What term describes organisms that can live with or without oxygen?

The term that describes organisms capable of living with or without oxygen is "facultative anaerobes." These organisms have the flexibility to use aerobic respiration when oxygen is present, but can also switch to anaerobic processes when oxygen is absent, allowing them to thrive in a variety of environments.

For example, facultative anaerobes can carry out fermentation or anaerobic respiration in oxygen-depleted conditions, thereby producing energy through different metabolic pathways. This adaptability gives them a significant advantage in fluctuating environments where oxygen levels may change.

In contrast, obligate anaerobes cannot survive in the presence of oxygen, as it is toxic to them, while obligate aerobes require oxygen for their survival and cannot carry out anaerobic processes at all. Microaerophiles have a specific requirement for low levels of oxygen and cannot survive in high concentrations.