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A lithotroph is an organism that uses an inorganic substrate (usually of mineral origin) to
obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or
energy conservation via aerobic or anaerobic respiration.
Known chemolithotrophs are exclusively microbes; no known macrofauna possesses the
ability to utilize inorganic compounds as energy sources.
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 lithoautotrophs are extremophiles, but this is
not universally so.
Photolithotrophs
Photolithotrophs obtain energy from light and therefore use inorganic electron donors only to
fuel biosynthetic reactions (e. g., carbon dioxide fixation in lithoautotrophs). (See navigation box
below)
Lithoheterotrophs versus lithoautotrophs
Lithotrophic bacteria cannot use, of course, their inorganic energy source as a carbon source for
the synthesis of their cells. They choose one of three options:
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Lithoheterotrophs do not have the possibility to fix carbon dioxide and must consume
additional organic compounds in order to break them apart and use their carbon. Only a
few bacteria are fully heterolithotrophic.
Lithoautotrophs are able to use carbon dioxide from the air as carbon source, the same
way plants do.
Mixotrophs will take up and utilise organic material to complement their carbon dioxide
fixation source (mix between autotrophy and heterotrophy). Many lithotrophs are
recognised as mixotrophic in regard of their C-metabolism.
Chemolithotrophs versus photolithotrophs
In addition to this division, lithotrophs differ in the initial energy source which initiates ATP
production:
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Chemolithotrophs use the above-mentioned inorganic compounds for aerobic or
anaerobic respiration. The energy produced by the oxidation of these compounds is
enough for ATP production. Some of the electrons derived from the inorganic donors
also need to be channeled into biosynthesis. Mostly, additional energy has to be invested
to transform these reducing equivalents to the forms and redox potentials needed (mostly
NADH or NADPH), which occurs by reverse electron transfer reactions.
Photolithotrophs use light as energy source. These bacteria are photosynthetic;
photolithotrophic bacteria are found in the purple bacteria (e. g., Chromatiaceae), green
bacteria (Chlorobiaceae and Chloroflexi) and Cyanobacteria. The electrons obtained from
the electron donors (purple and green bacteria oxidize sulfide, sulfur, sulfite, iron or
hydrogen; cyanobacteria extract reducing equivalents from water, i. e., oxidise water to
oxygen) are not used for ATP production (as long as there is light); they are used in
biosynthetic reactions. Some photolithotrophs shift over to chemolithotrophic metabolism
in the dark.