A lithotroph is an organism whose energy source is inorganic-chemical reactions; compare organotrophs, which use organic molecules, usually biomolecules, as their energy source. Most lithotrophs' energy-source chemical reactions can be interpreted as electron-transfer (redox) reactions; such reactions require an electron donor (reducer) and an electron acceptor (oxidizer).
- H2 -> 2H+ + 2e
- H2S -> 2H+ + S + 2e
- Fe++ -> Fe+++ + e
- O2 + 4H+ + 4e -> 2H2O
- NO3- + 6H+ + 5e -> (1/2)N2 + 3H2O
- SO4-- + 8H+ + 6e -> S + 4H2O
Organotrophs fit into this scheme because they use organic molecules as reducers -- and sometimes also as oxidizers (fermentation).
Methanogens use this reaction, in which carbon dioxide can be interpreted as the oxidizer:
- CO2 + 4H2 -> CH4 + 2H2O
Lithotrophs are usually autotrophic, and in some ecologies, like hydrothermal vents, lithotrophs are the primary producers.