| Composite class | Energy | Electron source | Carbon source |
|---|---|---|---|
| Photolithoautotroph | light | inorganic | inorganic carbon |
| Photoorganoheterotroph | light | organic | organic carbon |
| Chemolithoautotroph | chemical | inorganic | inorganic carbon |
| Chemolithoheterotroph | chemical | inorganic | organic carbon |
| Chemoorganoheterotroph | chemical | organic | organic carbon |
| Mixed or facultative forms | variable | variable | variable |
Representative classifications:
- green plants: predominantly photolithoautotrophic
- cyanobacteria: predominantly photolithoautotrophic
- animals: chemoorganoheterotrophic
- fungi: chemoorganoheterotrophic
- nitrifying microbes: commonly chemolithoautotrophic
- purple nonsulfur bacteria: frequently photoorganoheterotrophic, with substantial metabolic flexibility
The three axes must remain independent: light use does not prove autotrophy, organic-carbon use does not identify the electron acceptor, and inorganic-electron use does not prove carbon fixation. (PubMed Central (PMC))
Operational definition
Nomenclature rule.
A system for combining the energy-source, electron-source, and carbon-source axes into one metabolic descriptor.
The conventional order is:
energy source
+
electron source
+
carbon source
Thus:
- photolithoautotroph = light energy + inorganic electron donor + inorganic carbon;
- photoorganoheterotroph = light energy + organic electron donor + organic carbon;
- chemolithoautotroph = chemical energy + inorganic electron donor + inorganic carbon;
- chemolithoheterotroph = chemical energy + inorganic electron donor + organic carbon;
- chemoorganoheterotroph = chemical energy + organic electron donor + organic carbon.
The composite name is a compressed description of independent axes, not a fourth process. The separation of photo/chemo, litho/organo, and auto/heterotrophy is foundational to microbial energetic classification; mixotrophy represents combinations or conditional switching among those modes. (PubMed Central (PMC))