derive their energy from chemical reactions.[1] Chemolithoautotrophs are exclusivelymicrobes.Photolithoautotrophs includemacroflora such as plants; these do not possess the ability to use mineral sources of reduced compounds for energy. Most chemolithoautotrophs belong to the domainBacteria, while some belong to the domainArchaea.[1] Lithoautotrophic bacteria can only useinorganic molecules as substrates in their energy-releasing reactions. The term "lithotroph" is from Greeklithos (λίθος) meaning "rock" andtrōphos (τροφοσ) meaning "consumer"; literally, it may be read "eaters of rock." The "lithotroph" part of the name refers to the fact that these organisms use inorganic elements/compounds as their electron source, while the "autotroph" part of the name refers to their carbon source being CO2.[1] Many lithoautotrophs areextremophiles, but this is not universally so, and some can be found to be the cause ofacid mine drainage.
Lithoautotrophs are extremely specific in their source of reduced compounds. Thus, despite the diversity in using inorganic compounds that lithoautotrophs exhibit as a group, one particular lithoautotroph would use only one type of inorganic molecule to get its energy. A chemolithotrophic example isanaerobic ammonia oxidizing bacteria (anammox), which use ammonia and nitrite to producedinitrogen (N2).[1] Additionally, in July 2020, researchers reported the discovery of chemolithoautotrophic bacterial cultures thatfeed on the metalmanganese after performing unrelated experiments and named their bacterial speciesCandidatus Manganitrophus noduliformans andRamlibacter lithotrophicus.[3]
Some chemolithotrophs use redox half-reactions with low reduction potentials for their metabolisms, meaning that they do not harvest a lot of energy compared to organisms that use organotrophic pathways.[1] This leads some chemolithotrophs, such asNitrosomonas, to be unable to reduce NAD+ directly; therefore, these organisms rely on reverse electron transport to reduce NAD+ and form NADH and H+.[1]
Lithoautotrophs participate in many geological processes, such as theweathering ofparent material (bedrock) to formsoil, as well as biogeochemical cycling ofsulfur,potassium, and other elements.[1] The existence of undiscovered strains of microbial lithoautotrophs is theorized based on some of these cycles, as they are needed to explain phenomena like the conversion of ammonium in iron-reducing environments.[4] Lithoautotrophs may be present in the deep terrestrial subsurface (they have been found well over 3 km below the surface of the planet), insoils, and inendolith communities. As they are responsible for the liberation of many crucial nutrients, and participate in theformation of soil, lithoautotrophs play a crucial role in the maintenance oflife on Earth. For example, the nitrogen cycle is influenced by the activity of ammonium-oxidizing archaea, anammox bacteria, andcomplete ammonium-oxidizing (comammox) bacteria of the genusNitrospira.[4]
Several environmental hazards, such asammonium (NH4+),hydrogen sulfide (H2S), and thegreenhouse gasmethane (CH4), may be converted by chemolithoautotrophs into forms that are less environmentally harmful, such as N2,SO42-, andCO2.[4] Although it was long believed that these organisms required oxygen to make these conversions, recent literature suggests that anaerobic oxidation also exists for these systems.[4]
Lithoautotrophic microbial consortia are responsible for the phenomenon known asacid mine drainage, wherebypyrite present in mine tailing heaps and in exposed rock faces is metabolized, usingoxygen, to producesulfites, which form potentially corrosivesulfuric acid when dissolved in water and exposed to aerialoxygen.[5] Acid mine drainage drastically alters the acidity and chemistry ofgroundwater andstreams and may endanger plant and animal populations. Activity similar to acid mine drainage, but on a much lower scale, is also found in natural conditions such as the rocky beds ofglaciers, in soil andtalus, and in the deep subsurface.