You are digging in a swampy area, looking for new bacteria. you dig deep into the mud where little oxygen penetrates and take a sample back to the lab for culturing. you find one species that grows in the absence of oxygen and it also grows and uses oxygen when it is available. this bacterium is a

The bacterium, which is able to survive in both aerobic as well as anaerobic conditions are known as facultative anaerobes.

The facultative anaerobes are the organism, which can use oxygen for the synthesis of the ATP (adenosine triphosphate) molecule, but in the condition, where the oxygen is less or absent, they shift to fermentation or anaerobic respiration.

Hence, the correct answer is facultative anaerobes.

substitution is the answer in this case