Question

Some cells live where oxygen may or may not br available. Can you think of such situations (and organisms) where $O_2$ is not available ?

Answer 1

Some cells like anaerobic bacteria do not require molecular oxygen directly from the environment for survival. They use other chemosynthetic mechanisms for survival.

Examples of naturally occurring anaerobic places on earth are at the bottom of the ocean, ocean vents, hot springs that reach extreme temperatures, etc.