An organism that grows both in the presence and the absence of oxygen and uses oxygen when it is available is called a(n) .

Facultative anaerobe

Explanation:

Facultative anaerobes are the organisms that are able to grow both in oxygenic and anoxygenic environment but usually prefer oxygen to grow when it is present in the environment.  

Facultative anaerobes in absence of oxygen can switch to fermentation to form ATP and perform a metabolic function but they prefer aerobic respiration when oxygen is available. Bacteria living in our intestine are facultative anaerobes. The growth of these organisms is higher in aerobic conditions than anaerobic conditions.

Therefore the correct answer is a facultative anaerobe.