An organotroph is an organism which gets its energy from organic compounds, usually by consuming or breaking down other organisms. Nearly all organotrophs are also heterotrophs, meaning that they also get their carbon from that source. Organotrophs include all animals and fungi and many bacteria and protists.
Organotrophs' energy extraction can be either anaerobic (fermentation) or aerobic. Anaerobic metabolism involves rearrangment of organic molecules, like:
- Sugars -> Lactate
- Sugars -> Ethanol + CO2
By contrast, aerobic metabolism involves transferring electrons from organic compounds to some oxidizer (electron acceptor) like oxygen molecules, nitrate, or sulfate. Carbon is released as carbon dioxide, hydrogen as water, etc.
In aerobic organisms' energy metabolism, nearly all of the steps are anaerobic. Combination with oxygen is the last step, and one that was apparently developed or transferred several times among prokaryotes. Eukaryotes use mitochondria for aerobic metabolism; the rest of the cell is anaerobic.
Not surprisingly, some organisms are facultative anaerobes, being able to switch between aerobic and anaerobic metabolism as the available oxygen varies. This distinguishes them from obligate anaerobes and aerobes, which are committed to their nonuse or use of oxygen, respectively.
These considerations suggest that anaerobic metabolism was the original form, and that aerobic metabolism was later added in, which is consistent with life being present on Earth before its atmosphere became significantly oxygenated.