What defines a facultative anaerobe?

A facultative anaerobe is defined by its ability to adapt its metabolism depending on the presence of oxygen. This means that it can utilize oxygen for aerobic respiration when it's available, allowing for efficient energy production. However, when oxygen is scarce or completely absent, a facultative anaerobe can switch to anaerobic processes, such as fermentation, to generate energy. This versatility enables facultative anaerobes to thrive in diverse environments, making them highly resilient organisms.

In contrast, organisms that can only survive without oxygen or require oxygen exclusively do not have this metabolic flexibility. The adaptations of facultative anaerobes allow them to occupy a broader range of ecological niches compared to strictly anaerobic or aerobic organisms. This characteristic of being able to function effectively in both oxygen-rich and oxygen-poor conditions is what primarily distinguishes facultative anaerobes.