All living organisms can be categorized by their response to atmospheric oxygen as obligate aerobes, facultative aerobes, obligate anaerobes, and/or facultative anaerobes. What do the terms facultative and obligate mean in this context?

Facultative mean survival in presence or absence of oxygen;

Obligate mean survival either in the presence of oxygen or absence, NOT BOTH

Explanation:

An obligate aerobe is an organism capable of survival ONLY in the presence of oxygen. A good example is human being.

A facultative anaerobe is an organism capable of survival in either the presence of oxygen or absence. A good example is Escherichia coli

Obligate anaerobes is capable of survival ONLY in the absence of Oxygen, because they die in the presence of oxygen.