Chemotroph

Chemoautotrophs are autotrophic organisms that can rely on chemosynthesis, i.e. deriving biological energy from chemical reactions of environmental inorganic substrates and synthesizing all necessary organic compounds from carbon dioxide. Chemoautotrophs can use inorganic energy sources such as hydrogen sulfide, elemental sulfur, ferrous iron, molecular hydrogen, and ammonia or organic sources to produce energy. Most chemoautotrophs are prokaryotic extremophiles, bacteria, or archaea that live in otherwise hostile environments (such as deep sea vents) and are the primary producers in such ecosystems. Chemoautotrophs generally fall into several groups: methanogens, sulfur oxidizers and reducers, nitrifiers, anammox bacteria, and thermoacidophiles. An example of one of these prokaryotes would be Sulfolobus. Chemolithotrophic growth can be very fast, such as Hydrogenovibrio crunogenus with a doubling time around one hour.^[3][4]

The term "chemosynthesis", coined in 1897 by Wilhelm Pfeffer, originally was defined as the energy production by oxidation of inorganic substances in association with autotrophy — what would be named today as chemolithoautotrophy. Later, the term would include also the chemoorganoautotrophy, that is, it can be seen as a synonym of chemoautotrophy.^[5][6]

Chemoheterotrophs (or chemotrophic heterotrophs) are unable to fix carbon to form their own organic compounds. Chemoheterotrophs can be chemolithoheterotrophs, utilizing inorganic electron sources such as sulfur, or, much more commonly, chemoorganoheterotrophs, utilizing organic electron sources such as carbohydrates, lipids, and proteins.^{[7][8][9][10]} Most animals and fungi are examples of chemoheterotrophs, as are some halophiles.^[11][12]

Iron-oxidizing bacteria are chemotrophic bacteria that derive energy by oxidizing dissolved ferrous iron. They are known to grow and proliferate in waters containing iron concentrations as low as 0.1 mg/L. However, at least 0.3 ppm of dissolved oxygen is needed to carry out the oxidation.^[13]

Iron has many existing roles in biology not related to redox reactions; examples include iron–sulfur proteins, hemoglobin, and coordination complexes. Iron has a widespread distribution globally and is considered one of the most abundant in the Earth's crust, soil, and sediments.^[14] Iron is a trace element in marine environments.^[14] Its role as the electron donor for some chemolithotrophs is probably very ancient.^[15]

Methanogens are chemotrophic archaea that produce methane as a byproduct of their metabolic processes.^[16] Methanogens belong to the euryarchaeal domain.^[17] Methanogens are a part of an ancient monophyletic lineage within Euryarcheota phylum and can be classified into three classes, six orders, twelve families and thirty-five genera.^[18] Methanogenic metabolic pathways are thought to be present in some of the earliest organisms that survived on earth.^[19] Today, methanogens can be found in a wide range of environments, both oxic and anoxic and both terrestrial and aquatic, especially environments containing low sulfate.^[20][21]

Methanogens are identified by their ability when producing methane gas, to conserve energy needed to synthesize ATP (adenosine triphosphate, this difference is important to distinguish methanogens from other bacteria or archea that produce methane as a byproduct of metabolism.^[21] Methanogenic archaea are involved in the late steps of degradation of organic matter, where they produce methane.^[20][22] While different organisms may use different substrates, they all share methane as the final product of their metabolic processes, and they are all anaerobic processes.^[20] Some examples of the type of substrates methanogens use are acetate, formate, methylamine and other small molecules as well as carbon dioxide and hydrogen gas.^[23]Regardless of the substrate, all methanogenic bacteria utilize the enzyme methyl-coenzyme M reductase, which performs the final step of reducing methyl-coenzyme M to methane.^[17][24]

• Chemosynthesis
• Lithotroph
• Methanogen (feeds on hydrogen)
• Methanotroph
• RISE project – expedition that discovered high-temperature vent communities