Achemotroph is an organism that obtainsenergy by theoxidation ofelectron donors in their environments.[1] These molecules can beorganic (chemoorganotrophs) orinorganic (chemolithotrophs). The chemotroph designation is in contrast tophototrophs, which use photons. Chemotrophs can be eitherautotrophic orheterotrophic. Chemotrophs can be found in areas where electron donors are present in high concentration, for instance aroundhydrothermal vents.[citation needed]
Chemoautotrophs areautotrophic organisms that can rely onchemosynthesis, i.e. derivingbiological energy fromchemical reactions of environmentalinorganicsubstrates and synthesizing all necessaryorganic compounds fromcarbon dioxide. Chemoautotrophs can use inorganic energy sources such ashydrogen sulfide, elementalsulfur,ferrous iron, molecularhydrogen, andammonia or organic sources to produce energy. Most chemoautotrophs areprokaryoticextremophiles,bacteria, orarchaea that live in otherwise hostile environments (such asdeep sea vents) and are theprimary producers in suchecosystems. Chemoautotrophs generally fall into several groups:methanogens,sulfur oxidizers andreducers,nitrifiers,anammox bacteria, andthermoacidophiles. An example of one of these prokaryotes would beSulfolobus. Chemolithotrophic growth can be very fast, such asHydrogenovibrio crunogenus with adoubling time around one hour.[2][3]
The term "chemosynthesis", coined in 1897 byWilhelm Pfeffer, originally was defined as the energy production by oxidation of inorganic substances in association withautotrophy — what would be named today aschemolithoautotrophy. Later, the term would include also thechemoorganoautotrophy, that is, it can be seen as a synonym of chemoautotrophy.[4][5]
Chemoheterotrophs (or chemotrophic heterotrophs) are unable tofix carbon to form their own organic compounds. Chemoheterotrophs can bechemolithoheterotrophs, utilizing inorganic electron sources such as sulfur, or, much more commonly,chemoorganoheterotrophs, utilizing organic electron sources such ascarbohydrates,lipids, andproteins.[6][7][8][9] Most animals and fungi are examples of chemoheterotrophs, as arehalophiles.[citation needed]
Iron-oxidizing bacteria are chemotrophicbacteria that deriveenergy byoxidizing dissolvedferrousiron. 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 dissolvedoxygen is needed to carry out the oxidation.[10]
Iron has many existing roles in biology not related toredox reactions; examples includeiron–sulfur proteins,hemoglobin, andcoordination complexes. Iron has a widespread distribution globally and is considered one of the most abundant in the Earth's crust, soil, and sediments.[11] Iron is a trace element inmarine environments.[11] Its role as the electron donor for somechemolithotrophs is probably very ancient.[12]
1. Katrina Edwards.Microbiology of a Sediment Pond and the Underlying Young, Cold, Hydrologically Active Ridge Flank. Woods Hole Oceanographic Institution.
2. Coupled Photochemical and Enzymatic Mn(II) Oxidation Pathways of a Planktonic Roseobacter-Like Bacterium. Colleen M. Hansel and Chris A. Francis* Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115. Received 28 September 2005. Accepted 17 February 2006.