Operational definition
Classification axis.
The classification of an organism according to the principal origin of the electrons or reducing equivalents used in energy metabolism and biosynthesis.
The primary categories are:
- lithotroph — electrons come principally from inorganic donors;
- organotroph — electrons come principally from organic donors;
- mixed litho-organotroph — both contribute materially;
- electrotrophic modifier — extracellular conductive sources materially supply electrons.
The category concerns the donor’s chemical identity, not the terminal destination of the electrons.
12.1 Lithotroph
Electrons or reducing equivalents originate principally from inorganic donors.
Examples:
- H₂O
- H₂
- H₂S
- NH₃
- NO₂⁻
- Fe²⁺
12.2 Organotroph
Electrons originate principally from organic compounds.
12.3 Mixed litho-organotroph
Both inorganic and organic electron donors are used.
12.4 Electrotrophic modifier
Electrons are acquired directly or indirectly from extracellular conductive sources.
“Electrotroph” should be recorded as an acquisition modifier in addition to, rather than as a replacement for, the chemical identity of the electron source.