Final answer:
E. coli would continue to utilize aerobic respiration in the presence of O₂ due to the higher ATP yield. It can switch to anaerobic respiration or fermentation when oxygen is unavailable, showcasing its facultative anaerobic nature.
Step-by-step explanation:
E. coli are facultative anaerobes, meaning they are capable of surviving and growing in both aerobic (with oxygen) and anaerobic (without oxygen) environments. If E. coli were grown in the presence of O₂, it would likely continue to utilize the mechanism of aerobic respiration because it allows for much greater production of ATP, the energy currency of the cell, per glucose molecule compared to anaerobic respiration or fermentation. This is due to the fact that oxygen acts as a final electron acceptor in the electron transport chain during aerobic respiration, resulting in a more efficient production of ATP.
When conditions are anaerobic, E. coli will switch to using anaerobic respiration or fermentation pathways which do not require O₂. The pathways utilized, being fermentation or anaerobic respiration, depend on the availability of alternative electron acceptors in the environment. Evidence from early geological and biological evolution suggests that original life forms had to rely on anaerobic methods until the advent of photosynthesis, which increased atmospheric O₂, enabling the emergence of respiratory metabolism and obligate aerobes.