What is the defining characteristic of facultative anaerobes?

Facultative anaerobes are distinguished by their ability to grow in both the presence and absence of oxygen. This characteristic allows them to adapt to varying environmental conditions, making them versatile in terms of energy production. In an oxygen-rich environment, facultative anaerobes typically use aerobic respiration, which is more efficient in terms of energy yield. However, when oxygen is scarce or absent, they can switch to anaerobic metabolism, such as fermentation, to generate energy.

This adaptability provides a significant advantage in diverse habitats, allowing facultative anaerobes to survive and thrive in fluctuating conditions. Such organisms are commonly found in many environments, including soil and the intestines of animals, where oxygen levels can change.

The other options do not accurately capture the flexibility of facultative anaerobes. Some require oxygen for growth, while others thrive only in high carbon dioxide concentrations or strictly in the absence of oxygen. This adaptability uniquely characterizes facultative anaerobes, emphasizing their capability of utilizing both aerobic and anaerobic pathways depending on the available resources.