Obligate anaerobes, facultative anaerobes, and microaerophiles are terms referring to bacteria that require:

Obligate anaerobes, facultative anaerobes, and microaerophiles are classifications of bacteria based on their oxygen requirements. Obligate anaerobes cannot survive in the presence of oxygen; they thrive in environments devoid of oxygen and can even be harmed by it. In contrast, facultative anaerobes can switch between aerobic respiration when oxygen is present and fermentation or anaerobic respiration when it is not. Microaerophiles require a small amount of oxygen for growth but are poisoned by high concentrations.

The correct answer is decreased O2 because these types of bacteria demonstrate varying tolerances to oxygen levels. Obligate anaerobes need a complete lack of oxygen, while microaerophiles require low levels. Understanding these classifications is crucial in microbiology for cultivating bacteria in lab settings, determining the conditions necessary for their growth, and recognizing their ecological roles.