An aerobic organism (or aerobe) is an organism that has an oxygen based metabolism. Of course, all human beings are aerobes, and so the idea that this kind of metabolism requires special adaptations is not intuitively obvious. However, oxygen produces negatively charged ions when metabolized and, unless organisms have developed anti-oxidant pathways to counter these, using oxygen is dangerous. These oxygen radicals are toxic, and only organisms which express radical-scavenging enzymes (like catalase and superoxide dismutase) can survive their presence.
- Obligate aerobes strictly depend on energy production through aerobic respiration and are unable to survive for more than a few minutes in the absence of oxygen
- facultative aerobes can use oxygen, but are able to survive in its absence, through e.g. fermentation.
- Microaerophiles are organisms that may use oxygen, but only at low concentration.
- Aerotolerant organisms can survive in the presence of oxygen, but do not use it as electron acceptor.
Among the Prokaryotes, there are representatives of all the classes of oxygen metabolism in the Eubacteria. In Archea, the diverse group of prokaryotes forming a completely different line of descent from all other organisms, very few of the organisms are aerobes. A notable exception is Aeropyrum pernix.
Thus, most anaerobic organisms are Bacteria or Archaea.
Being an obligate aerobe, although being advantageous from the energetic point of view, means also obligatory facing high levels of oxidative stress.
Yeast is an example of a facultative aerobe. Individual human cells are also facultative aerobes: they switch to lactic acid fermentation if oxygen is not available. However, for the whole organism this cannot be sustained for long, and, therefore, even though we contain some cells that can survive oxygen deprivation, humans are obligate aerobes.
Examples of Obligate Aerobic Bacteria: Nocardia (Gram positive), Pseudomonas aeruginosa (Gram negative), Mycobacterium tuberculosis (Acid Fast), and Bacillus (Gram positive).